Audio Compact Disc (CD-DA) FAQ

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How does a CD work?

The design of the CD-DA (Compact Disk) Digital Audio, CD - digital sound) and the method of recording sound on it is described by the standard of the companies Sony and Philips that proposed it, published in 1980 under the name Red Book.

A standard compact disc (CD) consists of three layers: base, reflective and protective. The base is made of transparent polycarbonate, on which an information relief is formed by pressing. A metal reflective layer (aluminum, gold, silver, other metals and alloys) is sprayed over the relief. The reflective layer is covered on top with a protective layer of polycarbonate or neutral varnish - so that the entire metal surface is protected from contact with the external environment. The total thickness of the disc is 1.2 mm.

The information relief of the disk is a continuous spiral path starting from the center and consisting of a sequence of depressions - pits. The spaces between the pits are called lands. By alternating pits and gaps of various lengths, an encoded digital signal is recorded on the disk: the transition from gap to pit and vice versa denotes a unit, and the length of a pit or gap is the length of a series of zeros. The distance between the turns of the track is selected from 1.4 to 2 microns, the standard specifies a distance of 1.6 microns.

How is the audio signal represented on the disc?

The original stereo audio signal is digitized into 16-bit samples ( linear quantization) with a sampling frequency of 44.1 kHz. The resulting digital signal is called PCM (Pulse Code Modulation) because each pulse of the source signal is represented by a separate codeword. Every six samples of the left and right channels are formatted into primary frames, or microframes, of 24 bytes (192 bits) in size, arriving at a speed of 7350 pieces per second, which are encoded using a two-level CIRC code (Cross Interleaved Reed-Solomon Code). -Solomon with cross interleaving) according to the scheme: interleaving with a 1-byte delay, C2 level encoding, cross-interleaving with a variable delay, C1 level encoding, interleaving with a 2-byte delay. Level C1 is designed to detect and correct single errors, C2 - group errors. The result is a 256-bit block, the data in which is equipped with error detection and correction bits, and is also “smeared” down to the block, which leads to the recording of contiguous audio data in physically non-contiguous areas of the disk and reduces the impact of errors on individual samples.

The Reed-Solomon code has 25% redundancy and can detect up to four erroneous bytes and correct up to four lost or two erroneous bytes. The maximum length of a fully correctable error packet is about 4000 bits (~2.5 mm track length), but not every packet of this length can be completely corrected.

After the second interleaving, subcode bits are added to each received block - P, Q, R, S, T, U, V, W; each block receives eight subcode bits. Then, every 98 blocks with subcodes are formed into one superframe with a duration of 1/75 sec (the amount of pure audio data is 2352 bytes), also called a sector, in which the subcodes of the first two blocks serve as a sign of synchronization, and the remaining 96 bits of each subcode form the P-word, Q-word, etc. Throughout the track, the sequence of sub-code words is also called sub-code channels.

Words or subcode channels are used to control the recording format, indicate fragments of a soundtrack, etc. - for example, the P channel is used to mark audio tracks and pauses between them (0 - pause, 1 - sound), and the Q channel is used to mark the track format and sectors, TOC entries (Table Of Contents) and timestamps that track playback time. Channel Q can also be used to record information in the ISRC (International Standard Recording Code), which is intended to represent information about the manufacturer, release time, etc., as well as to divide the track into separate fragments (all on audio A disc can have up to 99 audio tracks, each of which can include up to 99 fragments).

Finally, frames designed in this way are channel encoded in pit-gap terms using 8/14 (Eight to Fourteen Modulation (EFM)) redundancy code, in which the source bytes are encoded into 14-bit words, increasing the intelligibility of the signal. Three link bits are inserted between words to maintain restrictions on the number of adjacent zeros and ones, which facilitates demodulation and reduces the DC component of the signal. As a result, 588 channel bits are obtained from each primary microframe, and the resulting bit stream is written to disk at a speed of 4.3218 (588x7350) Mbps. Since EFM coding produces a digital stream in which there are more zeros than ones, a system was chosen to represent units by the boundaries of a pit and a gap, and the number of zeros between ones by the length of a pit or gap, respectively.

At the beginning of the disc there is a so-called lead-in zone containing information about the disc format, the structure of sound programs, addresses of fragments, titles of works, etc. At the end the lead-out zone is recorded (track number AA) , which acts as the boundary of the recorded area of ​​the disk; The P code bit in this zone changes at a frequency of 2 Hz. A number of home players cannot recognize a disc without this zone, but many can do without it. Between the input and output zones, a program memory area (PMA) is recorded, containing the actual audio data. The program area is separated from the input area by a section of 150 empty blocks (2 seconds), which acts as a pre-gap.

The total recording time on a CD is 74 minutes, however, by reducing the standard track pitch and the distance between pits, you can achieve an increase in recording time - at the expense of reducing the read reliability in a standard disk drive.

How are CDs recorded and produced?

The main method of producing disks is pressing from a matrix. The original is formed from the original digital master tape, containing an already prepared and encoded digital signal, by a special high-precision machine on a glass disk coated with a layer of photoresist - a material that changes its solubility under the influence of a laser beam. When the recorded original is processed with a solvent, the required relief appears on the glass, which is transferred by electroplating to the nickel original (negative), which can serve as a matrix for small-scale production, or as a basis for making positive copies, from which, in turn, negatives are taken for mass replication.

Stamping is carried out using the injection molding method: a polycarbonate substrate with a relief is pressed from a negative matrix, a reflective layer is sprayed on top, which is varnished. Informational inscriptions and images are usually applied on top of the protective layer.

Recordable discs (CD-R, “blanks”) are made using the same method, but between the base and the reflective layer there is a layer of organic matter that darkens when heated. In the initial state, the layer is transparent; when exposed to a laser beam, opaque areas equivalent to pits are formed. To facilitate tracking of a track when recording on a disc, a preliminary relief (marking) is formed during the manufacturing process, the track of which contains frame marks and synchronization signals recorded with a reduced amplitude and subsequently overlapped by the recorded signal.

Recorded discs, due to the presence of an organic fixing layer, have a lower reflection coefficient than stamped ones, which is why some players (Compact Disk Player - CDP), designed for standard aluminum discs and not having a margin of read reliability, can play CD-R discs less reliably, than usual.

