The Compact Disk
It was the year 1982, for audiophiles loomed a new technological era.
The season of analog sound gave way to digital.
Turntable, cartridges, arms reading, knobs to adjust the stylus and tools of all kinds to remove all traces of dust and static electricity from the grooves of thediscs.
Only a few nostalgic still remember the ritual of the disc, the black one, whichis the analog one"played" on the turntable, or rather the "flat" after removing it from large (and readable) cover carefully avoiding touching it with yourfingers. All this now belongs to a bygone era, the vanished world of analog extinct bit cold to the sound of the CD.
The story of the Compact Disc
Rarely a technological innovation has had implications as wide as that of The early days, it was 1982: Sony and Philips, allied to the newstandard, presented the first readers and consumer electronics trade fairs, appeared the prototypes of the new standard.
The official launch date was August 31, when the Japanese took the veil inthe CDP-101 and Philips showed his creature: the CD100.
The diskette silver has been a technology whose advent has changed thestatus quo of an entire industry, and the record high fidelity impacting profoundly transforming the field of information technology.
After the introduction of the CD, historical names found themselves at a Under the impetus of the digital revolution as manufacturers of turntablesThorens, Technics and Dual.
Manufacturers of cartridges as Stanton, Ortofon and Shure passed fromleadership positions mass roles.
For many companies the consumer and professional audio cd has been the failure or the change of hands, or the substantial change in the business model and the conversion of production, perhaps by focusing on niche marketsas the disc-jockey.
The compact disc has transformed in a decade, an industry leading ease of use in homes a never seen before: think instant access to the tracks continuous listening to an album without having to turn and, above all, thequality sound, which to tell the truth at the beginning not enthusiastic listeners more attentive and refined.
In many, in fact, accused the compact disc to have a sound cold, certainly silent (no background noise), but less "addictive" than that offered by the best turntables.
But the cd is not only important for this and the implications for industry,is a technology milestone that represents over a century and a half of mathematical knowledge, physical and technical.
The compact disc (Compact Digital audio) in fact, brings two major innovations: the machine-readable through a laser beam, and thedigitization of music.
What do you mean digital?
Means that the "sound" is represented by a sequence of bits and a series of The grooves of the vinyl record is an exact mechanical transposition of the acoustic wave detected by the microphone in the recording studio.
From this "analogy" between the input signal and the output signal is thedefinition origin.
Analog turntables were obviously mechanical horn, whose basic principle (thephonograph by Thomas Edison drum of 1877) remained unchanged until the birth of the CD: a needle, and then the first mechanical piezoelectric ormagnetic, which follows a groove in a disk.
Cd in the bit sequence is engraved in a spiral, which goes from the center ofthe disc toward the outer edge, formed by a succession of pits (pits and lands) or less reflecting that the radius of thelaser are the numeric code in the formof 1 and 0 (ones and zeros).
With digital, the quality is no longer affected by the support and readingmechanism.
The combination of laser scanning and numerical coding (later decoding and digital-to-analog) offers high performance as long as the transposition between analog and digital (and vice versa) is accurate.
In fact, to digitize music you perform two operations of "discretization":sampling and quantization.
The first serves to detect the amplitude of a signal moment by moment, thesecond converts each pulse in a binary code.
The cd uses the technique Pcm Pulse code modulation which was invented by Alec Reeves in 1937 and tested for the first time in 60 years by the Japanese NHK broadcasting, using a magnetic tape.
Sony over the years developed many devices 70 conversion Pcm gainingexperience in this field.
The history of the cd has its roots in the early 19th century: the theoreticalfoundation dates back to 1841,when the mathematician Augustin Louis Cauchy The theoretical bases of the CD and digital signal processing are a staple in the work of Harry Nyquist in 1928 (the same year he was introduced to the disc LP33 rpm and 1/3)and Claude Shannon, who in 1948 publishes the mathematical Just to Shannon must be a fundamental theorem, which states that thesampling frequency must be at least twice the maximum frequency of the The compact disc uses a sampling rate of 44.1 kHz, this means that the sound is analyzed 44,000times per second and the maximum reproducible frequencyis 22,000 Hz (on the threshold of human hearing).
The quantization is instead equal to 16 bits used to represent 65,535 signal levels and thus adynamic range of over 92 decibels and a signal / noise ratio (index of the absence of noise) of at least 98 dB, much more of the moreexpensive the turntables.
