[mythtv] AVSync in master, Normal decoding, vga monitor and built-in audio analog stereo

John Pilkington johnpilk222 at gmail.com
Tue Dec 10 10:06:23 UTC 2019

On 09/12/2019 22:12, Mark Kendall wrote:
> On Mon, 9 Dec 2019 at 21:21, Gregorio Gervasio Jr.
> <gregorio.gervasio at gmail.com> wrote:
>>>>>>> John Pilkington writes:
>>> Hi Mark, Peter, Tim et al...
>>> I'm now running b5edda1b06e, today's master, under el7 with hardware
>>> as in the subject.
>>> A recording that allows good sensory evaluation of a/v sync shows
>>> noticeable drift that is not evident from the -v playback log.
>> [...]
>>          At least for me, it seems like the sync problems (where audio
>> runs ahead of video) are only happening with 1080i (MPEG-2) content and
>> only when I use the double-rate software deinterlacers.  If I disable
>> double-rate deinterlacing or if I enable "Prefer OpenGL deinterlacers",
>> I don't see the problem.  I'll need to dig deeper into it to be sure,
>> though ...
> Gregorio
> You may be on to something - I hadn't thought to look at the FFmpeg
> deinterlacers. They manipulate timestamps when run at double rate and
> as the filtering happens before the a/v sync - it could lead to drift
> if there is any small loss in precision.
> John - does that tie up with what you are seeing?
> Thanks and regards
> Mark

Hi Mark:  As I said in my reply to Gregorio's post,

> I have increased the AVSync2 increment to 4 ms and selected the low quality deinterlace options, and the drift does seem to have gone away.

The root cause seems to be a systematic and cumulative difference 
between what is being controlled and what ought to be being controlled, 
and if the suggested new 'loop gain' approach works with the timestamps 
as they actually reach the hardware that ought to improve things.

Testing of options is slow because the effects are slow to appear, and 
I'm not sure what to try now.

I should make clear that I don't expect my windowed displays on monitors 
to deliver the greatest AV experience.  They're just tools in the chain.

Thanks again,


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