How are CDs played?

During playback, an audio CD rotates at a constant linear velocity (CLV), at which the speed of the track relative to the playback head is approximately 1.25 m/s. The rotation speed stabilization system maintains it at such a level as to ensure the speed of the read digital stream equal to 4.3218 Mbps, so depending on the length of the pits and gaps, the actual speed may vary. Angular velocity The disk speed changes from 500 rpm when reading the innermost sections of the track to 200 rpm on the outermost sections.

To read information from the disk, a semiconductor laser with a wavelength of about 780 nm (infrared range) is used. The laser beam, passing through the focusing lens, falls on the reflective layer, the reflected beam enters the photodetector, where pits and gaps are determined, as well as the quality of focusing of the spot on the track and its orientation along the center of the track are checked. When focusing is disrupted, the lens moves, working on the principle of a loudspeaker diffuser (voice coil), and when it deviates from the center of the track, the entire head moves along the radius of the disk. In essence, the lens, head and spindle motor control systems in the drive are automatic adjustment systems (ATS) and are constantly monitoring the selected track.

The signal received from the photodetector in 8/14 code is demodulated, as a result of which the CIRC encoding result with added subcodes is restored. Then the subcode channels are separated, deinterleaved and CIRC decoded on a two-stage corrector (C1 for single errors and C2 for group errors), as a result of which most of the errors introduced by stamping violations, defects and heterogeneity of disk materials, and scratches on the disk are detected and corrected. surface, unclear definition of the pit/gap in the photodetector, etc. As a result, the stream of “pure” audio samples is sent to the DAC for conversion to analog form.

In sound players, after the corrector, there is also an interpolator of varying complexity, which approximately restores erroneous samples that could not be corrected in the decoder. Interpolation can be linear - in the simplest case, polynomial or using complex smooth curves.

To perform de-interleaving, any CD-reading device has a buffer memory (standard capacity - 2 KB), which is also used to stabilize the bit rate. Several different strategies can be used for decoding, in which the probability of detecting group errors is inversely proportional to the reliability of their correction; The choice of strategy is left to the discretion of the decoder developer. For example, a CD player with a powerful interpolator might choose a strategy that emphasizes maximum detection, while a CDP with a simple interpolator or CD-ROM drive might choose a strategy that emphasizes maximum correction.

What are the parameters of the audio signal on a CD?

Standard digitization parameters—sampling frequency 44.1 kHz and sample bit depth 16—determine the following theoretically calculated signal characteristics:

• Frequency range: 0..22050 Hz
• Dynamic range: 98 dB
• Noise level: -98 dB
• Total Harmonic Distortion: 0.0015% (at maximum signal level)

In real CD recording and playback devices, high frequencies are often cut off at 20 kHz to create a margin for the steepness of the filter's frequency response. The noise level can be as low as 98 dB with a linear DAC and a noisy output amplifier, or higher if resampled at a higher frequency using a Delta-Sigma, Bitstream or MASH DAC and low-noise amplifiers. The coefficient of nonlinear distortion strongly depends on the used DAC output circuits and the quality of the power supply.

A dynamic range of 98 dB is determined for a CD based on the difference between the minimum and maximum levels of the audio signal, but at a small signal the level of nonlinear distortion increases significantly, which is why the real dynamic range, within which an acceptable level of distortion is maintained, usually does not exceed 50-60 dB.

What is jitter?

Jitter is a rapid jitter in the phase of a digital signal relative to the duration of the period, when the strict uniformity of the pulse fronts is violated. Such jitter occurs due to the instability of clock generators, as well as in places where the clock signal is isolated from a complex signal using the PLL (Phase Locked Loop) method. Such selection takes place, for example, in the demodulator of the signal read from the disk, resulting in the formation of a reference clock signal, which, by correcting the rotation speed of the disk, is “adjusted” to the reference frequency of 4.3218 MHz. The frequency of the clock signal, and therefore its phase and the phase of the information signal, continuously fluctuates at different frequencies. An additional contribution may be made by the uneven arrangement of pits on the disk, caused, for example, by poor-quality pressing or unstable recording.

However, ripples in the disk signal are fully compensated for by the decoder's input buffer, so that any jitter or knock that occurred before the signal was placed in the buffer is eliminated at this stage. Sampling from the buffer is controlled by a stable oscillator with a fixed frequency, but such oscillators also have a certain, albeit much less, instability. In particular, it can be caused by interference in the power supply circuits, which, in turn, can occur when the ACS is activated and the disk speed or head/lens position is adjusted. On low-quality discs, these corrections occur more often, giving some experts reason to directly link the stability of the output signal with the quality of the disc, although in fact the reason is insufficiently good decoupling of CDP systems.

What do the abbreviations AAD, DDD, ADD mean?

The letters of this abbreviation reflect the audio waveforms used to create the disc: the first is during the original recording, the second is during processing and mixing, and the third is the final master signal from which the disc is formed. “A” denotes analogue form, “D” denotes digital form. The master signal for a CD always exists only in digital form, so the third letter of the abbreviation is always “D”.

Both analog and digital signal forms have their advantages and disadvantages. When recording and processing a signal in analog form, its “fine elements” are most fully preserved, in particular higher harmonics, but the noise level increases and the amplitude-frequency and phase-frequency characteristics (AFC/PFC) are distorted. When processed in digital form, higher harmonics are forcibly cut off at half the sampling frequency, and often even lower, but all further operations are performed with the highest possible accuracy for the selected resolution. A number of experts evaluate the signal that has undergone analog processing as “warmer” and “live”, but many modern methods Signal processing is only acceptably implemented in digital form.

Can two identical discs sound different?

First of all, you need to make sure that the discs actually contain an identical digital audio signal. A complete binary match between two discs at the pit and gap configuration level is virtually impossible due to minor material defects and distortions during die processing and pressing, but due to redundant encoding, the vast majority of these errors are corrected during decoding, providing the same “high level” digital stream.

You can compare the digital contents of discs by reading them in a CD-ROM drive that supports Read Long or Raw Read mode - reading “long sectors”, which are actually CD-DA superframes with a capacity of 2352 bytes each. You can read more about this in the CD-ROM FAQ or in the manual for audio reading programs (CD-DA Grabbers/Rippers). You can also compare discs using studio equipment that can read discs in digital form to a DAT tape recorder.

