[mythtv-users] Channel Mapping Issues on Analog Audio Output with Ticket #5900 changes

[Mark Goldberg]

It appears that when the changes from Ticket #5900 are applied to
0.21-fixes that
there are channel mapping issue when analog audio outputs are used.

This is on Fedora 10, with the stock Pulse installation, and it is understood
that there are issues with Pulse.

When an AC3 5.1 HD broadcast is recieved and 5.1 max channels is selected the
channel mapping appears to be incorrect. I believe it is due to the following:

Alsa Mapping is as follows:

0 FL
1 FR
2 RL
3 RR
4 C
5 LFE

One reference (Table 5.8 in http://www.atsc.org/standards/a_52b.pdf
audio coding mode 3/2) says AC3 is

0 FL
1 C
2 FR
3 RL
4 RR
5 LFE (not explicitly stated in table 5.8 but after the rest)

So I am guessing that the analog outputs of the new upmixer
are in AC3 order rather than Alsa order. If you are using a
digital output, it probably will work fine.

Toggling "Disable Upmixer" during playback does not correct the problem.
It can be corrected by either selecting max channels of stereo in setup, or
by remapping the channels in .asoundrc.

Unfortunately, for stereo sources the same remapping cannot be used and it
is necessary to go into setup to change max channels to either stereo or 5.1
and the output to either the remapped Alsa pcm, or the default Alsa pcm,
depending on the source. This obviously does not improve the WAF.

See http://www.gossamer-threads.com/lists/atrpms/users/12543#12543 for more
discussion  of this issue.

Also, it appears that if both the Myth built in resampler and the
Pulse resampler
are used, for example to play 44.1 kHz audio, the audio quality is
degraded, especially
on percussive sounds. Drumbeats are distorted. Disabling the resampling in setup
corrects the problem, and at least on Fedora, the Pulse resampler seems to be
decent.

Video playback also stutters if the myth resampler is enabled, even with 48 kHz
audio, but is smooth if the myth resampler is disabled. Even time stretch seems
to still work, even without the mythtv resampler.

Mark