DISC
TECHNOLOGY
23.1 INTRODUCTION
In the beginning there was videotape. It gave the public the opportunity to tape one program on the T.V while watching another. The media was inexpensive. There was a quite major problem at first though there were two different incompatible formats.
The consumer had to choose between Betamax and VHS. Betamax had a higher quality picture than VHS, but the problem with Betamax was that there was a shortage of pre-recorded videos, moreover there was plenty on VHS. And so Betamax died. VHS went global within a very short space of time.
After a while, though, some people started to notice a few problems with VHS. The picture is, well, quite rubbish to say the least! Nowhere near broadcast quality. And the sound, too, is not CD quality. There's no easy way to access different sections of programs. So, a solution called LaserDisc was born. Optical disc technology is a growing force in home video which became popular with the introduction of the audio Compact Disc. Today, various formats exist, which besides sound can also carry data, graphics and full motion video parts. Optical disc technology makes use of digital signal processing, contrary to the analogue audio and video carriers, such as the gramophone record and the magnetic video tape. But why digital?
In analogue transmission any imperfection during the registration, storage or reproduction phases of recording will decrease the quality of the audio and/or video signal. For example, a dirty record causes noise, an irregular revolution or winding speed causes problems, a worn needle or a dirty head causes distortion. These imperfections do not occur in digital registration.
In optical disc technology, analogue signals are converted into digital signals. During this process, the analogue signal (audio and/or video) is measured in parts and converted into a series of values, called sampling. Thus the analogue signal becomes a digital signal which is now a series of pulses: pulses for the '1's, and non-pulses for the '0's. For optical discs these pulse series are recorded on the surface of the disc as microscopically small pits and lands, with the help of a fine laser beam. Pits stand for '0's and lands stand for '1's.
LaserDisc fixed almost everything that was wrong with VHS. The picture quality was better, sound was CD quality, the discs were more durable, and it offered instant access to different sections of the film.
But the players were too expensive!! Those discs are a bit big, and you have to turn them over halfway through. AND you can't record. So LaserDisc failed to capture the public the same way VHS did and became the format for home cinema enthusiasts who cared more about picture and sound quality than price.
Later on, the manufacturers decided to digitise films on a CD using MPEG-1 compression and call the format VideoCD. This would give CD quality sound and improved picture quality, all in a compact medium. Unfortunately, with hardly any films available, and dubious picture quality, VideoCD flopped. What is the point of a format if you can’t get any recent films for it ??!!
What we need is a format that offers gigabytes of storage on a single CD sized disc with brilliant picture and sound quality. In 1994 there were two proposals, one from Toshiba/Time Warner called the SD (SuperDensity) disc and another from Philips/Sony called the MMCD (MultiMedia CD). The SD alliance proposed a double sided disc and Philips/Sony proposed a dual-layered disc.For a while it looked like there would be another VHS Vs Betamax situation, with two slightly different formats battling it out. This would be disastrous with a lot of confusion and lack of consumer confidence. So, with the intervention of IBM, the two systems were combined using both dual-layered and double sided discs. They called the new format DVD.
DVD is a high capacity CD capable of up to 133 minutes of video and sound per layer compressed using MPEG-2 and picture quality better than LaserDisc. There is also no need to flip sides or change discs as most films can fit onto a single layer DVD. Sound is well catered for as well, with capacity for 8 different soundtracks and 32 sets of subtitles. There are also other advantages. You can select your screen ratio. What this means is that people can watch the films in widescreen or, the DVD player can pan and scan each frame according to instructions on the disc. Also there is the option of choosing different endings on discs that support it. Soon, DVD will be recordable as well, giving perfect replicas of what's on screen.
Unfortunately, there have been a few problems. First there is the issue of copy protection. Hollywood didn't like the idea of people being able to make exact copies using their recordable DVD players. This would mean anybody could run off perfect copies of the latest films, which would lead to film companies not releasing new titles and DVD would die. There is a solution however. Each DVD will carry a piece of information that says it can't be copied. When rewriteable DVD players become available they will check for this and refuse to work if they find one. To stop recording onto video, DVD will use the MacroVision copy protection system which is used on commercial videotapes.
