USA OTA video audio sync vocabulary please? - AVS Forum
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post #1 of 13 Old 04-19-2013, 05:21 PM - Thread Starter
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Hello
I watch and record my television exclusively via a computer tuner from USA over the air television. I started having transcoding sync problems and I consequently found out that there was some sort of timestamp system for both the multiplex audio and video transmission on USA over the air television. However I have been finding it difficult to pin down just what are the digital fields which do this time stamping in both the audio and video streams. Maybe it's stupid but I thought the names of these fields and exactly where they are might be important in asking further questions on these forms on just what software honors and dishonors these fields . So can somebody help me out on this? When I do a Google or Bing search for say "USA OTA video audio sync " I get all sorts of outdated and erroneous links. So maybe it is not important but just exactly how does audiovideo synchronization work on US over the air television? What are the exact names of the fields employed to do this? And where are these fields exactly located? Perhaps a link to the answer may be adequate but I have not been able to find this information on my own so please help. And thanks.
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post #2 of 13 Old 04-20-2013, 10:30 PM
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Not sure what you are asking. Are you talking about timecode?

How is your system set up? If you are recording digitally, you shouldn't be getting any sync issues. It may be coming from your playback software, codecs, receiver, tv, etc.
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post #3 of 13 Old 04-21-2013, 06:10 AM - Thread Starter
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Originally Posted by sitlet View Post

Not sure what you are asking. Are you talking about timecode?

How is your system set up? If you are recording digitally, you shouldn't be getting any sync issues. It may be coming from your playback software, codecs, receiver, tv, etc.

Yes you are absolutely right under ordinary circumstances I am not getting any timing issues between my audio and video. But I know this is only possible via a hidden mechanism which my recording software and playback software observes! So I want to know how to refer to that hidden mechanism that keeps playback InSync.

I know this hidden mechanism for OTA recording and playback exists because of a question I asked about why does some of the transcoding of my OTA recordings sometimes result in desynchronization problem between my audio and video. The answer to this question refer to some of this timecode stuff, but I do not think it was in the exact universally accepted language.

I want the most universally accepted language to refer to this OTA resynchronization of playback and recording.

"VideoReDo TVSuite V4" refers to their software resynchronization mechanism this way "Automatic Repair Synchronization is for those situations where the source material may have transmission errors (Over-Air Broadcasts) or not have the perfect timing required by MPEG digital compression ( Camcorder or VHS to DVD )." I sometimes utilize this software for transcoding, but it is to CPU cycle hungry for my computer in less I have a lot of time for the transcode. So I want to ask about other less CPU hungry alternatives, but I also want to first get the language right so please help me get the language right that is my question.
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post #4 of 13 Old 04-21-2013, 03:39 PM
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Sounds like you are talking about the timestamping that goes on in MPEG2 Transport Streams, which both ATSC and DVB systems use to carry (and sync) multiple video and audio streams (along with digital subtitles, closed captions, text services etc.)

http://en.wikipedia.org/wiki/Presentation_time_stamp PTS may be of help?
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post #5 of 13 Old 04-21-2013, 10:03 PM - Thread Starter
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Ok sneals2000 so was I to ask the question - Does Mythtv (Windows Media Center alternative) along with its plug-in called Mythnuv2mkv (for transcoding) utilize timestamping MPEG2 Transport Streams keep everything in audio video sync even after transcoding like "VideoReDo TVSuite V4" does? I will have asked the question in the way that most people on these forms would understand it?

I don't know if I will get any response from this thread on that question, but were I to ask it in another thread perhaps in another forum that you would suggest would that be the way to ask that question?
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post #6 of 13 Old 04-27-2013, 09:19 AM
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Originally Posted by bbmiller View Post

Ok sneals2000 so was I to ask the question - Does Mythtv (Windows Media Center alternative) along with its plug-in called Mythnuv2mkv (for transcoding) utilize timestamping MPEG2 Transport Streams keep everything in audio video sync even after transcoding like "VideoReDo TVSuite V4" does? I will have asked the question in the way that most people on these forms would understand it?

