poggio@apple.com (Andy Poggio) (03/03/90)
CD Summary Part 4 CD-ROM Data Tracks Each CD-ROM data track is divided into individually addressable blocks of 2352 data bytes, i.e. one subcoding block or 98 frames. A header in each block contains the block address and the mode of the block. The block address is identical to the encoding of minute, second, and frame number in subcode channel Q. The modes defined in the CD-ROM specification are: Mode 0 -- all data bytes are zero. Mode 1 -- (CD-ROM Data): Sync Field - 12 bytes Header Field - 4 User Data Field - 2048 Error Detection Code - 4 Reserved - 8 Error Correction - 276 Mode 2 -- (CD Audio or Other Data): Sync Field - 12 bytes Header Field - 4 User Data Field - 2048 Auxiliary Data Field - 288 Thus, mode 1 defines separately addressable, physical 2K byte data blocks making CD-ROM look at this level very similar to other digital mass storage devices. Second Level Error Correction An uncorrected error in audio data typically results in a brief, often inaudible click during listening at worst. An uncorrected error in other kinds of data, for example program code, may render a CD unusable. For this reason, CD-ROM defines a second level of error detection and error correction (EDC/ECC) for mode 1 data. The information for the EDC/ECC occupies most of the auxiliary data field. The error detection code is a cyclic redundancy check (CRC) on the sync, header, and user data. It occupies the first four bytes of the auxiliary data field and provides a very high probability that uncorrected errors will be detected. The error correction code is essentially the same as the first level error correction in that interleaving and Reed-Solomon coding are used. It occupies the final 276 bytes of the auxiliary data field. Editorial: This extra level of error correction for CD-ROM blocks is one of the many reasons that CD-ROM drives are much more expensive than consumer audio players. To perform this error correction quickly requires substantial extra computing power (sometimes a dedicated microprocessor) in the drive. This is also one reason that consumer players like the Magnavoxes which claim to be CD-ROM compatible (with their digital output jack on the back) are useless for that purpose. They have no way of dealing with the CD-ROM error correction. They also have no way for a computer to tell them where to seek. Another reason that CD-ROM drives are more expensive is that they are built to be a computer peripheral rather than a consumer device, i.e. like a combination race car/truck rather than a family sedan. One story, probably apocryphal but not far from the truth, has it that a major Japanese manufacturer tested some consumer audio players to simulate computer use: they made them seek (move the optical head) from the inside of the CD to the outside and back again. These are called maximum seeks. The story says they managed to do this for about 24 hours before they broke down. A CD-ROM drive needs to be several orders of magnitude more robust. Fast and strong don't come cheap. --andy
allen@b11.ingr.com (John Allen) (03/06/90)
I didn't see a copy of CD summary 3, please re-post..
Thanks JEA
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