The final problem is that of audio formats. The new movie surround sound is that of Dolby Digital. This improves on Pro-Logic by offering different channels for surround left and right, plus a dedicated sub-woofer channel. Thanks to a dispute however, Dolby Digital isn't the universal surround sound format. There's a new one - Musicam. While Dolby Digital will be the standard in countries who use NTSC, Musicam will be the standard for PAL and SECAM countries. The first batch of European players cannot output Musicam, only Dolby Digital. It now looks like future players will be compatible with both Dolby Digital and Musicam, going some way towards solving the problem. It is not clear yet as to whether all European discs will carry Musicam, as opposed to Dolby Digital as originally thought. If they do, stand-alone Musicam decoders should become available and Musicam will become the main sound format for European DVD, much to the dislike of Dolby Digital fans and those who already have Dolby Digital decoders.
With a high capacity recordable version in the pipeline DVD should replace VHS if there is any justice, because it is far superior. The main benefits are excellent picture and sound quality. The video is compressed using MPEG-2 which gives a better quality picture than VHS and LaserDisc. The sound can be Dolby Digital, Musicam or both, and this can be mixed down into Dolby Surround to ensure backward compatibility. As well as this, you have all the benefits of CDs, e.g. instant access to different tracks and durability - no matter how many times you play a DVD the quality will always be the same. Add to this the option of multiple camera angles, different ratings, choice of aspect ratios (pan and scan or widescreen) and you have every movie lovers dream.At the moment DVD is not recordable. This means it can't replace the VCR - yet. A specification for a recordable version of DVD has been proposed by Sony which allows for 5.5 hours of DVD quality video and audio. This will not be available until at least the year 2000 however.
DVD-ROM is a version of DVD for computers, designed to replace the CD-ROM. It has the same capacity as DVD-Video which is far greater than normal CD-ROMs. DVD-ROM upgrade kits will soon be available and titles should then follow. To play DVD movies on a DVD-ROM drive you also need an MPEG-2 decoder board. Some upgrade kits will contain this too. A specification for a recordable version of DVD-ROM, called DVD-RAM, has been agreed upon and these drives should be available by the end of the year, although there are new proposals by other companies for rival formats.
23.2 CD-ROM
The Basic CD-ROM, which stand for Compact Disc Read-Only Memory, is now widely used as a vehicle to distribute information. "Read-Only" means that you can only view the contents of the CD-ROM, with a CD-ROM player, but you can't alter any of the information contained on the disc. CD-ROM's usually contain a woven stream of digital image, audio, video and text data or only audio data. Enhanced music CDs are currently all the rage, with the first track containing multimedia data -- text, images, audio clips and videos of and about the artist.
CD-ROM's are used to distribute a wide variety of information, from multimedia encyclopedias to living books to games to image and video libraries to product and sales presentations, and more. The advantage is that it's a portable media, and can contain a large amount of data. CD-ROM's are ideal for archiving, and makes specific information easily and quickly accessible. The development of CD revolutionized the audio world by introducing truly digital technology for the first time and set new standards in sound quality.
Fig. 23-1. The Compact Disc Player.
Digital technology describes the music signal in the form of series of numbers in binary notation, called bits. The bits are recorded on the disc in small grooves, called pits and lands. Pits and lands represent 0's and 1's, respectively. From the digital numbers, the CD computer reconstructs the original audio signal. The measuring of the audio signal bit by bit is called sampling. Bits make up digital words (numbers in binary notation). Binary notation has the base 2, and uses only 0's and 1's. Computers use binary notation since they can read a circuit as "on" or "off", as a "0" or as a "1". So the digital recording achieves high quality with low cost by using measured values instead of the analogue signal manipulation.
Digital recording of sound is intended to achieve superb sound reproduction with less cost. Superb reproduction has been possible using analogue methods, but only with a great deal of cost and expertise. The secret of the digital advance in quality is in the fact that digital recording does not manipulate the music signal itself, as is the case with analogue processing. Digital technology instead uses measured values to record sound.
Fig. 23-2. Digital technology uses measured values to record sound.
An analogue signal takes the form of a wave. At different times therefore, its amplitude will vary. When an analogue signal is converted into a digital one, the signal is measured at regular intervals. These measurements (or measured values) are converted into digital (binary) numbers (a series of '0's' and '1's) Example of a fragment of CD music encoded in binary notation: 1101 0111010 0010 1101.