I don't know if I will get any response from this thread on that question, but were I to ask it in another thread perhaps in another forum that you would suggest would that be the way to ask that question?

Isn't NUV the format used for analogue video captured and encoded in software? MythTV uses regular transport stream recordings for DVB/ATSC captures of MPEG2/MPEG4 encoded video (as encoded by the broadcaster on transmission).

I've used VideoReDo occasionally, but most of my transcoding and/or rewrapping stuff is done with FFMBC or FFMPEG. Sometimes errors cause some sync drift. I've also used TSDoctor to do some transport stream processing (it uses 5.1/2.0 AC3 switching to detect ad breaks on some platforms - when ads are 2.0 and the programme is 5.1) but not sure how ATSC friendly it it.
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post #7 of 13 Old 04-28-2013, 08:50 AM
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ATSC does indeed use MPEG-2 standard transport streams. Keeping elemental streams in sync will be done by MPEG-2, and any residual time code streams left over from the production process are most likely superfluous.

Because OTA reception varies, what is recorded may not contain the whole MPEG-2 TS preserved intact, hence the need for products like VideoReDo TVSuite. But it acts on the MPEG-2 streams themselves. At this point there's no difference between MPEG-2 data coming from ATSC, QAM or other RF modulation schemes, as these have been out of the equation for a long time.

If you're having problems with MythTV, that's a completely different subject.

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post #8 of 13 Old 04-28-2013, 09:38 AM
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Originally Posted by Speed Daemon View Post

ATSC does indeed use MPEG-2 standard transport streams. Keeping elemental streams in sync will be done by MPEG-2, and any residual time code streams left over from the production process are most likely superfluous.
Yep - It's very unlikely production timecode would survive through the broadcast chain - the only time you normally see anything timecode-y is when a VTR is mis-timed vertically and you see VITC in active. And it's not relevant to this as timecode isn't separately tied to audio and video streams in the broadcast stream in many cases - though in lots of cases these days audio is 'embedded' in an SDI/HD-SDI digital video signal in the studio (so it is routed as a single signal rather than separate audio and video connections. Timecode can be contained in this video signal as well as data.)
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Because OTA reception varies, what is recorded may not contain the whole MPEG-2 TS preserved intact, hence the need for products like VideoReDo TVSuite. But it acts on the MPEG-2 streams themselves. At this point there's no difference between MPEG-2 data coming from ATSC, QAM or other RF modulation schemes, as these have been out of the equation for a long time.
Though depending how filtered the MPEG2 transport stream is the ATSC and DVB streams can vary a bit in how they flag some aspects of the stream (EPG, start/end of programme triggers etc.) Hence some MPEG2 transport stream processing apps may not like ATSC-derived transport streams if they are looking for DVB-specific stuff AIUI.
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post #9 of 13 Old 04-28-2013, 10:33 AM
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That's a good point, sneals2000. It's possible that what the OP is describing is the PSIP data stream. PSIP data includes real time data, but not the kind of thing that I think of when someone says "time code". The OP's use of the word "multiplex" is also misleading. Back in the early days of stereo FM broadcasting in the US, "multiplex" was used to denote the stereo RF subcarrier. But in the digital age there are no RF subcarriers! Words like "multiplex" and "quadrature" still have meaning, but in a completely different context.

When the layperson starts using jargon terms without realizing it, big headaches follow! wink.gif

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post #10 of 13 Old 04-28-2013, 01:41 PM
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Quote:
Originally Posted by Speed Daemon View Post

That's a good point, sneals2000. It's possible that what the OP is describing is the PSIP data stream. PSIP data includes real time data, but not the kind of thing that I think of when someone says "time code". The OP's use of the word "multiplex" is also misleading. Back in the early days of stereo FM broadcasting in the US, "multiplex" was used to denote the stereo RF subcarrier. But in the digital age there are no RF subcarriers! Words like "multiplex" and "quadrature" still have meaning, but in a completely different context.
No - don't think the OP was talking about 'real time' - think they were talking about how separate audio and video streams stay in sync and avoid slipping.