The birth of the CD is the result, as already mentioned, the agreement between Philips and Sony for sharing in a uniform standard of their The Dutch house facility made available to the techniques of recordingon optical laser reading designed in 1969 by the physicist Klass Compaan and Piet Kramer in the laboratories of Philips, concepts that gave rise to the laser videodisc and compact disc, whose very first experimentalprototype (worked 14-bit) was introduced in 1979.
Sony, however, had been working since 1977 with Mitsubishi and Hitachi to format Dad (Digital audio disc), which provided optical discs 33 cm (12 cm of well-arching a cd) and 16 bit resolution.
Sharp instead was starting mass production of the laser diodes. Among otherthings, the laser was invented in 1958, the same year that was the A key element in the history of the cd is the work of Reed and Solomon, who in1960 had built a correction code used in players, compact disc read errors to Philips and Sony in the spring of 1981 defined a common standard (described in the so-called Red Book), which led to the introduction of the CD as it istoday.
Since then it's another story!
The compact disc digital audio (CD-DA) was the first optical disk toconquer the general public.
The first descent was the CD-ROM with 640 Mb (born between 1983-1985): an unprecedented success, so much so that today there are no reader pc.
Philips launched in 1986 the way of interactivity with the CD-I (Compact Disc In 1982 was introduced the CD-R technology to record CDs, a technology that only in the last 10years it has become popular undermining themonopoly of the cassette. Other formats are video CDs and photo CDs.
Was created in 1997 DVD (Digital Video Disk, later renamed "versatile") for movies, computer software and also audio. The DVD audio is, in fact, the alternative to high quality, superior to that of the CD.
And just to compete on this front(as opposed to the level of audio performanceof compressed digital 'Mp3) Sony introduced five years later the Super Audio The Technique of Compact Disk
Similarly to traditional vinyl records, CD-ROMs contain a single spiral trackthat starts from the center and grows outward.
This track, almost five and a half kilometers long, is divided into blocks ofequal length called sectors.
The data are stored by means of the alternation of small depressions of variable length (pits) and flat areas (lands).
The reading head is formed by a small laser (Light Amplification by Stimulated Emission of Radiation) at low power and a photodiode: when the laser beamhits a flat area and is reflected by a mirror strikes the photodiode, while when it is a depression hit the laser light is scattered and the photodiode receives no Contrary to what one might expect, lands and pits do not represent the binarydigits 0 and 1.
If so, then a single bit on the track would occupy a space of approximately300 nm, the CD should then have a very accurate clock face which correspond to each tick a range of 300 nm to the speed of rotation of the disc(which moreover is variable) .
With today's technologies, such a device is practically unworkable.
The method used is based on the alternation instead of lands and pits.
When the CD detects a transition from land to pit or reverse, it starts a timer:the subsequent transition the timer value is proportional to the number of bits represented by the pit or land in question.
Each transition from land to pit corresponds to a 1 (one), otherwise it has a 0 To avoid the problem of departure, transitions must be frequent, so you canreset the timer.
In practice, the minimum length of a pit or a land is approximately 900 nm(3 bits) and the maximum about 3300 nm (11 bit).
Because of this limitation can not have two consecutive 1, and the number of zeros between two 1 bits can go from a minimum of two to a maximum of ten.
Unfortunately, already a simple pair of bytes can take many values that violate the above rule, so you have to adopt a different encoding from what we areused to.
The technique used in the CD is called EFM (Eight to Fourteen Modulation)and made to correspond to each of the 256 possible values of a byte of a 14 bit code that is in accordance with the limitations described above.
Furthermore, between two consecutive codes are three bits of separation that have the purpose of limiting the length of a possible sequence of equalbits that can be created by combining two codes of 14 bits.
Stored in this form, the data is divided into sectors by 2352 bytes each.
A sector contains 12 bytes of data for synchronization, a header of 4 bytes2048 bytes of data with error correction and 288 bytes of codes to correct any errors (if the data are not sensitive to occasional errors, eg. Audio digitizedvideo or animation, you can do without the error correction and thus increase Given that all sectors have the same length and that the head must be able to read the data at the same speed? (CLV Constant Linear Velocity, as opposedto the method used in hard-disk, or CAV Constant Angular Velocity), the speed of the disc varies from about 500 r / min for the most interior areas about The need to change the rotational speed of the disk is one of the main reasonsfor the slow access time of the CD.
As regards capacity, despite the large number of bits "wasted" for the EFM andthe correction of an error (and the best is yet to come in the next section) A CD can hold 333,000 sectors well in fact, equal to 74 minutes at a standard speed of reading (75 fields per second).