23.2.1 Architecture of The Compact Disc Player
It consists of the laser beam which reads coded information from the compact disc. The reading is kept accurate by the servo processor. The data from the reading is sent to the decoder, where it is converted to regular digital information. A digital filter then removes noise. The Digital to Analog Converter, DAC, the most important part of the CD player, converts the digital data to an analogue audio wave. After an analogue filter which removes noise, this wave is sent to the loudspeakers (Left and Right) for reproduction as sound. The microprocessor controls features, such as volume, balance, tone, etc.
The CD is a plastic disc 1.2mm thick and 12cm in diameter, with a silver- colored surface which reflects laser light. The maximum playing time for music recorded on compact disc is 74 minutes. The CD has several layers. First, to protect the 8 trillion microscopically small pits against dirt and damage, the CD has a plastic protective layer that allows the laser beam to penetrate freely. Then there is the reflecting aluminum coating which contains the pits. Finally, the disc has a transparent carrier. Mechanically, the CD is less vulnerable than the analogue record, but that does not mean that it must not be treated with care.
Fig. 23-3.
The CD has several layers. Notice how the pits contain binary information.
The protective layer on the label side is very thin, only 0.002mm. Careless treatment or granular dust can cause small scratches or hair cracks, enabling the air to penetrate the evaporated aluminium coating. This coating then starts oxidising immediately at that spot. If the CD is played extensively, it may be advisable to protect the label side with a special protective foil, which is commonly available in shops.
A compact disc is scanned by a laser beam. When this laser beam hits a land, all of its light is reflected and the cell gives off current. When the laser beam shines on a pit, only half of the light hits the surface. The other half goes into the deep part of the pit. The difference in height between the two places is exactly a quarter of a wavelength of the laser beam light, so the original beam is totally eliminated by the interference between the beam reflected from the surface of the disc and the beam reflected from the pit. The photo cell does not produce current. The scanning must be very accurate because the track of pits is 30 times narrower than a single human hair. There are 20,000 tracks like the one shown in Fig 23-5 on one compact disc. The lens which focuses the laser beam on the disc has a depth of field of about 1 µm (micrometer = one-millionth of a meter).
Fig. 23-4.
The disc data is converted into electrical pulses (the bitstream) by reflections of the laser beam off of a photo-electric cell.
Fig. 23-5.
There are 20,000 tracks like this one on one compact disc.
Many CD players use three-beam scanning for correct tracking. The three beams come from one laser. A polarized prism projects three spots of light on the track. It shines the middle one exactly on the track, and the two other "control" beams generate a signal to correct the laser beam immediately, should it deflect from the middle track.
Fig. 23-6.
Cutaway view of the laser pickup. Depending on whether the laser beam hits a pit or a land, the laser beam is reflected and received by the photo-electrical cell.
Fig. 23-7.
Compact Disc player mechanism The laser pickup reads the disc from below.
Thanks to this optical scanning system, there is no friction between the laser beam and the disc. As a result, the discs do not wear, however often they are played. However, they must to be treated carefully, as scratches, grease stains and dust might intercept or diffract the light, causing whole series of pulses to be skipped or distorted. This problem can be solved, as during the recording the Cross Interleaved Reed Solomon Code (CIRC) is added, which is an error correction system, to be discussed later, that automatically inserts any lost or damaged information, by making a number of mathematical calculations. Without this error correction system optical disc players would not have existed, as even the slightest vibration of the floor would cause sound and image distortions.
An analogue to digital (A/D) converter translates these decimal values into binary notation (bits). Bits are made up of ‘1’ and '0's only, and by combining these ones and zeros in different combinations, decimal numbers can be expressed in binary notation. Thus the analogue signal becomes a digital signal which is now a series of pulses: pulses for the '1's, and non-pulses for the ‘0’s. For optical discs these pulse series are recorded on the surface of the disc as microscopically small pits and lands, with the help of a fine laser beam. Pits stand for '0's and lands stand for '1's. In most recordings every value (44,100 per second) converted into a string of 16 bits. This totals over one million bits per second. A 16-bit number of '1's and '0's can indicate no less than 65,536 different values (2 values possible for each bit = 216 = 65,536).
Digital comparators are used in the Analogue to Digital Converter (ADC) . Each comparator tells whether the voltage being measured is higher or lower than a set value. The comparators, each adjusted to a different level, are connected in series. To illustrate, imagine four comparators called A, B, C and D. A is adjusted to 1 V (=20). B is adjusted to 2 V (=21), C to 4 V (=22) and D to 8 V (=23). (Remember that we are working with base 2 notation.)
