The era of CDs is slowly but surely fading into the past. Now most modern users do not 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.

Development History of Optical CD Media

The first CD was developed by Philips. They are considered the pioneers in this field. At first optical discs had quite a bit of space to store the 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. Long time they were used as carriers. However, as time went on, technology developed, and very soon manufacturers introduced a rewritable CD-RW CD. This abbreviation (RW) comes from the English word Rewritable (with the ability to rewrite). Such optical media have become unrealistically popular among users. The very idea of ​​repeatedly writing to a disk seemed incredible. But there was one downside. The write speed on such media was very small. If a 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 fell out of fashion.

The advent of the DVD

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

DVD discs were used almost everywhere: programs, operating systems, movies and other information were recorded on them. Even music in lossless formats was written on DVD discs. And in this regard, DVD-RW discs looked the most one-stop solution. And soon two-layer DVDs appeared, which contained 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 and RW, so users recorded several films on them at once. And when they got bored, they re-recorded. This went on for quite some time. But the era of DVD came to an end.

Blu-ray era

Blu-ray media has replaced the classic and double-layer DVDs. They have a high capacity. About 25 gigabytes of information fit on one such disk. This is a lot. Around the same time, HD video formats also appeared. Movies in this format fit perfectly on BD. This determined the scope of such optical media - the film industry.

Indeed, it was somehow wrong to keep the library on the BD. Moreover, at the same time, the Internet developed rapidly and large-capacity USB drives appeared. No one needed discs, only BDs were still afloat. And that's only thanks to those who like to watch movies in maximum quality in home theater. Over time (as expected), double-layer BD and BD-RW discs appeared. The latter allowed to overwrite information on themselves. But given the volume of Blu-Ray media and the low write speed on RW, this option has not gained popularity. To this day, BD-RW remains just an interesting technology. But no more.

The relevance of Blu-Ray technology is also currently being rethought. There are new video resolutions - 2K and 4K. And they require much more space and will never fit on a classic BD disc. Probably, soon the era of Blu-Ray will be successfully completed. But that's a completely different story.

Conclusion

So, we talked about the features of RW disks and examined the history of the development of optical media. Classic CDs are already used exclusively in the music industry. No one has heard of the DVD for a long time. Now the ball rules Blu-ray technology. But judging by the latest trends in the world of multimedia entertainment, the days of the above technology are numbered. Perhaps now manufacturers are developing a new type of optical media. But what will happen next, we will tell sometime next time ...

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

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

What is a CD

Wikipedia gives this definition:

“CD 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-discs. The CD was originally created to store digital audio recordings (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 a 0.2 mm high annular protrusion that allows the disc placed on a flat surface not to touch this surface.

In the center of the disk there is a hole with a diameter of 15 mm.

Disc weight - 15.7 g.

Data storage format CD , known as red book("book"), was developed by the company Philips. In accordance with this format, a CD can record sound in 2 channels with 16-bit PCM and a sampling frequency of 44.1 kHz. Due to error correction using the Reed-Solomon code, light radial scratches do not affect the readability of the disc.

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

Data from a CD is read using a beam with a wavelength of 780 nm. The principle of reading information by 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 μm. If the light is focused between the pits (on the lens), then the photodiode registers the maximum signal. In the event that 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 sound recording). Beginning in 2000, 700 MB to record 80 minutes of audio.

CD-ROM 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 data storage. 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.

…Corporate Vice President Sony Oga (English) Ohga), who adored classical music, believed that the CD should be able to contain Beethoven's 9th symphony (the most popular musical composition in Japan in 1979, according to a specially conducted 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 running time 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 a decisive argument in deciding on the initial capacity of the disk - 650 MB information (or 74 minutes of sound recording).

…IN Philips And Sony until May 1980 there was no consensus on the outside diameter of the disc. From the point of view of engineers Sony, a diameter of 100 mm was sufficient, since it allows miniaturization portable player. From top management Philips the idea was to make a disc no larger than the diagonal size of a standard audio cassette (115 mm), which was a great success on the market. In May 1980, company executives compromised and approved a "final" disc diameter of 120 mm, a disc capacity of 74 minutes of audio recording, and a sampling rate 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 History of Rise and Fall, or Requiem CD

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

“When we started, there was no alternative,” Kramer recalls ( Kramer), head of the optical development department of the laboratory Philips in the 70s. 20th century - an attempt to bring digital sound in the masses was very risky ... ".

In 1982, mass production of CDs began at a factory in Langenhagen near Hannover (Germany). Release of the first commercial musical CD- it was an album CD "The Visitors" groups ABBA- was announced on June 20, 1982.