Multiplex in a digital TV environment usually describes the entire MPEG2 transport stream carrying multiple audio and video streams within a single RF channel. Certainly in DVB-T/T2 that is the standard term for the datastream carried in a single RF channel - which can carry multiple SD and/or HD video streams plus audio streams.

http://www.dtg.org.uk/industry/dtt_channels.html (The DTG is the UK Digital TV organisation that handles standardisation and receiver specs etc.)

One thing that I've noticed is that A/V sync can be massively influenced by errors in the received data stream.
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post #11 of 13 Old 04-28-2013, 03:08 PM
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Originally Posted by sneals2000 View Post

No - don't think the OP was talking about 'real time' - think they were talking about how separate audio and video streams stay in sync and avoid slipping.
I agree about that, but I think what the OP may have seen was the PSIP stream. I don't know how it's done in the UK, but at least for me and the OP (who said "USA") I think that's something that may have lead the OP to believe it played a role in A/V sync.

Does that make sense? biggrin.gif

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post #12 of 13 Old 04-29-2013, 04:05 AM
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Originally Posted by Speed Daemon View Post

I agree about that, but I think what the OP may have seen was the PSIP stream. I don't know how it's done in the UK, but at least for me and the OP (who said "USA") I think that's something that may have lead the OP to believe it played a role in A/V sync.

Does that make sense? biggrin.gif

Not sure - the OP was asking what digital fields or timestamps in a US OTA transmission keep the audio and video in a given mux in sync, as they are having a/v sync issues during transcoding.

Think they were referring to the functionality that PTS (Presentation Time Stamps) handles - which is how MPEG2 keeps separate audio, video, subtitle etc. PIDs in sync? This is a lower level of time stamping designed to ensure a non time-aligned packet distribution system (like MPEG2 ts) can allow reconstruction of time-aligned content, rather than the more 'clock time' data PSIP/EIT etc. provide for EPG, start/end of transmission for PVR recorders etc.

To answer the OP's original question - two issues spring to mind. How the transcoding software handles PTSs in general, and in particular how it copes with packet corruption (i.e. a missing audio or video packet) caused by a demodulation error (on a non-100% signal) I've seen many MPEG2 TS transcodes (and rewraps without re-encoding) slip sync on errors.
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post #13 of 13 Old 04-29-2013, 07:10 AM
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Originally Posted by sneals2000 View Post

Not sure - the OP was asking what digital fields or timestamps in a US OTA transmission keep the audio and video in a given mux in sync, as they are having a/v sync issues during transcoding.

Think they were referring to the functionality that PTS (Presentation Time Stamps) handles - which is how MPEG2 keeps separate audio, video, subtitle etc. PIDs in sync? This is a lower level of time stamping designed to ensure a non time-aligned packet distribution system (like MPEG2 ts) can allow reconstruction of time-aligned content, rather than the more 'clock time' data PSIP/EIT etc. provide for EPG, start/end of transmission for PVR recorders etc.

To answer the OP's original question - two issues spring to mind. How the transcoding software handles PTSs in general, and in particular how it copes with packet corruption (i.e. a missing audio or video packet) caused by a demodulation error (on a non-100% signal) I've seen many MPEG2 TS transcodes (and rewraps without re-encoding) slip sync on errors.
Right. It's all in the transport stream, but not as a separate elementary stream inside a program or transport stream, which was my point. I think we're all agreed. I think it's important to understand the role of the transport stream in broadcasting, and how a TS and PS are different things for different delivery systems. And since many people were going on about "timecode" I thought it was important to recognize that the MPEG-2 TS doesn't use SMPTE time code data that's used in production (and sometimes in Master Control) to sync up different sources. Also, VideoReDo TVSuite is one of the few available programs that will correct sync problems in TS files, and convert them to something more computer-friendly, like a PS.

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