[mythtv-users] A/V delay when recording HD

Brad Fuller bradallenfuller at gmail.com
Tue Oct 7 22:42:29 UTC 2008

On Tue, Oct 7, 2008 at 3:26 PM, Florin Andrei <florin at andrei.myip.org> wrote:
> I stumbled upon this post on doom9:
> http://forum.doom9.org/showthread.php?t=125966&page=326#6517
> eac3to is a Windows tool to demux A and V tracks from an HD file.
> The OP is correct, as far as I was able to verify - demuxing HD records
> made by my MythTV 0.21 server from Comcast cable in NorCal, using HDHR,
> does exhibit the same pattern: there's a 1000ms or more sync difference
> between A and V tracks.
> Now, the files are played correctly by MythTV - when displaying the
> recording, there's no A/V desync. Also, eac3to seems to extract the
> tracks correctly - when remuxed, they stay in sync (but a chunk of video
> is lost, presumably by eac3to when demuxing to fix the A/V timing
> difference).
> Therefore there appears to be no problem, but the OP is right - this is
> mystifying. Where does the timing difference come from? Is that how HD
> is being broadcast, or is it due to some buffering or something
> performed by MythTV, or is that how HDHR sends the A/V streams?
> I don't know if the OP uses an HDHR, or whether that matters at all. I'm
> just puzzled by the situation.

I read the post.
First, MythTV doesn't really record anything, right? That's left up to
the particular device's driver. Also, he doesn't mention what device
he uses nor what "digital HD cable" means. Is that a stream from a
settop box via fireware, or QAM256, or ? I would contend that "HD" is
not recorded but captured. Was there any transcoding going on

There are several questions that need to be answered first to
understand where the OP is coming from.

AFA your captured stream: I assume you captured a QAM256 stream from
the coax since you are using HDHR.

Brad Fuller

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