Re: ALSA-LIB: correct IEC958 preambles for PCM multichannel?

[Pavel Hofman <pavel.hofman@xxxxxxxxxxx>]

On 11. 02. 24 15:12, Pavel Hofman wrote:
> 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.

Hi, IMO the iec958 multichannel functionality could be fixed with

===================================================================
diff --git a/src/pcm/pcm_iec958.c b/src/pcm/pcm_iec958.c
--- a/src/pcm/pcm_iec958.c	(revision 
7e5de6fadd394cbbdd3a8d5d7d15081185b4d774)
+++ b/src/pcm/pcm_iec958.c	(date 1707745970448)
@@ -119,7 +119,7 @@
 		data |= 0x80000000;

 	/* Preamble */
-	if (channel)
+	if (channel % 2)
 		data |= iec->preamble[PREAMBLE_Y];	/* odd sub frame, 'Y' */
 	else if (! iec->counter)
 		data |= iec->preamble[PREAMBLE_Z];	/* Block start, 'Z' */


Test:

pcm.testiec {
  type iec958
  slave {
    pcm out
    format IEC958_SUBFRAME_LE
  }
}

pcm.out {
  type file
  file "/tmp/out.raw"


ALSA_PLUGIN_DIR=/usr/lib/x86_64-linux-gnu/alsa-lib/ 
LD_PRELOAD=~/personal-work/alsa-lib/src/.libs/libasound.so  aplay -v -D 
testiec -c 8 -f S32_LE  /dev/zero

Then it produces the format as in the attached screenshots from mvciew, 
i.e. IMO four interleaved IEC958 streams.

Attachment: Výběr_105.png
Description: PNG image

Attachment: Výběr_104.png
Description: PNG image