There can be several reasons for digital differences between discs that sound similar. Some CD-ROM drives and other digital CD-DA reading devices can, in order to prevent direct copying, introduce subtle distortions into the signal (for example, using smoothing polynomials), and most drives that support full frame reading commands do this inaccurately and inaccurately. When making copies (reprints) of audio discs, especially in a pirated way, they are often copied with resampling to another frequency (for example, 48 kHz in DAT) followed by resampling to the original one, or even through an analog path with double digital/analog conversion. A number of versions of CD-R burning software also intentionally or accidentally distort the original data so that the copy is not the same as the original.

It should be noted that even if the digital contents of two discs coincide when comparing them in some system (CD-ROM, special devices to compare the original/copy, etc.), this does not mean that identical digital signals will also be decoded from them on one or another CDP. Therefore, the most reliable way to determine the cause of differences in sound is to use a CDP with a digital output, from which recording is being carried out on some storage device while listening to both discs. Subsequent digital comparison of the resulting signalgrams will show at what point in the player the changes that are audible to the ear are introduced into the signal.

Of course, before comparing the original and the copy in this way, you need to make sure that the results of reading the same discs multiple times are repeatable. Various digital signalgrams in this case may indicate unreliable disk reading or poor operation of digital interfaces (receiver, transmitter, cable, connectors). Identity of digital data when replays several disks can be considered a sufficient sign of the reliability of both the disks themselves and the reading, decoding and intermodular transmission systems.

The auditory comparison of the sound of discs must be correct - the most recognized is the double-blind test. The essence of the method is that the expert (listener) should not see the manipulations with the equipment and the person performing them, and this person himself, who randomly changes the disks, should not know the features of their contents. In this way, any influence, including “subtle” and unstudied, of people on the equipment and on each other is eliminated as much as possible, and the expert’s opinion is considered extremely unbiased.

What is HDCD?

High Definition Compatible Digital is a “super-system” for CD audio encoding, using the standard CD-DA format. Sound signal with a higher bit depth and sampling frequency is subjected to digital processing, as a result of which the main part is separated from it, encoded, as usual, by the PCM method, and Additional Information, clarifying fine details, is encoded in the least significant bits of samples (LSB) and masked spectral regions. When playing an HDCD disc on a regular CDP, only the main part of the signal is used, but when using a special CDP with a built-in decoder and HDCD processor, all information about the signal is extracted from the digital code.

How to handle CDs?

Avoiding mechanical damage to any of the surfaces, exposure of the disc to organic solvents and direct bright light, impacts and kinks of the disc. Inscriptions on recordable discs may only be made with pencils or special felt-tip pens, excluding pressure and the use of ballpoint or fountain pens.

When removing a disc from the box, be careful not to bend it. One convenient and safe method requires the use of two hands - the thumb of the left hand presses lightly on the latch, loosening it, while the other hand removes the disc from the latch. The one-handed method, when the index finger loosens the latch and the thumb and middle finger remove the disk, requires more precise coordination of forces, without which it is easy to bend the disk or break the latch tabs.

A dirty disc can be washed with warm water and soap or a non-aggressive surfactant (shampoo, washing powder), or specially produced liquids. Shallow scratches on the transparent layer can be polished using polishing pastes that do not contain organic solvents and oils, or regular toothpaste.

What is a “green marker” and why is it needed?

Many users and experts claim that a disc treated in this way produces cleaner sound in high-end devices, attributing this to a more accurate reading of digital information from the disc, which in its original form supposedly cannot be reliably read in most drives. However, a carefully designed system (drive and decoder) is able to correctly read not only untreated discs, but also discs of average quality, and even slightly dirty and scratched ones, so possible reasons Sound improvements should not be sought in the disc. The most likely explanations for this phenomenon seem to be the same factors that create different sounds of copies of discs that match the digital content.

Where to find more detailed information by CD?

QUESTIONS AND ANSWERS: What you need to know about CD-R recordings and CD-RW discs.

1. What do you need to have to burn CDs?

You need a computer with a device installed in it called a CD-writer. This device connects to the computer using several possible ways. Most drives for recording discs have an IDE interface and are connected in the same way as regular CD-ROMs or hard drives and have an internal design. However, there are other designs, both external and internal - with SCSI interface, connecting to a parallel port or USB bus.
The second necessary part for burning discs is software. Its choice is very large - from the most popular commercial packages from Adaptec (Easy CD Creator, Easy CD Deluxe, Easy CD Pro) to shareware programs such as Nero or CDRWin.
And finally, you need a blank CD-R or CD-RW disc

2. What can you burn onto CD-R or CD-RW discs?

Traditionally, discs can store both sound and data. Data is stored on CDs in the same format as it is stored on a hard drive. It should also be noted that it is possible to create mixed discs, combining sound with data.

3. What is the difference between CD-R and CD-RW discs?

CD-R stands for CD-recordable, that is, “recordable.” This means that information recorded on such a disk cannot be deleted from there. The main difference between CD-RW (CD-rewritable) discs is that information from them can be deleted and recorded again. As a result, CD-RW discs, which are more flexible in use, are slightly more expensive than conventional write-once discs.

4. How much information can be recorded on a CD-R disc?

5. Why is the standard duration 74 minutes?

The general consensus is that this length was chosen because the CD developers wanted a format that would accommodate Beethoven's Ninth Symphony. They determined which diameter to use, and the length of some performances decided this issue.

Recorded discs can be used on the following devices:

Home CD Player Given that home CD players predate CD-R burners, there is no guarantee that all recorded music CDs will play in audio players. However, for best results, it is recommended to use CD-R discs, since their specifications are much closer to traditional music discs than CD-RW discs. DVD-ROM drive or DVD player The vast majority of DVD players and all DVD-ROM drives (with the exception of the first examples of these devices) are able to read information from CD-R and CD-RW discs. CD-ROM drives

All modern CD-ROM drives read excellently, both write-once discs and CD-RW discs. Nuances exist only with old drives, which in some cases do not read CD-R discs, or read these discs, but do not read CD-RW discs. If your old floppy drive marked as having Multiread function, this means that with its help you can cope with this task. A good indication that a drive is capable of handling recordable discs is how fast the drive reads data. If the speed is 24x or higher, then, as a rule, such a drive is quite suitable for working with CD-R and CD-RW discs.