Sales first CD-players began in the fall of 1982 in Germany, and they reached the US market only in the spring of the following year.

Corporations have made a significant contribution to the popularization of CDs Microsoft And Apple Computer. Scully, then in charge Apple Computer, said in 1987 that CDs would revolutionize the world. And he turned out to be right!

The rise in popularity of compact discs was greatly facilitated by the fact that they began to be used to record not only music, but also any (!) Data. And then everyone 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 discs!).

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

For nearly 30 years, the CD has led the market music carriers. But time does not stand still! With the growth of information volumes, new media formats appear - DVD, , Blu-ray. Many prefer to simply "" music (or video) on the Internet and download to your HDD than buying CDs.

There are many other reasons for the decline in the popularity of CDs: CD, etc. etc.

To be honest, on CD have been ignored for a long time, especially since the prices have fallen and their volume has 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 no advantages or prospects - so the recording industry should abandon CDs and come to grips with the distribution of music via the Internet.

save the format CD no innovations and tricks will be able, 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.

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

On April 1, 2009, the world's largest record store closed in New York Virgin Mega Store. A few weeks before its closing, the cult musical in Times Square announced a complete 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.

Flagship of the American entertainment industry Virgin Entertainment Group announced that he would close 5 more music stores - in San Francisco, Denver, Orlando, Hollywood and lower Manhattan. The music and video retail chain, once founded by billionaire Sir Branson, known for his extraordinary projects, could not compete 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...

Compact disc ("CD", "CD-ROM")- an optical storage medium in the form of a disk of small diameter (usually 120 mm) with a hole in the center with a permanent (non-erasable) recording of sound reproduced using optical (laser) players. In terms of quality characteristics, it is much superior to a gramophone record and a magnetic phonogram. The CD was created in 1979 by Philips and Sony. In 1982, mass production of CDs began at a factory in Langenhagen near Hannover, Germany. A significant contribution to the popularization of CDs was made by Microsoft and Apple Computer.
Compact disk - a disk for permanent data storage, which is an aluminum alloy circle covered with a protective transparent film. Data is recorded by a powerful laser beam in the form of mechanical microscopic mirror and scattering surfaces. Recording is done on a single spiral, very long track so densely that 640 MB of data fit on a 5-inch disc. Recording is carried out in stationary conditions on special devices, and then read-only CDs (CD-ROM - Compact disk read Only Memory "read-only storage device"). Reading is performed by a low-power (and therefore much cheaper) laser according to the same principle: the disk rotates with enough high speed, the laser beam is focused on the track, and the reader catches the reflected beam, which falls on the photodiode. If the beam hits the mirror surface of the disk, the intensity of the reflected beam is one, if it hits the scattering surface, it is different; this is the difference between zeros and ones, with the help of which the data is written.
Disks write-once, CD-R (they are usually called simply recordable, since the letter R is an abbreviation for the word Recordable - "recordable"), are much more complicated in the device. Modern recordable CDs good quality contain five layers:
- surface layer (Surface Layer) - performs decorative and to some extent protective functions; usually a coating of a special varnish or polycarbonate (a special type of transparent plastic);
- protective layer (Protective Layer) - designed to protect against damage to the two main, working, layers of the disk: reflective and informational; material may be different;
- reflective layer (Reflective Layer) - provides reflection laser beam passed through the information layer; gold and silver are usually used as reflective materials, less often - aluminum and its alloys;
- information (recording) layer (Recording Layer) - the main layer of the CD; can be made from various organic metal compounds. The most common today can be considered cyanine and phthalocyanine. It is the color of the recording layer that determines the color of the working surface of the disc: for example, cyanine gives a blue color;
- substrate (Substrate Layer) - plays the role of the base on which all other layers are applied; usually made of polycarbonate. When recording a CD-R disc, its information layer is processed by a focused high-power laser beam. Under the action of the beam, the "burnt" areas become opaque and begin to scatter light. Often such areas are called pits (from the English. "Pit" - a hole, a depression).
When reading data, the information layer of the disc is scanned by a laser beam of lower power.
All disc layers have a very small thickness (fractions of a millimeter), especially the recording layer, so the CD is very sensitive to mechanical, chemical and any other impact.
The structure of rewritable discs, CD-RW (RW is an abbreviation for ReWritable, that is, "rewritable"), is similar to the structure CD-R discs. The difference is only in the physical properties of the material of the information layer. It must be able to restore its original (transparent) state when performing an "erase" operation. It is produced using the same laser, but used at an intermediate power (lower than when writing, but higher than when reading).

