[mythtv-users] UK DVB-T radio timing problems since 0.25

[John Veness]

On 02/05/2012 15:12, John Veness wrote:
> Hi,
>
> I've just updated to MythTV 0.25 (previously 0.24) using the Mythbuntu
> packages, on Ubuntu 11.10. I didn't change the underlying Ubuntu version
> at the same time.
>
> Since 0.25, playing back programs on UK DVB-T (Freeview) radio channels
> shows screwy timecode problems. For example, a 35 minute recording plays
> as a 29 min 11 seconds recording. That is, the total length shows as 35
> minutes, but the current position marker will change from zero to
> 29min11 before stopping. During that time it will play the entire 35
> minute recording at normal speed. In other words, the current position
> marker is lagging behind real time.
>
> Also, when skipping forward or backward, e.g. by 30 seconds or by 10
> minutes, there is a noticeable delay of a second or so before the skip
> takes place and before the OSD pops up. Doing lots of skipping feels a
> bit like wading through treacle, like it is having problems finding the
> position in the file to seek to.
>
> Both of these things were fine on identical hardware and identical
> underlying Ubuntu version with MythTV 0.24. Note that these problems
> occur both with new recordings made with 0.25 and old recordings made in
> 0.24.
>
> Before I report this as a bug, I wondered if anyone else had seen these
> problems or knew of a fix?

Since I wrote the above, I've realised I haven't described the problem 
correctly. The second paragraph should be:

Since 0.25, playing back programs on UK DVB-T (Freeview) radio channels 
shows screwy timecode problems. For example, a 35 minute recording plays 
as a 29 min 10 seconds recording. That is, the total length shows as 35 
minutes, but the current position marker will change from zero to 
29min10 before stopping. The last almost 5 minutes 50 seconds of the 
recording will not play in Myth.

Note that if I watch the filesystem as the program is recording, I do 
see the filesize increasing during the last six minutes of the 
recording. Also, if I play back the file with something else, e.g. 
ffplay, it will play the full 35 minutes. So this is a problem with 
playback in Myth, not with recording.

I notice with interest that 35 minutes is 2100 seconds, and 29 minutes 
10 seconds is 1750 seconds. 2100/1750 is 1.2, or the same as 60/50. Is 
this some kind of 60fps vs 50fps mismatch?

These recordings have no video, just audio and MHEG. Maybe because they 
have no video, Myth is making a guess at 60 frames per second rather 
than (as it should be in UK) 50 frames per second. It then does some 
kind of calculation and plays what it thinks is the correct length but 
stops early. I haven't checked the code, so this is just a guess. Note 
that as far as I can tell, the recording is playing back at correct 
speed and pitch.

Does anyone want a sample file?

Cheers,

John

-- 
John Veness, MythTV user, UK, DVB-T