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Index:
The CD is the format that has brought digital quality music to the masses, but it was never designed to be a recordable media. What alternatives exist for users to record music digitally?
For starters, a recordable version of the CD is available which is something of a patch to the original design, but does allow for users to write their own CD’s that can be read by normal CD players. The writeable CD’s that exist are WORM (write once read many) devices that have a layer of material whose optical properties change when heated by a powerful write laser (usually a dye becomes transparent when heated). Once ‘burnt in’ the CD’s cannot be rewritten. The technology is quite expensive, although it is becoming cheaper as it becomes more popular. The CD’s themselves are quite expensive also at between 5 and 10 pounds, which is very effective for CD-ROM uses, but not much use for audio applications. There have also been questions of the reliability of the media which has been said to lose data if left in the sun, although this is little more than rumour.
Probably one of the first commercial standards to appear was the DAT (Digital audio
tape). This system has never really designed for the home user but was aimed more at the
professional and semi-professional market. The system does enable people recording
digital music in their own homes to record at very high quality, although bigger studios
are more likely to use higher resolution digital tapes. The system uses magnetic tape with
a very slow linear speed and a helical scan head, in much the same way as a video player.
In a conventional cassette the tape itself remains inside the case and the head is pressed
against it. In a DAT (and a video) the tape is pulled out and fed over the head (which is
contained inside a spinning drum) and held in with rollers. Because the head scans across
diagonally more information can be fitted into one track in one go on the tape. DAT is a
very flexible standard and allows sampling frequencies of 48KHz (record & playback,
high quality), 44.1KHz (same as CD - but for playback only) and 32KHz (for long play
mode to fit more time onto the tape, play or record). It also has two different quatization
systems - 16 bit linear quantization (same as a CD player for high quality recordings) and
12 bit non-linear (less detail for long play mode, but non-linear quantization provides
more resolution at levels where the ear tends to be more sensitive). DAT uses PCM
(pulse code modulation - the same as a CD player) which is the simplest form of putting
digital data onto media - it basically means that the data is directly placed on the media
with a pulse representing a one and a gap a zero. DAT uses a powerful double Reed-
Solomon error correction code which ensures that the playback is of very high quality.
Recently two new products were launched to give consumers recordable digital media at an affordable price. They aimed to improve upon the current technologies but the two competitors, Phillips and Sony, went about it in different ways.
Sony developed the MiniDisc - in some ways an improvement upon the design of the CD, although they do differ in quite fundamental ways. It is physically much smaller than the CD at only 6.4cm diameter although still retains a playing time of 74 mins using a number of clever tricks. The system used for modulation is called ATRAC (Adaptive transform acoustic coding) which reduces the bit rate of the 16 bit 44.1KHz input signal (same specification as for CD player) from 1.4Mbs to 292KBps. This system starts by using pyscho-acoustic coding to only record the noises the human ear can detect. It does this in two ways
The MiniDisc provides a number of additional features for the consumer. It has a User TOC (table of contents) area which allows the user to program in track names and track information. It also has a special memory buffer of three seconds to ensure uninterrupted play. This is possible because the data is read from the disc at 1.4MBps whereas the data rate is only 292KBps, so it reads faster than it needs to.
The MiniDisc employs the use of magneto optical technology which allows the user to rerecord on the media up to a million times. It works by having a layer of material which changes it'’s polarisation properties when magnetised (you can magnetise the material to North or South, representing a zero or a one). To write to the disc a powerful write laser heats up the material to 185 degrees and it’s polarity is then altered with a magnet. To read from the disc, laser beam has linearly polarised light, so using a prism to split the reflected beam into two, you can analyse the two beams (left and right) to see which way the beam was refracted and hence whether the information was a zero or one. Playback only MiniDiscs also exist that are manufactured using pits in the same way as CD’s. These can be read using the same laser but with some more decoding circuitry. This is possible because when the beam is reflected the two angles (left and right) will be the same because the polarisation isn'’t changed, so the two beams added together will be high if a one and low if a zero.
The MiniDisc uses an advanced version of the CD’s error correction code CIRC (cross interleaved reed-Solomon code). Because the MiniDisc is a disc the media doesn’t come into physical contact with the read head at any time, and because the design also includes a protective case similar to a floppy disc the media is very resistant to wear and tear.
Phillips produced the DCC (Digital Compact Cassette) which is effectively a digital update of the most popular music format in the world - the CC (compact cassette) and was specifically designed to be backwardly compatible with it’'s predecessor. It uses a clever head which has 9 tracks for digital recording and playback on one side and 2 tracks for normal analogue recording and playback on the other side. So, the digital tracks are on the other side of the tape from the analogue data. The system uses the same tape speed, width and case size as the traditional CC, but because more data must be crammed onto the tape, traditional analogue ferric tapes don’t provide enough resolution. However, because 9 tracks are recorded in parallel with individual bit rates of 96KBps (rather than the one track with a bit rate of up to 2.46MBps as in DAT) the tape used is quite simple and is the same material as used for video cassettes.
DCC uses PASC (precision adaptation subband coding) to modulate the data to put it onto the tape. This reduces the data bit rate by roughly a quarter to 384KBps. Because the system is dynamic it can obtain resolution equivalent to 19 bits of linearly quantified audio. The DCC format can operate at 48KHz (as DAT), 44.1KHz (as CD) or 32KHz (as long playing DAT). PASM uses the same psycho-acoustic tricks as MiniDisc of hearing threshold and masking of quiet sounds. What it does with the data after that is somewhat different though. It divides the data into 32 subbands of equal width (to make filters simpler). PASC analyses each subband to see which band of frequencies contains the loudest noises and then allocates the most number of bits to that band. Levels are given relatively to the threshold value, which is worked out separately for each frame (set of 12 samples) After error correction codes and 8/10 modulation (a coding technique to allow for the fact that tracks on magnetic media interfere with each other) the final bit rate to tape is 768KBps which is recorded in the 8 96KBps data tracks. This is a fixed bit rate, so if lower sampling frequencies are used the dynamic PASC system has more bits to allocate and so gives better resolution. A ninth track is used to record subcode information, such as block address, track name and artist information (in 7 different languages!)
The DCC uses a CIRC error correction code (same as the CD) but also interleaves the data on the tape in a chequerboard pattern so as to effectively turn burst errors into random errors. As with the DAT the DCC is a little more susceptible to wear and tear as the head has a large amount of contact with the media, although the design is an improvement upon the CC as there is a cover for the tape which also locks the reels when closed, stopping the tape from slackening or being chewed.
| Because it is very easy to make perfect clone copies of digital media a system was developed called SCMS (serial copy management system) which is used on CD’s, DAT’s, MiniDisc and DCC With this system a recording is in one of three possible states:
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DAT and other similar techniques have found a niche in recording studios and home music production as they give high quality at a relatively low cost. Looking through current hi-fi catalogues it is clear that even though these products have been on the market for some time, none of them have really taken off in the consumer market. If anything, MiniDisc does seem to be growing in popularity and is doing better than DCC. Perhaps these new standards don’'t offer enough of an advantage for users to upgrade their equipment and their collections once again as many people have already done from LP to CD. We will probably have to wait for the development of a re-recordable CD or perhaps a magneto optical version of the new DVD standard before recordable digital media becomes commonplace in the home.
Burst error : where a sequence of bits are corrupted - e.g. a long scratch in a CD
Random error : where individual bits are corrupted - e.g., gaussian noise in a transmission
channel.