Fig. 23-8.
The value of the signal is 1 V at the moment the measuring sample is taken (the moment of the sample pulse). The output of the 1 V comparator (A) is high: it emits a signal, a "1" in binary notation. The output of the next few comparators (B, C and D), which are all adjusted to a somewhat higher level, is 0. A digital signal is formed a digital word, 0001. At a signal value of 2 V, the comparator B has a high output. The digital word 0010 is therefore formed. At a voltage of 5 V (4+1), a high output appears at comparator A (for the "1") and C (for the "4"). The other two remain 0. The digital word is 0101. Remember that, in binary notation, each place to the left is worth one power of 2 more than the value immediately to it right. (The base is 2.). This is a 4-bit system for measuring different voltage values. Only steps of 1 V can be measured. The converter cannot be adjusted for smaller differences.
Fig. 23-9. With the 8-bit system, the waveform is rough after digital-analogue conversion.
The 4-bit system is too inaccurate to produce enjoyable music, so more than four digital comparators are commonly used. The system then has more than four bits. The absolute minimum for reproducing music is eight bits. The 8-bit system can measure 256 different voltage values. In digitising an audio signal of which the maximum voltage is 1 V, the 8-bit system can only measure differences larger than .003906 V (1/256). This is still coarse, although it is effective in practice. With the 8- bit system, the wave form is rough after digital-analogue conversion. Just as the ADC determines the quality of the digital recording, the DAC determines the quality of playback. With the standard 16-bit converter, the DAC converts the 16 bits of coded information into a series of voltages through a 16-step resistor ladder network to which a reference voltage is connected. This series of voltages is added by an "operational amplifier", which generates a certain output voltage.
Because of the analogue character of the resistors in the DAC, the accuracy is limited. Such a DAC, when going from a "0" to a "1" and vice versa, generates zero-crossing (or cross-over) distortion (distortion caused by amplifying the positive and negative half cycles separately), non-linear distortion and harmonic distortion. As a result, the sound loses part of its clarity and, especially in the silent passages, irregularities such as noise are heard.
Fig. 23-10.
Conversion process of an analogue signal to digital and back to analogue.
Quantisation is the conversion of digital information to an analogue signal. Quantisation noise is the result of the digital system working with voltage steps. The number of bits determines what the highest attainable, or theoretical, S/N ratio is. With 16 bits, this ratio is 96 dB S/N, allowing the CD to have a larger dynamic range than any other medium.
Fig. 23-11. Quantisation noise and modulation noise.
The S/N ratio is not a constant; it varies with the signal level. At low sound levels, the S/N ratio is lower; there is more noise. This can be heard in CD players of inferior quality. They produce a soft background noise as soon as they amplify even very weak signals. This is quantisation noise.
With the analogue tape or cassette recorder something similar happens. When the system is recording a tone, small side bands may be generated to the left and to the right of the actual frequency. These side bands cause modulation noise. With the digital signal, the many more voltage sources cause more side bands to be generated. Some side bands are generated at very low levels, yet nevertheless they may become quite audible, if not as noise, then as a certain coarseness of sound.
23.2.2 From Photo Cell To Error Correction
The signal coming from the photo diodes is not built up in clear, sharp pulses, but moves between low and high output, resulting in an imprecise signal. The shaper allows voltage jumps to be registered, so that this imprecise signal is corrected. A voltage jump is a jump between two states in a digital signal, i.e., from a "1" to a "0" or a "0" to a "1". If the signal jump is smaller than a certain number of volts, the shaper is off, and delivers no volts; if the jump is bigger than that number of volts, it is on and does deliver volts. Again, we see that digital technology works with these two values, "off" and "on": the "0's" and "1's" of binary notation.
These voltage jumps go to a buffer circuit, which is in fact a memory circuit. This correction system, invented by Reed and Solomon, is called Cross Interleaved Reed Solomon Code (CIRC). It is an essential element of the CD player. Without an error correction system, the CD player would not be possible, because every particle of dust, every spot of grease, however small, causes an interruption of the signal or deflection of the beam, causing pulses to drop out. Error correction pulses are immediately substituted for the missing pulses.
