Abstract
Each block is a fixed bit layout derived from (link mode, layout). It holds an 8-bit descriptor, then each mix’s codec resync field (5 bits for a stereo mix in the default mode, none otherwise), then every mix’s codec bits for four 4-sample groups, then per-mix parity, then an 8-bit keyed tag. Protection is unequal: in the default mode a shortened binary BCH code per mix covers only the descriptor, the tag, the lowest-frequency sub-band codes of both channels and the mix’s resync field; the other bands go unprotected. The tag is computed over the descriptor, the protected bits and the resync fields, then the BCH code is computed over the descriptor, the tag, the protected bits and the field, then everything is XORed with a keystream indexed by the block’s period number and the transmitter’s salt. The receiver removes the keystream, corrects every mix’s codeword, and only then checks the tag, so a hit the FEC repairs does not fail it. After any failed block the receiver re-arms: the next block that passes is still not played and only the one after it plays. A random block therefore reaches the decoder only by passing a 1-in-256 test twice in a row. Modes without correction carry two parity bits per group per mix and tag the descriptor and those parity bits.
Creative Commons License
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Recommended Citation
Anonymous, "A 38-byte audio block with unequal BCH protection, a codec state field inside the error-correcting codeword, period-indexed keystream, a one-byte keyed tag checked after correction, re-arm gating, and in-band control bits (second version)", Technical Disclosure Commons, ()
https://www.tdcommons.org/dpubs_series/12157