[mythtv] AVSync2 Refinements

Tim Pletcher pletchtd at gmail.com
Mon Dec 9 15:08:10 UTC 2019

Hi Mark,

Thank you for looking at this in detail.

You are exactly right in that last_fix doesn't get carried over between
calls. The last_fix value is actually needed to correctly do the signal
filtering to the control loop.  Over the weekend, I eventually figured out
the filtering wasn't actually working after adding more debugging detail to
the logging for better visibility of what was happening mathematically
between calls.  This is also the reason I arrived at the conclusion that
filtering is probably not really needed since performance is good anyway
and a filter brings the drawback of adding lag to the control response.

In reality, with the broken filter application, the code was effectively
using a control gain of (0.4) * (1 - 0.9) = (0.04).  This setting seems to
still provide rapid initial sync while preventing over-control in response
to natural audio timestamp jitter.

The patch looks great.  Based on discussion above, I think we should remove
the filtering and set:

s_av_control_gain = 04;

The last_fix and s_sync_fc references should go away alltogther and the new
control calculation becomes:

float fix_amount = audio_adjustment * sign * s_av_control_gain;

Should I still create a Trac ticket for this?


On Mon, Dec 9, 2019 at 9:06 AM Mark Kendall <mark.kendall at gmail.com> wrote:

> On Mon, 9 Dec 2019 at 00:32, Tim Pletcher <pletchtd at gmail.com> wrote:
> > Thank you for your insights Peter.  I have been testing and debating
> whether to actually include any filtering or not. I do think true
> proportional control with gain settings less than 1 is better as this
> minimizes the control oscillation once near setpoint while still giving
> somewhat faster initial sync. I do notice a significant difference in the
> amount of variability from my two intel apollo lake based front-ends using
> VAAPI versus my combined i5 frontend/backend using an NVidia GT1030 with
> NVDEC so I think the proposed change is more impactful on low end hardware.
> >
> > As David suggests, I will create a ticket and submit a patch.
> Tim
> Many thanks for this - very handy:)
> I recently got caught out by the fluctuating av sync on the raspberry
> pi. I've taken the liberty of producing a more extensive patch
> (attached). This adds your filtering as well as removing all of the
> the old a/v sync code and the settings for AVSync2 - i.e. it moves
> over completely to the new av sync code and removes the audio
> correction adjustment. (N.B. the patch is against today's  latest
> master - I pushed some cleanup of  MythDVDPlayer and the patch won't
> apply cleanly before that).
> It appears to work very well. On two (linux) desktop dev machines and
> a pi 4 I can't see any issues - playback is nice and smooth. As you
> say, initial sync seems to be much better as well.
> Certainly gets my seal of approval!
> One question - last_fix - should that be carried over between calls to
> AVSync()? At the moment it is a local that gets reset between each
> call - which I'm not sure is the intended behaviour.
> Thanks again and regards
> Mark
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