Literature:
1. PC hardware, O.V. Kolesnichenko, I.V. Shishigin. - 5th ed., [rev. and add.]. - St. Petersburg. : BHV-Petersburg, 2004;
2. PC iron, R.B. Thompson, 2nd edition, St. Petersburg: Peter, 2003.

A CD is a plastic disc with a round hole in the center. It uses a laser to write and read optical information in a digital format.

At first, such disks were used to store digital musical recordings, they are familiar to us under the name "Audio CD". But after a short time, the disks were adapted to store files containing digital information of various formats (video, text, programs, music, pictures and photos). Such disks began to be called CD-ROM or “read-only CD”, because information could be written to it only once, but many could be counted. A few years later, discs appear on which the user himself could write information (CD-R), as well as rewritable discs (CD-RW), information from which can be erased and rewritten.

The file formats for audio CDs and CD-ROMs are different. Because of this, players designed to read only Audio CDs are not able to play information from a CD-ROM disc, which requires a special reader.

The history of the CD begins in the 70s of the last century. It first appeared in 1979. It was a joint development Sony companies and Philips. Sony developed a signal encoding method (similar to that used in professional digital tape recorders), and Philips was manufacturing process, which used their proprietary laser disc technology.

IN industrial scale CDs began to be produced in 1982 in Germany by an enterprise located in the city of Langenhagen. The first music CD to be released to the public was introduced in June 1982. On this disc, the album of the group "ABBA" - "The Visitors" was released. Big influence giants such as Apple and Microsoft provided distribution of CDs.

True, there is another version of the origin of compact discs, according to which the American James Russell from Optical Recording was their inventor. Already in 1971, he showed his invention that allows you to store information. Push to develop by Russell optical discs was the desire to prevent damage to the pickup needles of vinyl records with his favorite musical compositions. And eight years later Philips and Sony repeated his invention.

Compact discs are 0.12 cm thick and 12 cm in diameter. They are made of polycarbonate coated with the thinnest metal coating (usually silver, gold, aluminum, etc.) and varnished. One side of the disc is printed with information and images related to the content (names of artists, names of albums, tracks, logos, etc.).

On the outer side of the disc there is a protrusion that encircles the disc, and does not allow the working surface with the recorded information to be scratched. In the center there is a round hole with a diameter of 1.5 cm. The CD weighs just under 16 grams.

At first, music was recorded on discs in the Red Book format. It was two-channel and had a sampling rate of 44.1 kHz, as well as pulse code modulation equal to 16 bits. Small scratches going to the edge of the disc from the center or vice versa do not affect the reading of information from the disc. This is possible thanks to the Reed-Solomon code, which makes it possible to correct reading errors.

Information is written to the disk in tracks (pits) that twist in the form of a spiral. The pits have standard widths and depths of 500 nm and 100 nm, respectively. But the length of the pits differ from each other and have a variation range from 850 nm to 3.5 microns.

There are the following types of discs: CD-ROM - read-only, CD-R - for one-time recording, CD-RW - rewritable. To record information on CDs, special writing devices (drives) are used. There are also Shape CDs, which are CD-ROM type optical media shaped like stars, hearts, airplanes, cars, etc. As a rule, people associated with show business use such discs as carriers of video or audio information. The Shape CD was patented by German producer Mario Koss in 1995. It is worth noting that disks of this type should not be used in computer drives, as they are faster than music ones, which can cause the disk to collapse and damage the drive.

FAQ on audio CDs (CD-DA)

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How is a CD arranged?

The design of the CD-DA disc (Compact Disk - Digital Audio, compact disc - digital audio) and the method of recording sound on it is described by the standard of the Sony and Philips companies that proposed it, published in 1980 under the name Red Book (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 metallic 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 lacquer so that the entire metal surface is protected from contact with the external environment. The total thickness of the disk is 1.2 mm.

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

How is the sound 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 - pulse code modulation, PCM), since each pulse of the original signal is represented as a separate code word. Every six samples of the left and right channels are formed into primary frames, or microframes, with a size of 24 bytes (192 bits), arriving at a rate of 7350 pieces per second, which are encoded using a two-level CIRC code (Cross Interleaved Reed-Solomon Code - redundant Reed code -Solomon with cross-interleaving) according to the scheme: interleaving with a delay of 1 byte, coding level C2, cross-interleaving with variable delay, coding level C1, interleaving with a delay of 2 bytes. Level C1 is designed to detect and correct single errors, C2 - group. As a result, a 256-bit block is obtained, the data in which is equipped with error detection and correction bits, and is also “smeared” to the block, which leads to the recording of adjacent audio data in physically non-adjacent areas of the disk and reduces the impact of errors on individual samples.