7. Why are the reflective sides of the discs different colors?

Different CD companies have patents on the various chemistries they use to produce the discs. Some companies produce disks themselves, others simply license their technology to them. As a result, the reflective side of CDs is different color. CD-Rs are available in the following composition combinations: gold/gold, green/gold, silver/blue, and silver/silver, and their many shades. The visible color is determined by the color of the reflective layer (gold or silver) and the color of the dye (blue, dark blue or clear). For example, green/gold discs consist of a gold reflective layer and a blue dye, so the disc is gold on the label side and green on the recording side. Many have come to the conclusion that "silver" discs are made of silver and, based on this assumption, have tried to speculate about the reflectivity and durability of the media. Until a manufacturer's representative comes forward with a statement about the actual composition of the disc, it is unwise to assume anything specific. Some CDs have an additional coating (such as Kodak's "Infoguard") that makes the CD more scratch-resistant but does not affect the way the information is stored. The top (label) side of the disc is the most important area to worry about because this is where the data lives and is the area that is most easily damaged on a CD-R. You can protect the disc from scratches by sticking a round CD sticker over its entire area. CD-RW discs have a completely different structure. The data side (as opposed to the label side) is a silvery dark gray color that is hard to describe. You can also give a short list of which companies produce which discs:

Taiyo Yuden produced the first "green" CDs. They are now also made by TDK, Ricoh, Kodak, and probably a few other companies.

Mitsui Toatsu Chemicals (MTC) produced the first "golden" CDs. They are now also made by Kodak and possibly others.

Verbatim produced the first "silver/blue" CDs.

Many brands of CD-R (such as Yamaha and Sony) are OEM versions of one of the major manufacturers. By and large, it is difficult to determine who produces what, since new plants have been built and sellers may change suppliers.

8. What do the speed numbers (for example, 6x4x32) mean in the parameters of CD burners?

Conventional audio players play music CDs in 74 minutes. This speed is taken as the basis when measuring the speeds of playing and recording CDs and is called a single speed (1-x). Single speed corresponds to a transfer of 150 Kilobytes per second. A CD-ROM drive with two times the speed (2x) transfers data at a speed of 300 Kilobytes per second.

Three numbers in the parameters of CD-writers indicate the speed at which this device can write CD-R discs, CD-RW discs and read these discs accordingly.
For example, 6x4x32 means that this device writes CD-R discs at a speed of 6x (900 KB/sec), writes CD-RW discs at a speed of 4x (600KB/sec) and reads any type of CD at a speed of 32 (4800 KB/sec)

9. What formats exist when burning CD-R discs?

This is the most difficult question to answer, given that many different CD formats have emerged over the past few years, while there are still historical formats that have been around for a long time and are used in specialized applications. Below is an overview of the main formats:

Music discs (Audi o CD) or CD-DA or "Red Book"

To burn regular music CDs, you need the disc you burn to conform to the CD-DA standard. When recording, standard WAV files (or AIFF - Apple Audio Interchange File Format) are used as a source.

ISO9660 Data CD

This standard defines the form in which conventional data is written to CD-R discs. This standard has many restrictions, namely, the maximum number of subdirectories cannot exceed 8, file names cannot be longer than 8 characters, and 3 characters are allocated for the file name extension. However, this standard is compatible with a large number of computers and operating systems.

A format proposed by Microsoft simultaneously with the advent of the Windows operating system"95. The length of the file name is limited in this standard to 64 characters, and this format is now supported both in the Windows environment and in MacOS and Linux. Joliet is based on the ISO9660 standard and disks, written in this format can be read by almost any computer, however the file names will be truncated to an 8+3 format.

This format strictly applies to Macintosh computers. HFS CDs can only be read on this type of computer.

UDForPocket Writing

UDF (Universal Disk Format) is a radical extension of the ISO9660 standard, somewhat reminiscent of Joliet. Adaptec DirectCD software (included with Easy CD Creator Deluxe, or sold separately for the Mac platform) and, for example, CeQuadrat PacketCD software allow you to burn discs in this format. UDF differs from other formats in that you can treat a CD like a large floppy disk by copying files to it using standard means Windows or MacOS. However, this format is not suitable for transferring discs to other people, because in order to read discs in this format, they will need to install special software for reading such discs.

ISO 9660 Rock Ridge

An extension of the ISO9660 standard, used exclusively in Linux and UNIX operating environments.

ISO Level 2

A slightly modernized ISO9660 format, simplified in terms of restrictions. For example, the file name length in it is limited to 31 characters. However low level The compatibility of this standard does not allow it to be used as widely as, for example, the Joliet format is used.

VideoCD or VCD or "White Book"

The VideoCD format was developed in the mid-90s and was intended for use in devices such as the Philips CD-I player. VideoCD discs contain video image and audio compressed using the MPEG1 standard. Despite the fact that the Philips CD-I player has not been produced for a long time, these discs can be used on the vast majority of DVD players if they support reading CD-R or CD-RW discs.

8. What format should I use if I want….

…. exchange data with friends who use an operating system similar to mine?

Everything is simple here. Windows operating system users should use the Joliet format, Mac users should use the HFS format.

…. share data with people who use different operating environments and platforms?

For maximum compatibility, it is recommended to use the ISO9660 format. However, if you need to store long file names on disk, then you should try using the Juliet format. Modern Macs and most operating systems now have the ability to read discs written in this format.

…. listen to music on a regular audio player?

Then you should burn the disc in CD-DA format, which will provide the highest level of compatibility with your audio player.

10. How to burn discs with mixed content?

There are two options for burning such discs:

Mode I- using of this format data is written at the beginning of the disk (at any known format), followed by recorded audio tracks. If you need to combine audio and data, then using this mode will provide the necessary level of compatibility with various devices and operating environments.
CD-XA (Mode II)- this mode differs from the previous one in that data and sound can be recorded in random order. However, this flexibility has a negative impact on the compatibility of the recorded discs.

11. What is a multisession CD?

This technology allows you to add data or sound to a disc that already has something written on it until the disc is closed. This was very relevant in the early 90s, when the cost of a blank CD-R disc reached $12, CD-RW discs did not exist, and hard drives had a small capacity.

