[flac-dev] FLAC frame boundaries and protocol

Miroslav Lichvar
Wed, 06 Mar 2019 10:56:54 -0000

On Tue, Mar 05, 2019 at 10:26:54PM -0800, Brian Willoughby
wrote:
> Frames start with a 14-bit sync code, which is 13 “one" bits and 1
“zero" bit. Subframes start with a 1-bit padding of “zero." Keep in
mind that FLAC is a bit stream, not a byte stream, so that 14-bit frame sync can
happen anywhere in a pair of bytes. You can’t simply scan memory bytes for a
frame sync, at least not unless you allow for 8 variations, apply bit masks, and
allow for non-word-address-aligned matches.

The specification says frames are aligned to bytes:

	Following the frame header are encoded subframes, one for each
	channel, and finally, the frame is zero-padded to a byte boundary.

The problem with finding frame boundaries is that the sync code and
the 8-bit CRC are too short, so the whole frame needs to be parsed
and the 16-bit CRC checked to be reasonably sure it really is a FLAC
frame.

--
Miroslav Lichvar