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

After the second interleaving, each received block is supplemented with subcode bits - P, Q, R, S, T, U, V, W; each block receives eight sub-code 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 a 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, to indicate fragments of a phonogram, 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 for marking the format of tracks and sectors, recording the TOC (Table Of Contents - table of contents) and timestamps by which playback time is tracked. The Q channel can also be used to record information in the ISRC (International Standard Recording Code - international standard recording code), designed to provide information about the manufacturer, release time, etc., as well as to divide the track into separate fragments (total on the audio A disc can have up to 99 audio tracks, each with up to 99 titles).

In the end, frames designed in this way are channel-coded in terms of "pit - gap" using redundant code 8/14 (Eight to Fourteen Modulation - EFM), in which the original bytes are encoded with 14-bit words that increase the intelligibility of the signal. Three tie bits are inserted between words to meet limits 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 rate of 4.3218 (588×7350) Mbps. Since EFM coding gives a digital stream in which there are more zeros than ones, and the system for representing units by the boundaries of a pit and a gap was chosen, and the number of zeros between ones is the length of a pit or a 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, fragment addresses, titles of works, etc. At the end, a lead-out zone is recorded (track numbered AA) , acting as the boundary of the recorded area of ​​the disk; the P-code bit in this zone changes at a rate of 2 Hz. A number of consumer players cannot recognize a disc without this zone, but many can do without it. Between the input and output zones, a program area (Program Memory Area - PMA) containing the actual sound data is recorded. The program area is separated from the introductory zone by a section of 150 empty blocks (2 seconds), which plays the role of a gap (pre-gap).

The total recording time on a CD is 74 minutes, but by reducing the standard track pitch and pit spacing, you can achieve an increase in recording time - at the expense of reducing the reliability of reading in a standard disk drive.

How are CDs recorded and produced?

The main way to manufacture discs 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 processing the recorded original with a solvent, the required relief appears on the glass, which is transferred by electroplating to a nickel original (negative), which can serve as a matrix in small-scale production, or as a basis for making positive copies, from which, in turn, negatives are made for mass replication.

Stamping is performed by injection molding: a polycarbonate substrate with a relief is pressed from a negative matrix, a reflective layer is sprayed on top, which is varnished. Information inscriptions and images are usually applied over the protective layer.

Recordable discs (CD-R, "blanks") are made in the same way, but between the base and the reflective layer 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 are formed that are equivalent to pits. To facilitate track tracking during recording, a preliminary relief (marking) is formed on the disc during the manufacturing process, the track of which contains frame marks and synchronization signals recorded with reduced amplitude and subsequently overlapped by the recorded signal.

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

How are CDs played?

During playback, an audio CD rotates at a constant linear speed (Constant Linear Velocuty - 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 provide a read bit rate of 4.3218 Mbps, therefore, depending on the length of the pits and gaps, the actual speed may vary. The angular speed of the disk in this case varies from 500 rpm when reading the innermost sections of the track to 200 rpm at the outermost ones.

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

The signal received from the photodetector in the 8/14 code is demodulated, as a result of which the CIRC encoding result with added subcodes is restored. Then 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 violations during stamping, defects and inhomogeneity of disc materials, scratches on it are detected and corrected. surface, fuzzy definition of a pit/gap in a photodetector, etc. As a result, the stream of "clean" sound 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 readings that could not be corrected in the decoder. Interpolation can be linear - in the simplest case, polynomial or using complex smooth curves.

To perform deinterleaving, any CD reader has a buffer memory (standard size - 2 KB), which at the same time is used to stabilize the bit rate. For decoding, several different strategies can be used, in which the probability of detecting group errors is inversely proportional to the reliability of their correction; the choice of strategy is at the discretion of the decoder developer. For example, a CD player with a powerful interpolator might choose a strategy focusing on maximum detection, while a CDP with a simple interpolator or a CD-ROM drive could choose a strategy focusing on maximum correction.

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

The standard digitization parameters - sampling frequency 44.1 kHz and sample length 16 - determine the following theoretically calculated signal characteristics:

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

IN real devices When recording and playing CDs, high frequencies are often cut off at 20 kHz to create headroom for the slope of the filter. The noise level can be either less than 98 dB in the case of a linear DAC and a noisy output amplifier, or more in the case of oversampling by more high frequency using a Delta-Sigma, Bitstream or MASH DAC and low noise amplifiers. The coefficient of non-linear distortion strongly depends on the used DAC output circuits and the quality of the power supply.

