Hi,
I wonder if the pcm_iec958.c correctly sets preambles for multichannel
PCM/LPCM. It looks as if coded only for stereo.
IIUC the X/Y preambles should alternate for even/odd channels.
This check
https://github.com/alsa-project/alsa-lib/blob/master/src/pcm/pcm_iec958.c#L122
:
if (channel)
data |= iec->preamble[PREAMBLE_Y]; /* odd sub frame, 'Y' */
else if (! iec->counter)
data |= iec->preamble[PREAMBLE_Z]; /* Block start, 'Z' */
else
data |= iec->preamble[PREAMBLE_X]; /* even sub frame, 'X' */
IMO the condition should be "if (channel % 2)", because for multichannel
only the first even channel ch0 gets Y, and the remaining even channels
ch2, ch4, ... get X. This is confirmed by analyzing the generated stream
captured with the file plugin.
Also I am not sure if counting the subframes for Z preamble in
https://github.com/alsa-project/alsa-lib/blob/master/src/pcm/pcm_iec958.c#L182
respects the multichannel. I do not know what "parallel IEC958 streams"
mean, how the Z preamble should be placed for PCM multichannel.
A) Either it can count all channel pairs sequentially up to 192, i.e.
preamble Z after 192 pair of (ch0, ch1), (ch2, ch3),...
If so, IMO the incrementation in
https://github.com/alsa-project/alsa-lib/blob/master/src/pcm/pcm_iec958.c#L229
should increment by channels/2, not by 1 (which fits for channels=2)
B) Or it can count each channel pair separately, i.e. 4 preables Z (for
8ch) in a row after 192 pairs of (ch0+ch1), 192 pairs of (ch2+ch3), etc.
If so, IMO the counter should be reset after looping for all channel
frames also for PCM, not only for nonaudio/single_stream
https://github.com/alsa-project/alsa-lib/blob/master/src/pcm/pcm_iec958.c#L232-L233
The reason I am looking at this code are the problems with channel swaps
at xrun handling of RPi HDMI
https://forums.raspberrypi.com/viewtopic.php?p=2187582#p2187540. I
suspect the iec958 plugin does not generate correct multichannel IEC958
stream which confuses the HDMI receiver at the stream recovery.
Thanks a lot,
Pavel.
