Re: [flac-dev] FLAC frame boundaries and protocol
Thanks, guys. Your answers helped and I think I have some ideas now on what to do. On Wednesday, March 6, 2019 Miroslav Lichvar wrote: 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 ___ flac-dev mailing list flac-dev@xiph.org http://lists.xiph.org/mailman/listinfo/flac-dev
Re: [flac-dev] FLAC frame boundaries and protocol
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 ___ flac-dev mailing list flac-dev@xiph.org http://lists.xiph.org/mailman/listinfo/flac-dev
Re: [flac-dev] FLAC frame boundaries and protocol
Hello Dave. 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. Whenever I have a question about the FLAC format, I always start by typing http://flac.sourceforge.net/ into my browser, which redirects to https://xiph.org/flac/ these days. There part you’re looking for is here: https://xiph.org/flac/format.html#blocking As for reference code, I recommend using the FLAC library. In other words, include the headers and link to the library, calling the API as it was designed. Don’t write your own code, that’s just asking for mistakes. I have not used the FLAC++ library, so I don’t know whether it has any peculiar issues. I’ve always used the Standard C FLAC library, and never have any trouble. You do have to pay attention to the sample format, particularly the bit justification for larger samples like 24-bit audio. I remember being interested in streaming, as opposed to file-based compression, but I can’t recall whether I ever finished that project. Brian Willoughby On Mar 5, 2019, at 10:36 AM, Dave Harris wrote: > Hello, > > I've set up and have been reading through the FLAC reference implementation > source code on Windows and stepping through it in the debugger. I've been > trying to understand how the protocol knows where the subframe and frame > boundaries are. Is there a good tutorial that discusses the ins and outs of > the flac protocol? Also, is there a piece of the reference code that shows > how frame and subframe boundaries are calculated? > > Thanks. > ___ flac-dev mailing list flac-dev@xiph.org http://lists.xiph.org/mailman/listinfo/flac-dev
[flac-dev] FLAC frame boundaries and protocol
Hello, I've set up and have been reading through the FLAC reference implementation source code on Windows and stepping through it in the debugger. I've been trying to understand how the protocol knows where the subframe and frame boundaries are. Is there a good tutorial that discusses the ins and outs of the flac protocol? Also, is there a piece of the reference code that shows how frame and subframe boundaries are calculated? Thanks. ___ flac-dev mailing list flac-dev@xiph.org http://lists.xiph.org/mailman/listinfo/flac-dev