The 98dB dynamic range is determined for CD based on the difference between minimum and maximum level sound signal, however, on a small signal, the level of non-linear 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 digital signal phase jitter that is fast in relation 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 extracted from the complex signal using the PLL method (Phase Locked Loop - phase lock loop, or phase locked loop - PLL). Such separation takes place, for example, in the demodulator of the signal read from the disk, resulting in a reference clock signal, which, by correcting the rotational 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 phase information signal while continuously oscillating at different frequencies. An additional contribution can be made by the uneven arrangement of pits on the disk, generated, for example, by poor-quality pressing or unstable recording.

However, disc ripples are fully compensated for by the decoder's input buffer, so that any jitter and knock that occurred before the signal was buffered is canceled out at this point. Sampling from the buffer is controlled by a stable fixed-frequency oscillator, but such oscillators also have a certain, albeit much less, instability. In particular, it can be caused by noise in the power circuits, which, in turn, can occur at the time of ACS operation and correction of the disk speed or head/lens position. On disks Low quality these corrections occur more frequently, giving some experts a reason to directly attribute output stability to disc quality, when in fact the cause is insufficient decoupling of CDP systems.

What do the abbreviations AAD, DDD, ADD mean?

The letters of this abbreviation reflect the forms of the sound signal used when creating the disc: the first - during the initial recording, the second - during processing and mixing, the third - the final master signal from which the disc is formed. "A" stands for analog (analog) form, "D" - digital (digital). 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 waveforms have their advantages and disadvantages. When recording and processing a signal in analog form, its “thin 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 processing in digital form, the higher harmonics are forcibly cut off at half the sampling frequency, and often even lower, but all further operations are performed with the utmost accuracy possible for the selected resolution. A signal that has passed analog processing is rated by a number of experts as more “warm” and “alive”, however, many modern methods signal processing is acceptable only in the digital version.

Can two identical discs sound different?

First of all, you need to make sure that the discs do indeed contain an identical digital audio signal. Complete binary matching of two disks at the level of pit and gap configuration is practically impossible due to small material defects and distortions during matrix processing and pressing, however, due to redundant coding, 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 - reading "long sectors", which are actually CD-DA multiframes 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 on studio equipment that can read discs digitally on a DAT recorder.

Reasons for occurrence digital differences there may be several between hearing-like discs. Some CD-ROM drives and other digital CD-DA readers can introduce subtle distortions into the signal in order to prevent direct copying (for example, by applying 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) with subsequent 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 does not match the original.

It should be noted that even if the digital content of two discs coincided when they were compared in some system (CD-ROM, special devices for comparing the original / copy, etc.), this does not mean at all that on one or another CDP they also identical digital signals will be decoded. Therefore, the most reliable way to find out the cause of the differences in sound is to use a CDP with a digital output, which is recorded to some storage device while listening to both discs. The subsequent digital comparison of the received signalograms will show in which place of the player the changes 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 multiple readings of the same discs are repeatable. Various digital signalograms in this case may indicate unreliable disc reading or poor performance. digital interfaces(receiver, transmitter, cable, connectors). The identity of digital data during repeated playback of several discs can be considered a sufficient sign of the reliability of both the discs themselves and the reading, decoding and intermodule transmission systems.

Auditory comparison of the sound of discs should be correct - the most recognized is the double-blind test (double-blind test). The essence of the method is that the expert (listener) should not see the manipulations with the equipment and the person who produces them, and this person himself, who randomly changes disks, should not know the features of their contents. Thus, any influences, including “subtle” and unexplored ones, of people on the equipment and on each other are excluded as much as possible, and the expert’s opinion is considered to be extremely unbiased.

What is HDCD?

High Definition Compatible Digital is a "super-system" of audio encoding on CD, using standard format CD-DA. An audio signal with a higher bit depth and sampling frequency is subjected to digital processing, as a result of which the main part is extracted from it, encoded, as usual, by the PCM method, and additional information, clarifying small details, is encoded in the least significant bits of samples (LSB) and masked spectral regions . When playing an HDCD disc on a conventional CDP, only the main part of the signal is used, while 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, contact with organic solvents and direct bright light, shock and kinks of the disk. Inscriptions on recordable discs may be made only with pencils or special felt-tip pens, excluding pressure and the use of ballpoint or fountain pens.

When removing the disc from the box, be careful not to bend it. One of the comfortable safe methods requires the participation of two hands - the thumb of the left hand slightly presses the latch, loosening it, while the other hand removes the disk from the latch. The one-handed method, with the index finger loosening the latch and the thumb and middle finger removing the disc, requires more precise force matching, without which it is easy to kink the disc or break the latch tabs.

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

What is a "green marker" and why is it needed?

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

Where can I find more information on CDs?