Discs recorded using this technology have some compatibility problems, and therefore it is not recommended to use it without good reasons. The UDF format makes this technology unnecessary; using Direct CD and similar software you can record data without worrying about compatibility. If you need to give the disc to other people, then just burn it at once in Joliet format

12. What is "closing" a disk?

"Closing" a disc means that after this procedure is completed, nothing can be done on that CD-R disc. If you never use the "multisession" feature, then there is no point in thinking about it, because the disk will close automatically after the end of writing information to the disk. Many older CD-ROM drives and audio players have problems reading unsealed discs, so it makes sense to "seal" the disc for greater compatibility.

If you want to write something to "closed" CD-RW disc just perform the "erase" function and you can write data to this disk again. If you use the UDF format, then there is no concept of “closing” the disk in the traditional sense of the word - just copy and delete files from such a disk, as from a simple floppy disk.

The era of CDs is slowly but surely becoming a thing of the past. Now most modern users don’t even know how they differ from standard R and ROM. In order to understand the difference, you need to remember the history of their creation. Only then will it be possible to determine their main difference from classic CDs.

History of the development of optical CD media

The first compact disc was developed by Philips. They are considered pioneers in this field. At first optical discs had quite a bit of space to accommodate data. The initial volume of such a “blank” was 640 megabytes. But over time it increased to 700. The first optical discs in the compact format were called CD-R. This meant that data could only be written to them once. For a long time they were used as carriers. However, time passed, technology developed, and very soon manufacturers introduced the CD-RW rewritable compact disc. This abbreviation (RW) comes from the English word Rewritable (with the ability to rewrite). Such optical media have become incredibly popular among users. The very idea of ​​reusable recording to disk seemed incredible. But there was one minus. The recording speed on such media was very low. If the standard R disc was written at x53 speed, then RW Classic discs needed to be written at x6 speed. But this did not last long, as standard CDs soon went out of fashion.

The advent of DVD

The decline of classic "compacts" is directly related to the emergence of a new format - DVD-R. These optical drives were distinguished by their gigantic volume (compared to CDs). They could fit 4.5 gigabytes of information. It was a breakthrough. As expected, some time after successful start classic DVDs, DVD-RW discs have appeared that allow you to record on one or another medium several times. And this solution has become incredibly popular.

DVD discs were used almost everywhere: programs, operating systems, films and other information were recorded on them. Even music in formats without loss of quality was written on DVD discs. And in this regard, DVD-RW discs looked the most universal solution. And soon double-layer DVDs appeared that could hold almost 10 gigabytes of information. This was truly a breakthrough. For a long time, DVDs were used everywhere. Special players were also released. They could read RW as well, so users recorded several movies on them at once. And when they got tired of them, they re-recorded them. This went on for quite some time. But the DVD era has come to an end.

Blu-Ray era

Classic and double-layer DVDs have been replaced by Blu-Ray media. They were distinguished by increased capacity. One such disk held about 25 gigabytes of information. That's a lot. Around the same time, HD video formats also appeared. Films in this format fit perfectly on BD. This determined the area of ​​application of such optical media - the film industry.

Indeed, keeping the library on BD was somehow wrong. Moreover, at the same time, the Internet developed rapidly and large-capacity USB drives appeared. Nobody needed disks anymore; only BDs were still afloat. And that’s only thanks to those who like to watch movies in maximum quality in their home theater. Over time (as expected), dual-layer BD and BD-RW discs appeared. The latter allowed information to be overwritten. But considering the volume of Blu-Ray media and low speed recordings on RW, this option has not gained popularity. To this day, BD-RW remains just an interesting technology. But nothing more.

The relevance of Blu-Ray technology is also being rethought. New video resolutions have appeared - 2K and 4K. But they require much more space and will never fit on a classic BD “blank”. The era of Blu-Ray will probably soon end successfully. But that's a completely different story.

Conclusion

So, we talked about the features of RW disks and looked at the history of the development of optical media. Classic CDs are already used exclusively in the music industry. No one has heard of DVD for a long time. Nowadays Blu-Ray technology rules the roost. But judging by latest trends In the world of multimedia entertainment, the days of the above technology are numbered. Perhaps manufacturers are now developing a new type of optical media. But we’ll tell you about what happens next next time...

Ministry of Education of the Republic of Belarus

GRODNO STATE UNIVERSITY

NAMED AFTER YANKA KUPALA

Abstract on the topic:

Structure

CD

in the subject “System Software”

prepared by math student

faculty 5th group 2nd year

Krizhak Alexander Stanislavovich

TeacherLivak Elena Nikolaevna

Grodno 2003

Introduction

Compact discs (CD-ROMs), originally developed for lovers of high-quality sound, have now firmly established themselves in the computer device market. Due to their small size, high capacity, reliability and durability, they are successfully used as external memory devices.

Optical music CDs replaced vinyl records in 1982, around the same time as IBM's first personal computers. Two giants of the electronics industry - the Japanese company Sony and the Dutch Philips - developed a special standard known as the “Red Book”, according to which a compact disc should be designed for only 74 minutes of sound, or more precisely 74 minutes and 33 seconds. When 74 minutes were converted into bytes, it turned out to be exactly 640 MB.

The two above-mentioned companies also played a leading role in the development of the first specification for digital compact discs - the so-called “Yellow Book”. It served as the basis for the creation of compact discs with a comprehensive presentation of information, that is, capable of storing not only audio, but also text and graphic data (CD-Digital Audio, CD-DA). The second standard for digital CDs was the HSG (High Sierra Group) specification, or simply High Sierra. This document was proposed by the major digital compact disc manufacturers in an effort to provide at least some compatibility. This specification already defined both logical and file formats CDs.

The international standard ISO 9660 for digital compact discs, adopted somewhat later, in principle coincided with the main provisions of the HSG. Note that all CDs that comply with the ISO 9660 standard, which defines their logical and file formats, are compatible with each other. In particular, this document defines how to find the contents of a CD on a Volume Table Of Contents (VTOC).

Physical structure of a CD

In the structure of a compact disc, four main layers can be distinguished (the fifth is an image applied to the surface of the disc), applied in stages.

