[mythtv] Jumpy video playback

Mark Kendall mark.kendall at gmail.com
Tue Jan 14 09:09:17 UTC 2020


On Sun, 12 Jan 2020 at 09:44, Klaas de Waal <klaas.de.waal at gmail.com> wrote:
> I got a bit further on this; the problem is in the frame rate!
> It looks like that the recording is incorrectly identified as 24Hz instead of as 50Hz.
>
> When everything is OK it works like this as I understand it:
> (1) Broadcast signal is 50Hz interlaced
> (2) The playback info box shows this as 25.00fps
> (3) De-interlacing then generates 50 fps.
> (4) The Nvidia driver is set to 50Hz by MythTV/xrandr
> (5) This is shown in the System Status / Display box (very useful new addition)
> (6) The actual framerate is also shown on my TV as 50Hz

That's a fair summary of how it should work.

> With the problematic recording steps (1) to (3) are the same.
> The difference is:
> (4) The Nvidia driver is set to 23.98Hz by MythTV/xrandr
> (5) This is shown in the System Status / Display box (very useful new addition)
> (6) The actual framerate is also shown on my TV as 50Hz

I'm not sure how we can set 23.98Hz but the display still shows 50Hz -
unless the xrandr call failed (anything in the logs?)

But more importantly - if the video is detected as 25.00fps
(regardless of interlacing) - why is it trying 23.98?

> When the first 10 megabytes of the recording are removed then the playback is correct.
>
> This happened twice in the last two weeks in about 100 recordings or so.
> The broken recording is created with multirec; the previous recording on that
> channel was still running at the moment the new recording started.
> I have also in the past observed that the start of playback of recordings created
> by multirec has sometimes blocks.

Multirect aside - the playback code should hopefully handle modest
corruption issues.

> My tentative conclusion at this moment is:
> - there is something not correct in the first 10MB of the file
> - the frame rate is obtained from the first 10MB of the file
> - this frame rate is used to set the Nvidia with xrandr
> - later the correct frame rate is obtained as shown in the playback box
> - but the Nvidia framerate setting is never updated

Sounds very plausible.

thanks and regards
Mark


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