What is PASC? How does it work?

PASC is an abbreviation for 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. See the Philips DCC page for a detailed description of how PASC works. Experiments with files recorded with a DCC-175 have shown that PASC is identical to MPEG-1 layer 1, but PASC uses a rare (and not very well-documented) feature of the ISO11172-3 (MPEG Audio) standard: padding. Padding is necessary to make sure that the bitrate of the data stream on tape is exactly 384kbps, and it is only necessary for MPEG1L1 compression of 44.1kHz audio: it ensures that each block of data is exactly 420 bytes on tape, even when only 416 bytes are in use for data. Unfortunately many software MPEG decoders don’t support the padding bit in the MPEG header and choke on the files that the DCC-Studio produces. The good news is that it’s easy to remove the padding and make the .MPP files decodeable and playable on all MPEG players. If you use Windows 3.1, you can find it here, if you use Windows 95 or later you can find it here. Note that some MPEG players comply better to the ISO 11172 standard than others: it may not even be necessary to convert your files.

How does PASC affect sound quality?

A lot of people are worried that PASC degrades sound quality because it removes parts of the audio. Also, there are people who claim they can hear the difference between the original CD and a digital copy on DCC. I don't know if that is possible - I never met anyone who could.

How much does PASC degrade sound quality after several generations of copying?

PASC was designed with one generation of compression in mind. When a DCC is copied digitally to another DCC (which is in most cases prevented by SCMS), the PASC signal is decoded to PCM (Pulse Code Modulation - a direct real-time binary representation of the wave form of the audio) by the player and encoded back to PASC by the second recorder. In theory, the compression algorithm is the exact reverse operation of the decompression algorithm. Hence, the second-generation recorder would find that there is nothing to remove from the input signal (it was already taken out in the first generation) and the PASC data of the second generation (and beyond) should be exactly the same as the first tape. However it is not clear if this is indeed the case.

Does PASC degrade sound quality when I use my DCC recorder as A/D converter or D/A converter?

Studies of the SAA3323 service documentation (the SAA3323 is a chip used in second-generation DCC recorders and decks) have revealed that the PASC compression and decompression only happens when the digital signal is transferred to or from tape. You can use your DCC recorder as Digital-Analog converter or as Analog-Digital converter without fear for degradation in quality.

Last updated: 25-Nov-2003.
Author: Jac Goudsmit (jacg (a)
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