Let's take the path of making a disk. Initially, the plastic base of the disc is made - polycarbonate (E), which makes up the main part of the CD-R and gives it the necessary strength and shape. Next, the active layer (D) (dye) is applied to the finished plastic mold. It is this layer that allows recording to the disk and determines its reliability and quality of reading information in the future (in simple CD-ROM On disks, this layer is absent, and the necessary information is recorded directly at the manufacturer). Today, two types of active layer are widely used: cyanine and phthalocyanine.

After dye has been applied to the polycarbonate blank, the disc is covered with a special layer of reflective material (C). Conventional CD-ROMs use aluminum for this purpose, CD-R discs Pure silver is used, which allows achieving a 65-80% reflection coefficient.

The final stage of disc manufacturing is the application of a protective layer (B), onto which images (A) can subsequently be applied. The most common and easiest to manufacture protective layer is a special varnish. Disc varnishing does not provide a 100% guarantee of data safety when exposed to external mechanical or chemical influences. However, many Chinese “manufacturers” often skimp on varnish, or apply it in such a way that concentric waves in the form of waves form on the disc, which indicates either an incorrectly calculated application speed or an incorrect drying mode, which makes the discs practically defenseless against influence of external influences.

CD format

All CD-ROM have the same physical manufacturing format and a capacity of 650 MB. Disc with a diameter of 120 mm, a thickness of 1.2 mm and a central hole with a diameter of 15 mm. The central area around the 6mm wide hole is called the attachment zone ( Clamping ). After it there is a calibration ( Program Calibration ) zone. It is used in CD-R discs to adjust the laser power of the recording device. Registration area ( Program Memory ) is also only available on recordable discs. The coordinates of the beginning and end of each track are temporarily recorded here when the disc is removed from the recording device without closing the session.This is immediately followed by a header area ( Lead-in ), containing the disc's table of contents ( TOC - Table Of Content ), -- a 4 mm wide ring (diameter 46-50 mm) closer to the center of the disk (up to 4500 sectors, 1 minute, 9 MB). Consists of 1 track (Lead-in Track). Contains TOC (absolute temporary addresses of tracks and the beginning of the output area, accuracy - 1 second). Next is a 33 mm wide area intended for data storage and physically representing a single track. The final area is the terminal area ( Lead-out ) 1 mm wide. In addition, there is also an outer (protective) rim of the disc with a width of 3 mm.

The data storage area can logically contain from 1 to 99 tracks, but disparate information cannot be mixed on one track. Digital information is stored on CD-ROM in the form of pits alternating along the spiral, applied to the surface of polycarbon plastic. The pit is perceived by the laser beam as a logical zero, and the smooth surface as a logical one.

Each byte of data (8 bits) is encoded into a 14-bit character on the medium (EFM encoding). Characters are separated by 3-bit spaces, chosen so that there are no more than 10 consecutive zeros on the medium.

From 24 bytes of data (192 bits) a frame (F1-frame) is formed, 588 bits of media, not counting the spaces:

· synchronization (24 bit media)

· subcode symbol (bits of subchannels P, Q, R, S, T, U, V, W)

· 12 data characters

· 4 character verification code

· 12 data characters

· 4 character verification code

Decoding may use different strategies for detecting and correcting group errors (probability of detection vs. reliability of correction).

A sequence of 98 frames forms a sector (2352 information bytes). The frames in the sector are shuffled to reduce the impact of media defects. Sector addressing originates from audio discs and is written in A-Time format - mm:ss:ff (minutes:seconds:beats, fractions per second from 0 to 74). The countdown starts from the beginning program area, i.e. the sector addresses of the input area are negative. The subchannel bits are assembled into 98-bit words for each subchannel (of which 2 bits are synchronization). Subchannels used:

· P - marking the end of the track (min 150 sectors) and the beginning of the next (min 150 sectors).

· Q - additional information about the track contents:

o number of channels

o data or sound

ois it possible to copy

oa sign of frequency pre-emphasis: artificial increase in high frequencies by 20 dB

osubchannel usage mode

§ q-Mode 1: The input area stores the TOC here, the program area stores track numbers, addresses, indexes and pauses

§ q-Mode 2: disk catalog number (same as on the barcode) - 13 digits in BCD format (MCN, ENA/UPC EAN)

§ q-Mode 3: ISRC (International Standard Recording Code) - country code, owner, year and serial number of the recording

§ CRC-16

A sequence of sectors of the same format is combined into a track (track) from 300 sectors to the entire disk. A disc can have up to 99 tracks (numbered 1 to 99). A track may contain service areas:

· pause - only subchannel information, no user data

· pre-gap - the beginning of the track, does not contain user data and consists of two intervals: the first, at least 1 second long (75 sectors), allows you to “build up” from the previous track, the second, at least 2 seconds long, sets the format of the track sectors

· post-gap - end of track, does not contain user data, at least 2 seconds long

The lead-in digital area must end with a post-gap. The first digital track must start from the second part of the pre-gap. The last digital track must end with a post-gap. The output digital area does not contain pre-gap.

There are many standards and formats for CDs, depending on the purpose and manufacturers. I'll bring you for example, not all existing ones : Audio CD (CD-DA), CD-ROM (ISO 9660, mode 1 & mode 2), Mixed-mode CD, CD-ROM XA (CD-ROM eXtended Architecture, mode 2, form 1 & form 2), Video CD, CD-I (CD-Interactive), WITH D-I-Ready, CD-Bridge, Photo CD (single & multi-session), Karaoke CD, CD-G, CD-Extra, I-Trax, Enhanced CD (CD Plus), Multi-session CD, CD-Text, CD -WO (Write-Once).

CD-ROM file structure

The initial data track on a CD begins with a service area necessary for synchronization between the drive and the disk. Next is the system area, which contains information about disk structuring. The system area also contains directories for this volume with pointers or addresses to other areas of the disk. A significant difference between the structure of a CD and, for example, a floppy disk is that on a CD, the system area contains the direct address of files in subdirectories, which should make them easier to find. The international standard ISO 9660 describes the file system on a CD-ROM. ISO 9660 level one resembles the MS-DOS file system: file names can contain up to eight characters, the file name extension (consisting of three characters) is separated from the file name by a dot. File names cannot contain special characters ("-", "~", "=", "+"). When naming files, only uppercase characters, numbers and the "_" character are used. Directory names cannot have extensions. Each file has a version - the version number is separated from the extension by a ";". Directories can have a nesting depth of 8. The ISO 9660 level two standard allows up to 32 characters in file names, imposing the restrictions described above. Disks created using this standard cannot be used in a number of operating systems, including MS-DOS.

