[mythtv-users] Audio edge case

John Pilkington johnpilk222 at gmail.com
Sun Jul 28 12:56:18 UTC 2019

On 28/07/2019 11:41, Mike Perkins wrote:
> Okay. I'm running 29-fixes on plain Debian Stretch using the 
> deb-multimedia packages. I plan to upgrade to Buster+30-fixes within the 
> next week, assuming I can get time with everything else that's going on.
> The problem: As I do every year, I record all the BBC Proms - DVB-T, 
> OTA. Normally all works fine, except for one or two awkward programs 
> that happen to start at 19:00 on BBC4. I get occasions when there is no 
> audio.
> It isn't just the Proms: this can happen to any program that is the 
> first one broadcast when BBC4 starts up. This is literally an edge case, 
> since I have a standard 5 minute pre-roll and therefore the recording 
> begins before the transmission begins. I have tested these recordings 
> with VLC and, provided one jumps the initial gap, the record is fine 
> from then on.
> What seems to happen is that the stream gets sampled by mythtv at the 
> beginning and whatever is found for audio and video is taken to be set 
> for the whole transmission. In the old analog days, this was fine, since 
> it was the encoder setting the format and that wasn't going to change 
> from one end of the file to the other.
> Unfortunately, with DVB you get what you are given and the format can 
> change from moment to moment throughout the transmission, audio and 
> video both.
> My question is: has this edge case been looked at in 30/31/master? Will 
> an upgrade solve this issue or is there really a problem here that needs 
> some attention?
It has 'always' been a feature in recordings made at the start, or 
sometimes the end, of part-time DVB channels.  The simple solution is 
not to record over the transition.  I find, with master aand BBC FOUR, 
that a start at 1858 is obviously affected but still plays for me.  The 
audio graph display in the editor then shows stuff that is clearly not 
audio.  A start at 1859 is ok.  I haven't looked recently for an ffmpeg 
option to move the probe point.

Given an affected recording, you could remove its start with dd, but I 
have my mythDVBcut script already in the toolbox and use that.

John P

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