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

Brad Fuller bradallenfuller at gmail.com
Wed Oct 8 00:21:31 UTC 2008

On Tue, Oct 7, 2008 at 5:06 PM, Geoffrey Schwartzreich
<atomik_kat at yahoo.com> wrote:
> I do think Comcast applies Dynamic-range compression on the audio of most
> channels, or the feeds have it already.

Really? Is Comcast really compressing the AC3 stream before it goes out?
And, how does this compression screw up sync?

Florin: can you do some experiments on other channels? Especially on
the 4 major networks. That could give some more clues. The major
networks' primetime programs will most likely include metadata (like
dialnorm) that providers like Comcast should pay attention to.   I've
heard that not every mixer (a human mixer) doesn't always add
dialnorm. I hear it's a problem, but, I'm not a broadcast engineer ..
so take my comments with a ton of salt.

> TNTHD is awful about timecode problems. Sometimes it crashes my TiVo, and
> files that I move from the TiVo and play back have numerous stream errors
> and break VLC sometimes.
> So, I think it's safe to say that Comcast or the upstream provider is
> mucking around with the audio track or the timecoding is just overall
> inaccurate on some feeds.

I really don't think you can make that claim w/o some data to back it
up. There are so many other things that can effect sync. Heck, it
could be HDHR hw or the drivers that act strange in a particular

> Geoff
> On 10/7/08 4:52 PM, "Florin Andrei" <florin at andrei.myip.org> wrote:
>> Brad Fuller wrote:
>>> AFA your captured stream: I assume you captured a QAM256 stream from
>>> the coax since you are using HDHR.
>> Well, forget the OP, explain my situation. :-)
>> So yeah, it's a QAM256 (I *think* it's 256), recorded by MythTV from an
>> HDHR tuner, from a regular Comcast cable feed. MPEG2 1080i. Upon
>> analysis, the A and V tracks in the .mpg file turned out to be not
>> aligned, there's an A/V timing difference slightly bigger than 1000ms.
>> MythTV and pretty much any player is aware of the difference and the
>> file is played in sync. So it looks like the timing diff is done on
>> purpose by some component of the recording chain, and the whole thing is
>> done properly.
>> My question was - why are not A and V muxed in sync? Why there's a time
>> delta at all? For some reason, I expected a file dumped from a broadcast
>> feed to have A and V in sync, with a 0 ms difference in between.
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Brad Fuller

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