[pulseaudio-discuss] review+pull-request: Passthrough support

[pl bossart]

I finally managed to figure out why eac3 wouldn't work in passthrough
mode. Apparently my HDMI receiver will only lock on eac3 data if the
AES0 control is set to 0x06 (non-audio, ie iec61937).

ffmpeg -acodec copy -i csi_miami_5.1_256_spx.eac3 -f spdif outfile.pcm
aplay -Dhdmi:AES0=0x6 -fs16 -c2 -r192000 outfile.pcm

the same file plays well through pulseaudio if I modifed default.conf to
[Mapping hdmi-stereo]
device-strings = hdmi:%f,AES0=0x06
channel-map = left,right
priority = 4
direction = output

Somehow we need to find a way to set this AES0 byte when opening the
iec958 or hdmi device in the passthrough code, and reset it back to
0x04 for PCM playback (should be fairly easy to do by looking at the
code of iecset in alsa-utils). Beats me why this was not needed for
plain ac3, but setting the audio bit is actually the correct thing to
do.

So far all the tests seem ok with the ffmpeg examples, with the
exception of two files:
- 7_pt_1: works well with ffmpeg, aplay but the sound is way too fast
with pulseaudio. Lots of rewind messages seen in PulseAudio.
- serenity_english_5.1_1536.eac3 file: no sound out, and again lots of
rewind messages (the receiver still shows the D+ logo though). The
same file converted with ffmpeg and played with aplay seems fine.
looks like a buffering issue more than a payloader problem really.

-Pierre