Abstract
Each block is a fixed bit layout derived from (link mode, layout). It holds an 8-bit descriptor, every mix’s codec bits for four 4-sample groups, per-mix codec resync fields, per-mix parity, and 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 and the lowest-frequency sub-band codes of both channels; the other bands go unprotected. The tag is computed over the descriptor and the protected bits, then the BCH code is computed over the descriptor, the tag and the protected bits, 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 concealed 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. A block whose descriptor names another plan is re-checked under that plan’s layout and reported as a plan change only after two consecutive authenticated foreign blocks.
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Recommended Citation
Anonymous, "A 38-byte audio block with unequal BCH protection, period-indexed keystream, a one-byte keyed tag checked after correction, re-arm gating, and control blocks sent in place of audio", Technical Disclosure Commons, ()
https://www.tdcommons.org/dpubs_series/12129