How does this system work? Basically, it detects and, if possible, corrects errors in order to regain the original signal after disturbances or drop-outs. When recording extra bits, the correction system adds parity bits to groups of audio bits. The audio bits are not passed on one after the other, but are ordered in certain relations in small groups. A parity bit is added to each group. The relation between the bits ("0's" and "1's") in the group (also called the matrix) is such that adding certain series of classified audio bits produces only one kind of result, e.g., an even number of "1's". The parity bits are adjusted to this result. As soon as one bit drops out during playback, there is no longer an even number of "1's". The "1" in question (which has become a "0") is immediately converted into a "1" again. The error is corrected.
Fig 23-12.
The correction system adds parity bits to groups of audio bits.
Two CIRC’s may be used to maximize the efficiency of the ingenious CIRC system by "cross-interleaving" the two codes in matrix. The effectiveness of this system can be demonstrated by pasting a piece of paper several millimeters wide across the CD, and playing the CD. A good correction system will produce normal sound. The CIRC system will first try to correct the signal errors produced by the paper. If the error is too large (if too many bits have dropped out), the system will calculate and insert the missing parts. If this approach fails because of the size of the interruption, the system suppresses disturbance. The music will skip.
Thanks to CIRC system, the CD player works free of disturbances, even if the disc is dusty or even slightly damaged, or if the machine is bumped during playback, or if the surface on which the CD is placed vibrates. However, the more error correction is used, the more the final sound quality is affected. So a CD should always be handled with care. It is not as robust as most people think!
During signal processing of the photo diode via the CIRC correction system to the DAC, harmonics are generated as a result of the sampling. On both sides of the sampling frequency of 44.1 kHz, upper and lower side bands spontaneously generate, each measuring from 2 kHz to 20 kHz.
The same thing happens at each successive harmonic. If this complete signal were to be passed to the DAC in this form, all these harmonics would influence each other in a unacceptable way and cause distortion. That is why all frequencies over 20 kHz have to be filtered out as stringently as possible. This is done with a low-pass filter.
This filter must meet very high demands. The filter should allow the frequency of 20 kHz to pass freely, since 20 kHz is the upper limit of the audible range. The filter should also block out completely the frequency of 24.1 kHz (the limit of the lower side band of 44.1 kHz). There is then a difference of only 4.1 kHz between what the filter should allow to pass freely and what it should block out completely.
In the early days of CD, single-analogue filtering (brick-wall filtering) was used. This filtering system placed a filter at a sharp angle behind the DAC. This, however, had side effects which affected the pure stereo image, such as unacceptable phase shifts. As every processing stage causes a time delay, filtering systems cause delays or phase shifts if the original signal is compared with the output signal.
Oversampling is a simple, inexpensive device to filter out conversion side effects. Sometimes two, three, or even four oversampling devices are applied in one machine. The great value of oversampling lies in the possibility of applying a simple filter that leaves the high quality of the signal intact. Due to oversampling, a low-priced digital filter can be applied. As a CD player reads information from the disc, the information is converted from its digital form ("1's" and "0's") to its analogue form (a series of electrical current values). This conversion, called quantisation, causes noise. So before sending the signal to the loudspeakers, it is necessary to remove the quantisation noise with a filter. However, some types of filters affect the quality of the music reproduction.
The brick-wall filters employed in many first-generation CD players cut off any information above a certain point, 24 kHz. Unfortunately, these filters were themselves found to cause ringing, a type of distortion. Ringing is actually a remainder of the sampling frequency in the output signal. Ringing causes the stereo image to become unclear.
The technology of oversampling virtually eliminates this problem through ultrasonic noise reduction. If the problem of ringing with brick-wall filters lies in their proximity to the audible range, why not move the filter further away from the audible range? By oversampling the digital data four times (at 176.4 kHz instead of 44.1 kHz), the quantisation noise is moved further away from the audible range. The noise is then removed by an analogue filter that also eliminates ringing. The rough analogue wave form of simple sampling is refined by oversampling.
Fig. 23-13.
By oversampling the digital data four times (at 176 kHz instead of 44.1 kHz), the quantisation noise is moved further away from the audible range. The noise is then removed by an analogue filter.
In practice, 2-time oversampling works very well, as many CD players prove. Even better is 4-time oversampling which most higher-quality CD players have. The more expensive models aim at 8-time or even 16-time oversampling, to attain even further refinement. Some CD players even have a 256-time oversampling system.
Fig. 23-14. The rough analogue waveform of simple sampling is refined by oversampling.
