[mythtv-users] 0.23-fixes ac3 passthrough intermittent problem

[Andre]

Hi,

recently upgraded to 0.23-fixes and finding a strange problem with ac3 soundtracks. 0.23.0+fixes23858-0ubuntu0+mythbuntu1 on 32bit Ubuntu 9.10.

I have an external surround sound 5.1 amp connected to MythTV via optical spdif, speakers are set to 5.1, AC3 & DTS passthrough ticked, no advanced audio settings.

Most of the time AC3 audio works fine, sync is all over the place from show to show but I'm used to dealing with that. Sometimes when listening to 5.1 AC3 the sound will go a little quiet and indistinct and stay like that for the remainder of the show! I have now spotted that when this happens the amp indicates that it's receiving stereo PCM not AC3 any more, if I skip back to the previous ad break (which is usually stereo) then when the show starts again the amp will switch back to AC3 audio and everything sounds fine.

I tried switching on the upmixer and a similar thing happens, the sound goes muddy and indistinct but the AC3 indicator stays on, rewind to the ad break and most of the time it comes back clear again.

So seems like MythTV is switching to downmixing the AC3 track to stereo and my amp is switching to Pro-Logic, hence the muddy audio. I can go through the same show three times and it does it in different places, the one consistent thing is that it seems to happen when there's a glitch in the audio (and/or video), maybe quarter of the time it happens. Oh and when this happens lip sync changes dramatically by >200ms, also I get a loud crack from the speakers, sometimes I get that anyway when a show changes from stereo to 5.1 or back again.

I can find many threads about 5.1 problems but haven't seen one that sounds like this. It seems to happening at some point in most HDTV that has 5.1 audio across a variety of channels and it didn't happen on 0.21 or 0.22

Any ideas?

Andre