Since the last 14 minutes you are at the outermost part of the disc, which isthe most difficult to influence, it is not uncommon to find CDs that contain "only" 270,000 sectors (60 minutes).
In a nutshell, how would a well-known judge, 333,000 sectors will be of 650 MB of data (742 MB without error correction).
The compact disc player is the protagonist of the system of Hi-Fi nineties infact it has, compared to the traditional system of vinyl records, numerous benefits such as: increased dynamic range, lower distortion, better frequencyresponse and, most important, the discs are not consumed during use as there is no direct contact between the reading system and the disk surface due to their particular construction technology.
Before describing the apparatus for reading the discs we see how they aremade and how they are engraved compact disc.
The compact disc (also called optical discs for the reading system that providesfor the use of laser light) are presented as small discs (diameter of 5 inches equivalent to 12 cm of aluminum surfaces with extremely glossy and reflective. The music information are found only on one side, contrary to what occurs for normal vinyl records, even if the total duration of a compact can reach over 74minutes.
The signal, in digital form, is encoded by a series of microscopic traces 0.1 thousandths of a millimeter in width, 0.5 thousandths of a millimeter deep For the manufacture of such discs will require extremely sophisticatedprocedures to the scope of a few industries, it is therefore a final cost quite The manufacture of the compact disc begins with the production of the Master it is obtained by stretching on a disc of pure glass a layer of photosensitivematerial, then a laser beam impresses this layer with a series of pulses that follow the musical recording made earlier.
The points of film etched by the laser are removed and the surface is metallized. At this point the master obtained is used, as for vinylrecords, which mold to produce the discs definitive; these, after which the incision is made under a press, are covered with a layer of transparentplastic that will protect the microscopic traces from dust and abrasions.
In addition to program real musical compact discs contain a numberof ancillary information that helps you make while playing, fast searches, All compact disc data and music in the form of digital signals, but the process of recording, mixing and editing may have been obtained with classicaltechniques, known as analog, such as recordings on magnetic tape.
The different procedures used for the production of the CD are highlighted, on most of the packs of discs, through the acronyms whose meaning is specified AAD (Analog Analog Digital).
This symbol indicates that the original recordings, mixing and editing were obtained in analog form; burning in digital form.
ADD (Analog Digital Digital).
This symbol indicates that the original recordings were obtained in analogform, mixing, editing and burning in digital form.
DDD (Digital Digital Digital).
This symbol indicates that all the steps were performed with digital signals.
In the case of the CDs with AAD or ADD will be visible, during playback, the hiss due to magnetic tapes used in the different stages of production, the typeof discs DDD will be absolutely free from hiss.
How the data is stored?
Since CDs have a standard transfer rate of 150 K / sec and audio CDs are digitized with 16-bit per channel at 44.1 kHz, in order to play a CD needs totransfer well 172 K / sec.
We already know that the reading speed is 75 fields / sec and that a sector contains 2352 bytes,and precisely 75x2352 is equal to 172 K.
It would seem to have to do without the byte dedicated to error correction, but The data stored on CD first of all are divided into 24 byte packets called The method used for the correction of errors is called the Cross Interleave The effectiveness of this system is remarkable: for each frame, the 24 bytes ofdata are sent to the first Reed-Solomon decoder, using the first 4 byte extra is capable of detecting and correcting an error every 32 bytes.
Subsequently, the 24 bytes of data and the remaining 4 extra bytes are sent at different intervals, according to the RS decoder.
The interleaving of the byte enables to decompose burst type errors, that is on many consecutive bytes, in many errors involving a single byte for each block.
If there are errors the second RS decoder uses the last 4 bytes to adjustthe 24 bytes of data. After being de-interlaced to restore the starting order of the data can be finally sent out.
The specifications for the CD allow to have up to 220 errors per second.
The algorithm CIRC can almost always perfectly correct these errors and toprovide correct data.
Errors extended up to 450 consecutive bytes, for about interleaving, are corrected without loss of information.
It 'still possible that something escapes, but usually these are small “click” We can now specify the statements of the previous paragraph: a sector contains 98 frames, for a total of 98x24 = 2352 bytes corrected by the CIRC.
Since the CD-ROM data integrity is more important than in the audio CD, thelast of these 2352bytes 288 are reserved for an additional layer of error In fact we have 4 bytes of EDC (Error Detecting Code) that contain the CRC of previous data and 276 bytes of ECC (Error Correcting Code) that the techniquestill using interleaving and Reed-Solomon codes are used to correct errors The probability that they remain undetected or uncorrected errors becomes Each sector has a mode byte that specifies the type of data it contains: So a single CD can contain both data and audio (if the data should be first).
