Alexandre Ratchov: > > I would, but there shouldn't be anything required to make it work. > > The point of AC3/DTS pass-through mode is that it sends what looks > > like 16-bit stereo audio at the S/PDIF level. It's only the receiver > > that, on seeing a signature bit pattern, re-interprets the data and > > decodes it according to AC3 or DTS. > > So, I don't even understand how AC3 would work at all. I mean, a lot > of programs (ie all using sio_onmove(3)) expect that one second of > s16le at 48kHz be exactly 192000 bytes of data. If AC3 is used, that > wouldn't be true anymore (AC3 is 320kbit/s iirc) and audio would be > broken.
Padding, of course. A video player like MPlayer will format the AC3 or DTS audio (both of which support numerous bit rates) in such a way that the result is a 192000 byte/s stream. The data on a DTS CD is already preformatted to 176400 byte/s and just needs to be passed through. I put the disk in, "cdio cdplay", and the drive, cdio(1), and azalia(4) all think they are dealing with 44.1 kHz 16-bit stereo audio when in fact it is 6-channel DTS. As designed, S/PDIF only transmits 20(!)-bit stereo audio at various sample rates. However, ISO/IEC 61937 specifies how to carry various non-PCM audio formats over S/PDIF by formatting them like a 16-bit stereo stream. This is widely implemented for AC3 and DTS, rarely for MPEG audio. > And this can't be fixed without adding AC3 specific code. It is handled at the application level. Check out libmpcodecs/ad_hwac3.c in the MPlayer source. There is nothing OpenBSD-specific about this. It's the same for FreeBSD, Linux, and probably others. > > We don't use any "AC3 mode" for azalia(4) either and pass-through > > works there. > > I bet it only works in few special cases, probably because the clock > is not involved And you will lose that bet. There is no theoretical reason why it shouldn't work, and in practice it does with everything I've thrown at it: * DVD audio tracks, AC3 (2.0, 4.1, 5.1; 192/224/384/448 kbit/s) and DTS (5.1; 768 and 1536 kbit/s) * DTS CD (44.1 kHz) * AC3 from digital television (2.0 and 5.1; 192/256/384/448 kbit/s) * DTS extracted from Blu-ray (at the maximum 1536 kbit/s) -- Christian "naddy" Weisgerber na...@mips.inka.de
