[pulseaudio-discuss] "passthrough" audio (eg. AC3 / DTS / WM9)

pl bossart bossart.nospam at gmail.com
Mon Oct 5 14:06:24 PDT 2009

> If you need ac3 pass-thru then you need to bypass PA. Just make sure
> you are not using the SPDIF port for PA (use g-v-c or pavucontrol and
> make sure the sound card is notconfigured for any of the 'digital
> iec985' modes).

If you use AC3 pass-thru, what you are really sending over the SPDIF
interface is a PCM stream. The AC3 content is formatted to follow the
recommendations of IEC 61937: the compressed content uses 16 of the
24-bit words, there are additional headers and lots of padding with
zeroes. In terms of timing, this is really PCM. You may need to set a
number of control bits in the PCM interface but nothing serious. As
long as you don't mix and don't modify the bitstream, you could handle
pass-thru w/ PulseAudio without major modifications.
Indeed things are completely different when the content is not
reformatted to meet the PCM bitrate required by SPDIF (BT, MP3, etc).
- Pierre

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