The standards
The standard for CD-ROM has been defined by a group of companies known as The format proposed by this group has in fact spread rapidly, and in 1988 was accepted and integrated into the document signed by the ISO-9660 ISO.
Also known as the "Yellow Book", allows multiple operating systems to read the same CD-ROM providing the specifications of the file-system and theorganization of data on a CD.
Following this standard has been extended to meet the needs of more advanced multimedia applications, the CD-ROM XA (Extended Architecture) allows interleaving of data and audio compressed with the latter techniqueADPCM (Adaptive Delta Pulse Code Modulation ) and extended to a maximum For CD-DA (Digital Audio) instead, the standard is called the "Red Book". The audio is sampled at16 bits with a frequency of 44.1 kHz, which allows adynamic range of about 96 dB.
Very similar to the XA CD is the CD-I (Interactive) or "Green Book" developed by Philips in 1986.Direct competitor of the CDTV (Commodore Dynamic Total Vision) is implemented as a stand-alone system based on a CPU Motorola680x0 class and allows you to store data, images, videos(MPEG), and audio.
Finally we have the Kodak Photo CD that makes use of rewritable discs tocompress and store up to a hundred photos on the same disc.
Being based on the same structure of the CD XA and CD-I, CD players that support these standards can also read the Photo CD using an appropriate The Photo CD, in any case, it is also available as a stand-alone connected to aregular television.
The library CDA
One of the most interesting possibilities made available by CD-ROM on PC is certainly that they can also play standard audio CDs, supported by most of thedrives currently in circulation.
Our aim is precisely to integrate this capability in our applications.
To see a CD-ROM drive as DOS is no need for two drivers: the first changesdepending on the make and model of the CD-ROM drive installed, the second is usually called Mscdex and uses the API provided by the first driver to To save space, Mscdex does not explicitly support the audio functions of thedrive, so an application needs to talk to the low level driver.
The library CDA, along with a CD-player that uses it allows programs writtenin high-level languages in a natural way to use the CD-ROM as CD-DA without as having to worry about what happens "behind the scenes".
The Readers Compact Disc
The compact disc player (also called CD Player) consists of three basic parts: the reading system,constituted by the laser pick up, the systemof conversion of the signal from digital to analog, consists of special electronic circuits and the motor that turn the platter is placed where the disc.
The reading system, in addition to the laser pick up comprises a series of mechanisms that control the functions of the laser.
The disc reading, unlike the case for vinyl records, takes place from the inside outwards and at different speeds, in fact, given that the amount of informationread per second must be constant, hand in hand that the laser beam is moves from inside to outside the rotation speed of the plate must decrease.
The laser light generated by a diode, is focused through a lens system, on the surface of the disk where there are traces; provides a photodiode to measurethe amount of light reflected from the disk surface transforming this measure When the laser beam hits a flat area of the disc all the light is reflected on the photodiode; vice versa when the beam hits a track, the light is partiallydispersed and therefore a smaller amount of it will reach the photodiode.
Operating diagram of a CD player
The absolute insensitivity of the compact disc to the effects of the powderis due to the fact that the beam is focused on the disc surface where there are the tracks, and not on the external surface which, as seen, formed by alayer of transparent plastic.
So that the reading of the tracks are made properly are needed many controlsperformed by specialized circuits, such as the control of the focus, the adjustment of the rotation speed, the correction of reading errors, etc.
in particular the error correction is possible thanks to the special encoding in which the data are recorded in the tracks.
Some devices use for reading the disc three beams of laser beams instead of one, of these one serves for reading real and the other two, which explore themargins of the track, serve to keep the well focused beam of detection in order There are often multiple charger appliances, they come with CD tray can accommodate more disks (typically 6), which will be read sequentiallyor according to a schedule determined by the user.
The front panel of the CD players is considerably complex for many of thecommands and the display of which is usually supplied.
The main commands relate to the search of the indices (each index is associated with a piece of music), the possibility of fast forward and slow, the ability to program certain sequences, the possibility of random access, thepossibility of repeating parts of the disk or l 'entire program.
The display provides information about the duration of the entire disc, the duration of individual songs, the index number currently read, the remaining time and others in relation to different types of devices.
The different models of readers differ in the accessory functions that arecapable of performing and for the quality of the materials used in their manufacture, in particular the disc tray should be sturdy and has a good suspension system in order to avoid movements and vibration when thedisc.
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