[mythtv-users] While watching a show in Time Stretch, audio lags behind on 720 recordings

Mike's JdJ stepsisters at comcast.net
Fri Aug 22 14:30:28 UTC 2014



On 14-08-22 06:41 AM, Jean-Yves Avenard wrote:
> On 22 August 2014 11:07, Mike's JdJ <stepsisters at comcast.net> wrote:
>>
>> While watching a show with a Time Stretch of 2.0, audio starts lagging
>> behind the video until unwatchable.
>
> what do you use for video decoding ?
>
> When using VDPAU, the hardware decoder can only decode, depending on
> the codec, only about real time.
> It can't decode much faster. Certainly not 2x as fast.
>
> Time Stretch then can be an issue (a codec like h264 requires you to
> decode all frames between a reference frame to get a single picture)
>
> When that happens, you will see in the log errors about video frames
> being dropped
>
> I wouldn't rely too much on the A/V Sync value... is it in sync, or else?
>
> what do you mean by unwatchable? too many frames are dropped, the A/V
> sync is no good (like audio is too far forward/behind)...
>


Thank you for your reply Jean-Yves.


I went to my Front End setup for General Playback.  It was set 
for High Quality (ffmpeg and xvideo).  I changed it to both 
VDPAU and VDPAU High Quality, but when I tried to play a 
recording I got "Failed to initialize video output".


While going through my recordings, I observed that the 
high-speed long lag only occurs on 720 recordings and does not 
always occur on 720 recordings.

Over The Air recordings can reach a constant A/V Sync of 140. 
Some recordings are near zero A/V Sync.

Comcast recordings can reach a maximum A/V Sync of 1500 over a 
period of almost one minute, then jump to zero and start 
increasing again.

The A/V Sync pattern on 720 recordings appears to be:
OTA:  minimum 0.3, maximum 140; constant values
Comcast:  minimum 10; maximum 1500 varying.

The larger A/V Sync values appear to be associated with certain 
Comcast channels and content:
AMCHD 30
ABCFHD 1500
BIOHD 114
CBETHD 110
FXXHD 1500, 30, 10
HALLHD -0.5
NGCHD 15

I'm guessing that A/V Sync is roughly the milliseconds of audio 
delay from video; it seems to correspond.  At A/V Sync values of 
around 100, lip-syncing is lost, but it is still watchable.  At 
A/V Sync values of 1500, the audio lags the video by a couple of 
seconds so that dialogue comes from another actors mouth, sound 
effects are disjoint with actions, and sound can be coming from 
previous scenes.  Again, this is on Time Stretch playbacks of 
2.0 - 1.5; less at 1.5 than at 2.0.


More information to be added to my configuration:
Motherboard - GigaByte GA-Z68MA-D2h-B3
no video board, using the on-board video ports


Mike


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