[mythtv] DVD playback issues, patch

Mark Spieth mark at digivation.com.au
Mon Nov 16 00:10:56 UTC 2009

>> also on dvd pts change, avsync_delay should be set to avsync_avg so that 
>> there is no discontinuity in avsync_avg
>> determination.
>I'm not sure if I understand what you mean by pts change. The following 
>             if (! (avsync_delay > 2000000 && player_ctx->buffer->isDVD()))
>                 avsync_avg = (avsync_delay + (avsync_avg * 3)) / 4;
>... changes avsync_avg only if there is no large discontinuity in the video 
>timecode. Doesn't that have the same effect? (Or were you referring to 
>something else altogether? - when does pts change?)

it probably should be something like this

            if (avsync_avg == MAXINT)
                avsync_avg = avsync_delay;
            // prevents major jitter when pts resets during dvd title
            if (labs(avsync_delay) > 2000000 && player_ctx->buffer->isDVD())
                avsync_delay = avsync_avg;
            #define AVSYNC_TC 8
            avsync_avg = (avsync_delay + (avsync_avg * (AVSYNC_TC-1))) / 

and in the constructor
        set avsync_avg to MAXINT.
        alternatively use a bool to indicate avsync_avg has not been set.

also notice the labs for dvd check. I think that is also important though 
its only a guess.
however is 2 seconds enough for dvd?
I like the idea to smooth avg a bit more. would require some experimentation 
to determine good values.

>> a big problem is that the initial value of avsync_avg is 0 which is not 
>> correct.
>> somehow it should be set to avsync_delay if it hasnt been set before.
>That would certainly be possible but I think the real problem is the 
>av_delay calculation doesn't take the previous delay into account. That is, 
>instead of (in pseudo code):
>     avsync_delay = video.timecode - audio.timecode
>It should be:
>     avsync_delay = video.timecode - audio.timecode + avsync_adjustment

no as avsync_adjustment is for realigning, not measuring and is based on the 
refresh rate not the delivered rate.
avsync_delay is the instantaneous estimation of avsync.
avsync_avg is smoothed so that any minor variations dont cause excessive 
frame double/skip jerkiness.

avsync_adjustment is not part of the delay estimation algo and shouldnt be.

I do see some jerkiness currently watching say motor racing at 1.8. or 1.75. 
if the delived rate is not a close multiple of the refresh rate you notice a 
bit more jerkiness. not that it's too noticable though.


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