Audio Skipping Issue Live TV

[jamhandman]

I'll post logs when I get back home, but wanted to throw this out there...

When I tune to a few channels (Clear QAM) The channel comes up and the audio is there, but the audio track is skipping. If I do a short rewind, the audio skipping goes away and I can watch the channel. This is not an issue for main channels, like 4.1, but for the other sub channels, like 4.2/9.3 as example.

I also notice there is an audio/video sync issue on the same channels, where after a few minutes I have to skip back to resync the audio.

Thoughts? Thanks.

I'm using the AC3 Filter with the Microsoft DTV decoder.

I can try a different combination.

[Zeb]

Your decoders should be fine. Try setting MPEG1 Audio to Disabled. If you need MPEG1 to play analog recordings, there's a Config.xml setting that would be better.

[jamhandman]

I'll try that. I actually never thought about it, but since these channels come in as stereo they might not go through the AC3 Filter...

[Zeb]

AC3Filter can handle 2-channel AC3, but NextPVR's default live TV filter graph seems to confuse it with MPEG1. The simplest tweak is to banish the MPEG1 filter.

[Reddwarf]

AC3Filter can handle 2-channel AC3, but NextPVR's default live TV filter graph seems to confuse it with MPEG1. The simplest tweak is to banish the MPEG1 filter.

I seem to remember that AC3Filter can also handle MPEG audio, in case it really is MPEG.

[johnsonx42]

the issue is that with live tv, NPVR doesn't know until it tunes the channel whether the audio stream is going to be MPEG or AC3 (simplified to just those two since this discussion is about the US). When it first starts it has to pick something to set the reference clock to, and with both MPEG and AC3 enabled it defaults to the MPEG renderer. Once it tunes a channel and sees that the audio is AC3, it tells windows to switch the reference clock to the AC3 renderer. The problem is, Windows doesn't actually do what it's told; the reference clock stays on MPEG; as this is the wrong clock reference, a/v sync drifts around, and if the audio plays a little fast it gets far enough ahead of the video that it bumps into the end of the stream. That makes it stutter, and that's why skipping back a little fixes that.

Disabling MPEG-1 audio causes NPVR to build the graph from the start with the AC3 renderer set as the reference clock, and all is well.

If you do need both MPEG and AC3 enabled, then you can force NPVR to start with the AC3 renderer instead (thus moving the audio sync/stutter problem to your analog channels). In config.xml, set "<ForcedLiveTVClock>AC3 Audio Renderer</ForcedLiveTVClock>" in the <Renderers> section.

[Zeb]

The only mystery that remains is why this issue plagues 2-channel AC3 but not 6-channel or even mono. My cable provider used to deliver most SD channels as mono, but has now switched to stereo, however I was ready for them thanks to this old thread: http://forums.nextpvr.com/showthread.php...lter-Graph. I presume this has something to do with the inner workings of sub's NPVRTSReader? Anyway, with U.S. digital channels there should be no need for any audio format other than AC3.

@johnsonx42: If you set ForcedLiveTVClock as specified, what happens to live TV from an analog tuner that encodes audio as MPEG, silent or choppy? (I have no analog channels here.)

[Reddwarf]

@johnsonx42: If you set ForcedLiveTVClock as specified, what happens to live TV from an analog tuner that encodes audio as MPEG, silent or choppy? (I have no analog channels here.)

I was also pondering the same question, isn't it just as good to use AC3Filter for MPEG audio too?

[mvallevand]

I use the registry setting to default to AC3 and then M2A works fine

Martin

[Reddwarf]

I don't have analog channels but I have a bunch of channels with MPEG stereo audio, and I haven't had any snc issues since I started using AC3Filter for mpeg audio as well.