Het PASC geluidscompressie systeem dat Philips ontwikkelde voor DCC past een compressie toe van ongeveer 1:3. Dat betekent dus een forse reductie van de data. Door toepassing van verschillende sporen op het magnetische bandoppervlak kan op deze manier gemakkelijk 90 minuten digitale audio op een DCC worden opgeslagen. DCC kan digitale signalen op 32 kHz, 44.1 kHz of 48 kHz opslaan (vergelijkbaar met DAT). Het materiaal waarvan het 'tape' in de beschermde DCC cassette's is gemaakt is ofwel Crome Dioxide of Cobalt-dropped Ferric Oxide, vergelijkbaar met videotapes.
PASC (Precision Adaptive Subband Coding)
It is a lossy compression method for audio, i.e. some information that you can't hear anyway is lost in the compression. The trick for compression schemes like PASC is to keep the parts that you hear and drop the parts that you don't. DCC uses the PASC system to compress the audio information so that it will fit on an audiotape and produce CD sound quality. PASC concentrates on maximising the efficiency of the digital coding, by taking into account two factors not previously considered in digital audio:
The ear hears only sounds above a certain loudness (dB) level, called the hearing threshold. The threshold of hearing depends on the frequency of the sound (since the ear is more sensitive to mid- range frequencies) and on the individual. Consequently, it is only necessary to record sound above the hearing threshold, provided that the threshold is taken as the reference for both recording and playback.
Louder sounds hide (mask) softer sounds. A whisper, perfectly audible in a quiet room, will not be heard in a busy street. In fact, louder sounds dynamically adjust the threshold of hearing. With computer techniques, it is possible to track this threshold adjustment, making it necessary for only the sounds above this dynamic threshold to be recorded. Of course, this applies to both recording and playback.
PASC achieves very efficient sound recording indeed. It needs only one quarter of the bit rate of PCM (of CD). This level of efficiency creates adequate room for precise recording of what the ear actually hears. The sound quality of DCC is in every way comparable with Compact Disc.
Tracks and tape frames
DCC signals are recorded on nine parallel tracks on the cassette tape. Eight "Main Data" tracks contain all the PASC data, error correction data and system information. The ninth, "Auxiliary Data" track holds mainly track and time information, similar to compact disc, with extra tape markers for easier operation. Start markers, for example, make track access easy, while reverse markers are used to initiate auto reverse. The auxiliary data can be scanned during high-speed search, making operation faster and more straightforward.
All the DCC data on tape is grouped into self-contained tape frames, separated by InterFrame Gaps (IFGs). To accommodate small deviations in the sampling frequency during recording, IFGs can vary slightly in length. They also help to locate the start points of the tape frames. Each DCC tape frame contains 12,288 bytes of information (not including synchronisation). This is composed of: 8,192 bytes of PASC data, 128 bytes of system information (data for text-mode displays and information such as copyright and tape type), and 3968 bytes of error detection and correction information.
The PASC data is spread across the tape frame in a checkerboard pattern which stops drop- outs (missing signal on the tape due to damage of the magnetic layer), influencing the quality of the audio performance. Even large drop-outs will not impair sound quality (see figure 7.6). This can be compared to interleaving used in CD players, which compensates for any interruptions to the signal caused, for example, by dirt or grease.
A Cross Interleaved Reed Solomon Code (CIRC) protects the main data against random and burst errors. The two layers of CIRC data are spread across the eight main data tracks. This powerful error correction code allows for correction of drop-outs even up to 1.45mm in diameter. It can even compensate for a drop-out bigger than a completely missing data track.
In support of the revolutionary PASC, all the techniques which have made compact disc synonymous with audio excellence are applied to DCC. All are closely integrated, and optimised for the tape medium. They are fundamental to the extreme reliability and quality of this digital audio system.
In audio terminology, azimuth is the position and angle of the recording or playback head in relation to the tape. Azimuth alignment is the position of the head gap in relation to the position and direction of the tape. Azimuth difference is a slight discrepancy between the position of the recording head gap and the position of the playback head gap. Azimuth error refers to problems in playback that arise because of azimuth differences. If the azimuth is not adjusted well, the head will not be in the best position to read the information on the tape and the sound will be negatively affected.
DCC has incorporated an important advance to ensure azimuth alignment and prevent azimuth differences and errors: Azimuth Locking Pins (ALPs). In conjunction with the Fixed Azimuth Tape Guidance (FATG) mechanism fitted to the head assembly, the ALPs ensure not only improved wrap-around tape- to-head contact (see left inset of figure 7), but also consistent azimuth alignment (see right inset of figure 7).
The ALPs improve the wrap-around angle of the tape against the head. This extends the tape-head contact area and optimises the physical conditions for signal recording and reading. The exclusion of gaps in the head mechanism means less friction and so less wear on the tape (which will therefore last longer). The tape is also stiffened in this crucial tape guidance area, and this contributes to the high accuracy of the FATG mechanism.
In the FATG mechanism, special slots are mounted either side of the head assembly. The two top edges of the slots are reference surfaces to align the tape with the head. Meanwhile, the sloping profiles of the lower ports of the slots gently force the stiffened tape upwards against both reference surfaces. This simple device eliminates azimuth error.
The ALPs/FATG design requires no complicated mechanisms or close tolerances. Its very simplicity ensures permanently accurate tape-head alignment.
The DCC sound signal is recorded on eight parallel tracks, each 185 Ám wide. The track width required for playback, on the other hand, is only 70 Ám wide. This width factor helps to reduce the sensitivity to azimuth error. An additional track carries control and display subcode information.
To achieve these miniature dimensions, the DCC record/playback head assembly calls on the advanced thin-film head technology already well proven in multichannel professional recording. In one single head element, three sets of head elements are combined:
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