[mythtv] Jitter with time stretch

Peter Bennett pb.mythtv at gmail.com
Sat Jul 14 21:27:41 UTC 2018

Hi David

I do not use time stretch, so I do not know what is good and what is 
bad. I tested time stretch with the current master version on Linux (no 
patches), VAAPI and "-v playback" . With a 30fps recording it can be 
speeded up to 2x with no log messages about aout of sync audio. With a 
60fps recording (720p) from Comcast, it shows hundreds of errors, a 
bunch of messages saying "Video is 20.0819 frames behind audio (too 
slow), dropping frame to catch up." followed by a bunch of messages 
saying "Video is 17.7729 frames ahead of audio, doubling video frame 
interval to slow down.". This repeats.

On the shield, this type of thing also happens, including with 30 fps 
interlaced content, perhaps because it is actually shown at 60 fps.

On the shield if you set your audio device to NULL it seems to be 
perfectly smooth when speeding up to 2x.

I suspect what you are seeing is a pre-existing bug, not related to the 
shield specifically, but showing up because of the frame doubling that 
is happening on the shield.

I think the audio speedup may not be perfect, or maybe showing video at 
higher rates than 60fps is problematic.


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