[alsa-devel] How to handle HBR pass-through? (new PCM_FORMAT?)

Anssi Hannula anssi.hannula at iki.fi
Mon Aug 2 09:40:17 CEST 2010

pl bossart kirjoitti maanantai, 2. elokuuta 2010 06:18:35:
> > What this means is that if just the needed HBR bits are set in the audio
> > driver, one can play back e.g. TrueHD by passing the IEC-61937 stream to
> > ALSA as 192kHz 8-channel 16-bit PCM data; of course normal 8-channel PCM
> > playback wouldn't work anymore then. So the driver would need to know if
> > this is 8- channel PCM or 8-channel compressed audio.
> > 
> > Which brings us to my question: How should the user be able to do this,
> > i.e. inform the driver whether this is 8-channel PCM data or compressed
> > audio data?
> > 
> > 
> > One easy option would be to have a separate IEC61937_HBR format in ALSA
> > which could be used for IEC-61937 streams represented as 8x 16-bit
> > channels.
> > 
> > If that would be considered too specific, we could add a more generic
> > IEC61937 format, which is the same as above except that "normal" ( = "2
> > channel") IEC-61937 streams could be used as well. However, as normal
> > IEC-61937 streams can be used with S/PDIF as well, we'd AFAICS need to
> > add such format bit to all S/PDIF drivers or have a no-op converter to
> > the more widely supported 16- bit PCM format.
> Could we require that the user sets the channel status bits (with the
> AESx controls) in case this is compressed. The driver would then make
> an informed decision to use HBR packets instead of regular PCM 8-ch
> layout2?

Indeed, the non-audio flag (AES0 & 0x02) could be used for this.

It seems the only always-available way to set those is as parameters in the 
device string, correct?

Will it always be reset after the user closes the device?

> At any rate, supporting such formats entails changes to higher levels
> of the audio stack, for example gstreamer/alsasink would need
> additional caps. Changes are not limited to the alsa driver I guess.

We are in the process of adding support in ffmpeg for the TrueHD-in-IEC61937 
muxing and after that I'll add support into XBMC for using that functionality.

Looked at gstreamer, it seems to only have ac3-in-iec61937 support currently. 
However, I'd guess gstreamer would additionally need some ways to 1) detect if 
HBR is supported, and 2) detect if the connected receiver supports the 
specific codec..

Anssi Hannula

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