MPEG2 file structure and handling. Is the truth out there?

[Dietmar]

Desperately looking for an expert answer about MPEG2 file structure and handling.

My Compro DVB-T300 card gives excellent reception on screen. I only use it for digital TV reception and recording. It can be used like a VCR, allowing selection of programs from different stations to be recorded off air as well as powering up and powering down the PC!

The card's performance is as good as my set-top-box with the TV but cost only half as much as the set-top-box.

BUT when I record the Compro reception to harddisk to watch at a later time about 50% of recordings have lipsync problems.
The sound then is ahead of the picture.

The Compro manual states: "The digital TV transmission is recorded to disk directly without any recoding".

Trying to get some help the dealer, where I bought the card, says the problem is in my computer sound system or the hard drive is too slow or my video card or whatever.... meaning he doesn't know.

Looking at the recorded file size and length of playing time the data rate is only about 700 kilobyte per second and the drive light gives a short flick about once per second so the drive can't be the problem.
And I can't see the video card or the sound card at fault either, as the data handling is fully digital and the lipsynch problem doesn't always appear.

How is the sound and picture synchronised in MPEG2 files and how can they drift from each other during transfer from digital TV tuner to hard drive?

I have done many test recordings of about 70 minute length, all with the PC freshly booted, and yet some have good and others bad lipsync. There also does not seem to be any correlation between good or bad reception.

There is some lipsync correction because when watching live, occasional
interference may cause momentary loss of lipsync but this corrects itself
immediately, sometimes accompanied by a slight flicker of the picture.

Could it be that when recording, the software writes the raw data to disk without these corrections and that this correction mechanism is not active on playback?

I have tried to send a fault report to www.comprousa.com and received some barely understandable (very poor english) reply about difference of PAL and NTSC and that I gave them a wrong serial number (with 19 digits a mistake could easily be made).
Not very encouraging.

Of all the digital TV cards I have either tried or seen demonstrated I think
the Compro is by far the best, the software is very user friendly and I would love to keep it, provided this lipsync problem can be licked

Basil Brush

[Soopafresh]

Looks like the card records Mpeg2 Transport streams. Do the saved files have .TS or .TP extensions ?

What kind of program are you using to play the files ?

I'm wondering if the problem is with your playback software rather than the capture card.

I've got some suggestions, but please answer the above questions first

[Dietmar]

Looks like the card records Mpeg2 Transport streams. Do the saved files have .TS or .TP extensions ?
Neither, just oldfashioned .mpg
What kind of program are you using to play the files ?
Compro provides own player, but 'Power DVD' or ULEAD VS9 player all give same result:
OK sometimes, poor lipsync at other times
I'm wondering if the problem is with your playback software rather than the capture card.
I've got some suggestions, but please answer the above questions first

Thanks for the reply, 'Soopafresh'.
As forum / posting etc is totally new to me I send this reply
with everything crossed. :-}
Basil Brush

[Soopafresh]

Thanks for the info. If you could post a small (5-20 mb) section of a bad file, we can take a look. Download MPG2Cut2 and save a slice of bad file. You can upload it to http://rapidshare.de . Just post the Url once your upload is complete.

[Dietmar]

Sorry Soopafresh, but I only have POTS (plain old telephone service) access to the internet and cannot upload a segment of the faulty mpg file.
After checking some 'good' files I think that even these have uncertainty about their lipsync, just not enough to interfere with watching them.

Surely there has to be some mechanism to lock sound with picture?
I am aware that mpg2 video compression varies with picture content while
the sound compression follows its own rules, but what locks both together?
After all DVDs or digital TV never have any lipsync problems.

[SatStorm]

There are many reasons this happens. I won't try even to explain. But here is a simply practical solution in case you intereset to solve this

Keep in mind that DVB transmissions use CBR mp2 sound.

Load your DVB file to Virtualdub mpeg2. Then, adjust the audio to mach the picture. When you correct this, save the audio as .wav.
Re-encode that wave file to mp2 (using dbamp for example, or TMPGenc) Finally, re-mux your DVB file with the new mp2 audio. TMPGenc Free is excellent for this.

The new file must have correct audio.
My experience in this matter, show me that trying to solve this issue with patches, fixes and other methods, may help on the playback, but when you later author your file, on the DVD standalone has again the same problem

[Nelson37]

There is no real mechanism for ensuring synch. The video and audio streams are recorded seperately, and time indexed for playback. Broadcast errors can affect synch, recording over commercial breaks can affect it. It is Extremely Important that no PC backround tasks interrupt the recording of the streams in any way. Read the sticky on Dropped Frames.

There are other problems such as GOP errors, often around commercial breaks. Can you record an uninterrupted movie, or several short segments between commercials, and see if synch is lost? Does it happen at all on short recordings, or only long ones?

Some recording softwares use a Padding Stream to correct for errors, some playback softwares ignore this. Some, but not all, Authoring and remuxing progs will use this stream during file creation. Try running your file thru TmpGenc Merge and Cut to see if this helps.

[ronnylov]

Have you tried demux/fix the recorded video with PVAStrumento or PjojectX? These programs make wonders fixing my DVB recordings before authoring it to DVD. But my recordings are in a format called pva, but it should work with mpg input also.

When you run it through PVAStrumento you get demuxed video and audio (separated files) and you can multiplex them to mpg again with bbmpeg or TMPGEnc to check the sync. I think there is an option to make mpg directly also from PVAStrumento but it has been buggy at least in the past, but you can try it.