Note that for most CDs, all information subsequently stored on them is recorded in one technological cycle, or session (single session). As already mentioned, the table of contents of the disk, that is, the index of where and how information is stored on it, is contained in the VTOC. However, after the technology appeared that made it possible to add information to special (appendable) CD-ROMs, we started talking about multi-session CDs and corresponding drives (multi session).

The main elements of the CD-ROM file structure are:

· primary volume descriptor (PVD - Primary Volume Descriptor); it is always located in the sixteenth sector of the session and contains links to the path table (PT - Path Table) and the root directory (RD - Root Directory);

· the path table (PT) contains directory addresses (DF - Directory Files).

If the file structure spans more than one session, then links from the root directory of subsequent sessions include links to directories of previous sessions, and thus the directories of previous sessions become available in subsequent sessions. This is the basis for the ability to update files. Despite the impossibility of erasing, the effect of "overwriting" is preserved for the user: this is achieved by overwriting in a subsequent session the directories containing references to the file being replaced. The file is, of course, also recorded in the subsequent session, and a link to it is included in the new edition of the catalog. Standard file access will use links from the root directory of the last session and the file will appear updated, although you will still be able to access a previous version using a special link.

It is also possible that the session recorded later is independent, in which case the links to the sessions will be similar to the links to different sections of the physical disk. For normal operation file system With CD-R, a drive that “understands” multi-session discs is highly desirable. It is easy to check whether a drive has such capabilities - just look at the directory of a multi-session disk: a primitive player will “see” only the directories and files of the first session.

As you can see, recording formats turn out to be quite closely related to the design of the CD-ROM drive.

CD- RWdisks

The term CD - RW denote a relatively new type of recordable disc that went on general sale in 1997. Unlike CD-R disks (i.e. disks on which information can only be additionally written), CD-RW disks allow you to partially or completely erase/rewrite information at least 1000 times. The basic principle of rewriting is based on the fact that the substance used as the working layer can be in one of two stable states - crystalline or amorphous, respectively, transmitting the laser beam to the reflective layer and back, or scattering the light.

If the substance is in the first (crystalline) state, then the reading laser beam passes unhindered through the working layer, is reflected from the reflective layer and ultimately hits the photodetector, which corresponds to logical “1”. If the substance is in an amorphous state, the beam is scattered, ultimately not hitting the photodetector, which corresponds to a logical “0”.

To transfer a substance from one state to another, special modes of heating and cooling with a laser beam are used. First, the substance is heated to a high temperature T 1, in this case it loses its structure, locally becoming amorphous at the focusing point of the laser beam; if you then turn off the laser completely, i.e. quench T 1 >> T room , then the substance will remain in an amorphous state as it cools. If you do not turn off the laser, but only reduce its power and turn it off completely only after some time, then due to 2-stage cooling T 1 >> T 2 >> T room, the substance of the working layer has time to crystallize.

Conclusion

The CD-ROMs that exist today were “born” from audio discs, the technological readiness for release of which has existed for more than 15 years. During this time, new technological opportunities have emerged, as well as a sufficient market for creating a device focused on efficient data storage and convenient means of accessing it. The possibilities of the format based on the Red Book are almost exhausted (merely storing the table of contents in a Q-frame of a subchannel with empty sectors cuts to the root the possibility of using small sessions). Naturally, the world is striving to create more modern CDs. Such CDs have been waiting on the market for a long time; they not only came up with a name (High Density Compact Disk - HD CD), but also managed to change it to MMCD (Multi Media CD). It is expected that by reducing the wavelength of the readout laser, it will be possible to reduce the size of the pit and the distance between the tracks. Together with improved information storage structure and more modern error correction tools, it may be possible to achieve a capacity of 3.7 GB per disk. Multisurface technology promises even greater capacity, in which recording is carried out on several (two, to begin with) layers located one above the other. The choice of the readable layer is ensured by focusing the beam precisely on it, and extremely short-focus optics makes it possible to reduce interference from another layer to an acceptable value.

Thus, CDs are firmly rooted in our lives, because... are by far the most universal information carriers in the modern information world, and a detailed examination of the structure of these devices is an integral part in the study of this mysterious computer Universe.

Literature

1. http:// referat 2000. bizforum. ru/komp/25. htm

2. http://www. ixbt. com/storage/cdr. shtml

3. http://blhard. people. ru/books/cd. html

4. http://referat. ru/document/12944

5. http://www. comizdat. com /3/4/90/363/378/

6. http://www. transelectro. ru/glossary/cdrw. html

And they merged with the thick mass of darkness that gravitated over them. The CD was dying...

(Based on: N.V. Gogol. The lantern was dying)

What is a CD

Wikipedia gives this definition:

“A compact disc is an optical storage medium in the form of a plastic disk with a hole in the center, the process of writing/reading information to/from which is carried out using a laser. Further development CD- steel discs DVD-disks. The compact disc was originally created to store audio recordings in digital form (known as CD-Audio), however, later it became widely used as a medium for storing any data (files) in binary form (the so-called. CD-ROM(English) Compact Disc Read Only Memory, read-only CD)...".

The CD is a polycarbonate substrate 1.2 mm thick and 120 mm in diameter, covered with a thin layer of metal (gold, silver, aluminum, etc.) and a protective layer of varnish, on which a label representing the contents of the disc was applied.

On the outer surface of the CD there is an annular protrusion 0.2 mm high, which allows the disc, placed on a flat surface, not to touch this surface.

There is a hole with a diameter of 15 mm in the center of the disk.

Disc weight – 15.7 g.

Data storage format on CD , known as Red Book("book"), was developed by the company Philips. In accordance with this format, audio can be recorded onto a CD in 2 channels with 16-bit pulse code modulation and a sampling rate of 44.1 kHz. Thanks to error correction using the Reed-Solomon code, light radial scratches do not affect the readability of the disc.

