Trim pre-padding for part-time channels

[bgowland]

I know this has been mentioned before but did anyone ever write a utility to do it?

The problem being channels like BBC THREE and BBC FOUR which "time-share" with other channels. A recording that starts at 7pm (with pre-padding) shows the "BBC THREE gets going at 7pm" message with no sound, it records OK but the sound never kicks in when trying to watch it (either the recording or playback graphs get confused???).

If somebody did write a utility to trim the pre-padding for the first programmes on this sort of channel then point me at it otherwise I'll knock something up.

Cheers,
Brian

[mvallevand]

Do these shows play ok on the NMT? If not it could be more than padding, it would by the program inform changing and you'd have to do a cut when the the program changes? There's also a good chance the timeline will change when the new stream starts, which probably the best place to cut.

Note I was working on something like this to recreate a Timing.Info which you probably should do. It is not trivial for h264 streams.

Martin

[imilne]

None of the UK timeshared channels are h264. For now.

Iain

[bgowland]

Do these shows play ok on the NMT? If not it could be more than padding, it would by the program inform changing and you'd have to do a cut when the the program changes? There's also a good chance the timeline will change when the new stream starts, which probably the best place to cut.

Note I was working on something like this to recreate a Timing.Info which you probably should do. It is not trivial for h264 streams.

Martin

No they don't play on the NMT and it can be easily fixed just by setting pre-padding to 0 on the first programme of the evening.

The problem with the "time-share" is two different channels (BBC THREE and CBBC, for example) use the same AV ES PIDs. It was a while ago when I last looked at this (back with gbpvr, I think) but what happens is the audio ES disappears for a short while during cross-over and doesn't re-appear until somewhere between 50 seconds to 30 seconds before transmission starts. The result is there's never any audio playback.

As long as the PC clock is accurate to a few seconds, there's normally a short bit of announcer pre-amble with the AV streams present before the actual programme starts so no danger of timing info going awry.

I'm sure I started something like this years ago - I'll dig into my old code vault.

Cheers,
Brian

[mvallevand]

As long as the PC clock is accurate to a few seconds, there's normally a short bit of announcer pre-amble with the AV streams present before the actual programme starts so no danger of timing info going awry.

I'm not talking timing.info I am talking the mpeg timeline. If you can get me a chuck of one of these files with padding and a bit of playback, I'd like to have a look I think 20MB should do for SD.

Martin

[bgowland]

I'm not talking timing.info I am talking the mpeg timeline.

Sorry Martin, bad choice of words on my part - I wasn't talking about timing.info either, there was a deliberate <space> between the two words. What I mean is there isn't any issue with any timing of any sort.

Just to explain further, the two channels don't share all of the same tuning info so it isn't a case of watching CBBC until 7pm and then BBC THREE starts without having to change channel.

They share the same UHF frequency, TSID and AV ES PIDS but their SIDs are different and they have different LCNs. When CBBC goes 'offline' (a few minutes before 7pm) the AV elementary streams disappear from the TS and CBBC gets an MHEG image saying it's offline until...blah. Shortly after that, BBC THREE's MHEG (which will have been showing since it went offline) disappears and it gets the video ES but no audio ES until 50-30 seconds before 7pm. At that point both streams are running and timing isn't an issue.

If you can get me a chuck of one of these files with padding and a bit of playback, I'd like to have a look I think 20MB should do for SD.

OK, I've set a recording with pre-padding for 7pm tonight. I'll upload a chunk to my web server and email you with a link.

Cheers,
Brian

[mvallevand]

Yes your sample shows this nicely. Personally I'd just use ffmpeg to remux (-acodec copy -vcodec copy newfile.ts), it's amazing how much non-video is in the mux.

Martin

[bgowland]

Personally I'd just use ffmpeg to remux

Yes always an option although I don't want to run any automated post-processing on recordings (I was glad to leave that behind when moving from gbpvr to npvr).

The idea is simply to run a utility in PostUpdateEPG.bat which checks for recordings on the known channels and if they're the first programme of the night, set the prepadding to 0 minutes.

Cheers,
Brian

[mvallevand]

Facepalm: I read "write a utility to trim the pre-padding" and that you actually wanted to trim it from the file.

You don't really need a utility just a command line sql call something like

Code:

update scheduled_recordings set pre_padding = 0 where channel_oid = ### and status = 0

The problem might be in PostUpdateEPG because it runs after recordings are scheduled so there really is no way to insert something after the update of the EPG and before recordings are scheduled. I don't know if NRecord will reload this.

Martin

[bgowland]

Facepalm: I read "write a utility to trim the pre-padding" and that you actually wanted to trim it from the file.

Yes, I realised the use of the word 'trim' was perhaps not the best.

You don't really need a utility just a command line sql call something like

Sure but to make it more manageable (for other users) I'm looking at a simple text file that can easily be managed with one <chanName>:<startTime> key/value pair per line, e.g., BBC THREE:1900.

The problem might be in PostUpdateEPG because it runs after recordings are scheduled so there really is no way to insert something after the update of the EPG and before recordings are scheduled. I don't know if NRecord will reload this.

Actually that's a valid point - I was thing with a 14 xmltv source, everything would be adjusted well in advance but as the recordings are deleted/recreated each day that could be a problem. Hmmm.

Cheers,
Brian