Higher oversampling levels are not, however, always better. Sampling systems which make use of higher rates can actually degrade the quality of music produced, since many DAC’s cannot "settle" from one input value to the next. Converters can only process a certain amount of information in a given period of time. Beyond a certain point, the faster the process is attempted, the more errors are introduced. Imagine that a person is told to compute the equation 2 + 2 in ten seconds. This is a simple matter. But if the same person must calculate a series of 20 equations in the same ten seconds, the chance of error is much greater. So it is with the hundreds of thousands of calculations which the DAC must perform. Beyond a certain point, sound quality is diminished rather than increased.
The digital signal transmitted by the CD contains the complete stereo information, both the left (L) and right (R) channels. This information is passed on alternately. After oversampling, the digital signal reaches the DAC, which converts it into an analogue signal. If only one DAC is applied, as is the case with many simple CD players, the signal is very quickly switched from L to R behind the DAC. This system appears to work well at first sight, but it results in a small time difference between L and R. This small time delay leads to phase shifts. Human ears are very sensitive to these differences, however small they may be. Stereo information is located precisely in these phase relations of the high tones. Through faults in the conversion procedure, a considerable part of the real depth of the stereo image is lost.
If, on the other hand, two DACs are applied, things will be quite different. At playback, the same phase relation as was recorded by the microphones will be preserved exactly. Then the playback will have real spaciousness, depth and complete naturalness. A double DAC is an essential feature of a good CD player.
Left and Right signals do not reach the DACs at the same time. By inserting a delay circuit in one of the two channels, the delay of the second signal is perfectly counterbalanced. Because this is done in the digital section, the approach is the same for all frequencies, so that higher frequencies are never delayed more than the lower frequencies.
Fig. 23-15. A double DAC is an essential feature of a good CD player.
Behind the single or double DAC there is a single or double analogue filter, as we saw above. We have the intact sound signal at our disposal, after it passes through this filter. We only have to connect the CD output to the AUX or CD input of the stereo amplifier.
23.2.3 CD Mechanics
Electronics is such an important part of the CD player, that the mechanical part is easy to forget. Nevertheless, the mechanical part is very important. Mechanical speed fluctuations such as wow and flutter do not occur with Philips CD players, because our CD players first store the signal in memory for a fraction of a second, and then read it with quartz precision.
The reproduction of the music is more natural as the CD is more stable and free of vibration. The tray construction should not be too light, but should be made of either metal or firm plastic, because the smallest vibration of the CD puts the pits out of focus, and the error correction must come into action.
23.2.4 CD Features
In addition to producing quality sound, the capabilities of a CD player also extend to displaying information about the track, index and time (elapsed/left) in the form of Control and Display (C & D) bits. There are also all sorts of search, programming and repetition possibilities and random playback, which plays back the tracks of the CD in random order.
Possibilities for digital communication between a CD player and a cassette deck connected to the same Hi-Fi set are increasing. This communication makes dubbing from CD to cassette automatically perfect, with the option of automatic fade-in and fade-out. With Record sync, for instance, the cassette deck begins recording automatically as soon as the CD music starts. The Edit system determines as accurately as possible how many tracks of the disc can fit into the available recording time of the cassette. The tape length (C60, C90, etc.) of the cassette is entered into the CD player memory. If at any point in the recording, the next track is too long to be recorded completely, the CD player automatically switches to pause, so that the cassette can be reversed. There are also Edit systems that arrange the tracks in such a way that the available tape length is optimally used, without having to break off any track.
Remote control is increasingly gaining popularity, and CD changers are available which allow continuous playback for hours. CD changers also help to solve the problem of the in-car CD player vibrating. A CD changer system allows the set to be mounted in the boot of the car, a more stable position than the dashboard. It also helps to prevent theft.
The connection to the amplifier is mostly analogue, simply by a cinch cable. It is advisable to use a high-quality cable, and a suitable cable is commonly supplied with the CD player. Better CD players have a digital output that can be used to record the CD digitally with a Digital Audio Tape (DAT) or a Digital Compact Cassette (DCC) recorder. Because the record industry demanded that their recordings should be copy-protected, such DAT or DCC copies can only be made once. The DAT recording cannot therefore be further copied digitally.
CD drive uses the digital output to connect the digital signal to a high- quality amplifier with built-in, high-quality DACs. Some of the better CD players are equipped with opto coupling, which transmits the signal optically to the amplifier.