Information is written to the disk in the form of a spiral track consisting of pitov(English) pit– depression, “crater”, depression – non-reflective spot on the surface CD-ROM, representing the binary "0"), extruded into the polycarbonate base. Each pit is approximately 100 nm deep and 500 nm wide. Pit length varies from 850 nm to 3.5 µm. The spaces between pits are called Landom(English) land– contact pad, contact area – , reflective spot on the surface CD-ROM, representing binary "1"). The pitch of the tracks in the spiral is 1.6 microns.

Data from the CD is read using a beam with a wavelength of 780 nm. The principle of reading information with a laser is to register changes in the intensity of reflected light. the beam is focused on the information layer into a spot with a diameter of 1.2 microns. If the light is focused between the pits (on the landing), then the photodiode registers the maximum signal. If light hits the pit, the photodiode registers a lower light intensity.

Read/write speed CD 150 KB/s is indicated (that is, 153,600 bytes/s). For example, a 48-speed drive provides maximum read/write speed CD equal to 48 x 150 = 7200 KB/s (7.03 MB/s).

CDs originally held up to 650 MB information (or 74 minutes of audio recording). Since 2000, disks with a capacity of 700 have become increasingly widespread. MB, allowing you to record 80 minutes of audio.

CD-ROM-th legends

...There is a legend that the CD was not created Philips And Sony, and the American Russell, who worked in the company Optical Recording. Allegedly, already in 1971 he demonstrated his invention for storing data. He did this for “personal” purposes, wanting to prevent scratching his vinyl records pickup needles. 8 years later, a similar device was “independently” invented by companies Philips And Sony.

...Vice President of the Corporation Sony Oga (English) Ohga), who adored classical music, believed that a CD should be able to contain Beethoven's 9th Symphony (the most popular piece of music in Japan in 1979, according to a special survey!). In this case, in his opinion, up to 95% of classical works can be distributed on disks. Further research showed that the 9th Symphony, performed by the Berlin Philharmonic Orchestra under the direction of von Karajan, had a duration of 66 minutes. And the longest performance was the symphony under the direction of Furtwängler, performed at the Bayreuth Festival - 74 minutes. Allegedly, this served as the decisive argument when deciding on the initial capacity of the disk - 650 MB information (or 74 minutes of audio recording).

…IN Philips And Sony until May 1980, there was no consensus on the outer diameter of the disc. From an engineering point of view Sony, a diameter of 100 mm was sufficient as it allows miniaturization of the portable player. From senior management Philips The idea was to make a disc no larger than the diagonal size of a standard audio cassette (115 mm), which had great success on the market. In May 1980, company executives compromised and approved the “final” disk diameter of 120 mm, a disk capacity of 74 minutes of audio recording, and a sampling frequency of 44.1 kHz.

...Another legend says that the disc diameter of 12 cm was chosen because it corresponds to the size of a standard Dutch backing...

A story of rise and fall, or Requiem for a CD

CD Compact Disk, CD) was developed in 1979 by a Dutch company Royal Philips Electronics together with Japanese Sony. Philips developed technological process production of compact discs and turntables. Sony has improved its recording method (signal encoding), previously used in professional digital tape recorders, which ensures error-free reading of data from the disk ( Pulse Code Modulation, PCM– pulse-code modulation of the signal).

“When we started, there was no alternative,” recalls Kramer ( Kramer), head of the optical development department of the laboratory Philips in the 70s XX century “The attempt to bring digital audio to the masses was very risky...”

In 1982, mass production of CDs began at a plant in Langenhagen near Hannover (Germany). Release of the first commercial music CD– it was a CD with a recording of the album "The Visitors" groups ABBA– was announced on June 20, 1982.

First sales CD-players began in the fall of 1982 in Germany, and they reached the US market only in the spring of next year.

Corporations made a significant contribution to the popularization of compact discs. Microsoft And Apple Computer. Sculley, then head Apple Computer, said in 1987 that compact discs would revolutionize the world. And he turned out to be right!..

The rise in popularity of CDs was greatly facilitated by the fact that they began to be used to record not only music, but also any (!) data. And then everything began to be equipped with a drive CD-ROM. In addition, discs intended for recording at home have become widespread: CD-R (Compact Disc Recordable; CD+R, CD-R) – for single and CD-RW (Compact Disc ReWritable; CD+RW, CD-RW) – for multiple recording.

The CD was a resounding success: by 2004, worldwide CD sales CD, CD-ROM, CD-R, CD-RW reached 30 billion pieces. By 2007, about 200 billion were sold worldwide. CD(for every inhabitant of the Earth, including babies and old people, at least 30 disks!).

But in 2007, the end of the CD era began - sales CD fell by 15%!..

For almost 30 years, the compact disc led the music media market. But time does not stand still! With the growth of information volumes, new media formats appear - DVD, , Blu-ray. Many people prefer to simply "" music (or videos) online and download to their hard drive rather than buy CDs.

There are many more reasons for the decline in the popularity of CDs: small volume CD, etc. and so on.

Honestly speaking, on CD They haven’t been paying attention for a long time, especially since they fell in price and their volume increased significantly. In addition, it allows you to quickly download large amounts of information.

Analysts Gartner think that CD has lost its commercial appeal - it has neither advantages nor prospects - so the recording industry must abandon CDs and seriously focus on distributing music via the Internet.

Save the format CD no innovations and tricks will be able to, including the one proposed in 2007 by the company Walt Disney format CDVU+ (CD View Plus), which in addition to music tracks may include other multimedia content.

The largest British company Linn Products was the first to abandon the release of home and professional CD- , after their sales fell by 40% in 2 years.

On April 1, 2009, the world's largest music store closed in New York City. Virgin Megastore. A few weeks before its closure, the iconic Times Square music venue announced a full sale. But, nevertheless, there was no influx of buyers, despite the fact that discounts on goods reached 60%, and the store itself is located in the center of Manhattan, where it is always very crowded.

The flagship of the American entertainment industry Virgin Entertainment Group announced that it will close 5 more record stores - in San Francisco, Denver, Orlando, Hollywood and lower Manhattan. The retail network of music and video products, once founded by the billionaire Sir Branson, known for his extraordinary projects, could not withstand competition with the Internet market. Sales fell from $230 million in 2002 to $76 million in 2008.

So, His Majesty the CD is dead, long live...