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Sound and picture out of synch?

N

nocomradio

Guest
I've noticed on occasion, that when watching TV, and this seems to be later at night, sometimes the sound is out of synch with the picture. Its very noticeable when someone is speaking as their lips and the sound don't match. I used to see this on DirectTv when I had that a few years back, and I still see it on OTA TV now. Is this a byproduct of digital television? Is the sound still broadcast on a separate carrier like the old days? What causes this?
 
nocomradio said:
I've noticed on occasion, that when watching TV, and this seems to be later at night, sometimes the sound is out of synch with the picture. Its very noticeable when someone is speaking as their lips and the sound don't match. I used to see this on DirectTv when I had that a few years back, and I still see it on OTA TV now. Is this a byproduct of digital television?
Click to expand...

Yes.

Is the sound still broadcast on a separate carrier like the old days?
Click to expand...

No.

What causes this?
Click to expand...

If we knew, we'd fix it ;)

_________________________________________________

Seriously:

In a way, digital television isn't television at all. It's the broadcast of a series of 188-byte packets of data. That data may consist of bits of digitized video, bits of digitized audio, program guide information, information about the audio & video packets needed by the receiver to reassemble them, even data that has nothing whatsoever to do with TV programs.

All of which is transmitted over a single carrier. Which means that the picture of someone firing a gun is not sent at the same time as the audio bang of that gun going off.

The audio and video packets do include a "presentation time stamp" (PTS) which basically tells the receiver "sound packet #20652 goes with video packet #43125, play them at the same time".

I think in most cases "lip sync" issues arise *before* the point where the PTS is generated; the video is passed through a piece of processing gear that delays it, while the audio goes straight through. At one time our station had to install a device to intentionally delay the audio to make up for the delay in a video titling device.
 
It's not uncommon for the audio to slip out of sync with the video after being tuned to a particular channel for several hours, like after a night of viewing.
Some sets do better than others, with time-stamping.
You just have to occasionally tune to a different station for a moment.
 
Could this not also be an issue of audio and video being encoded differently - that is to say, if the video is encoded as an MP4 stream, but the audio is an AAC stream at 44.1 K? I know as a video editor, I've sometimes had issues with video footage that contained audio encoded at a different sample rate.
 
I have noticed this somewhat regularly on HD broadcasts of my local stations on DirecTV.
Tuning away and then back does not appear to fix it, so I presume the problem is with
their stream and not my set. It always goes away, eventually.
 
Thank you for all the replies!

The technical side of digital TV is something I haven't really begun to understand yet, so its good to get some perspective and insight.

The length the TV is on (in my case) doesn't seem to be as much an issue as the time of the evening. During the day, I never seem to notice it, so I did wonder if the signal may be affected by weather, or atmospheric conditions as well. And yes, sometimes its very noticeable to the point of almost being hard to watch. Especially with live news or something like that.
 
I've noticed it at times with certain channels on Charter, like WMC and WKNO in Memphis, and also on the Weather Channel at times, but it comes and goes.
 
nocomradio said:
The length the TV is on (in my case) doesn't seem to be as much an issue as the time of the evening. During the day, I never seem to notice it, so I did wonder if the signal may be affected by weather, or atmospheric conditions as well. And yes, sometimes its very noticeable to the point of almost being hard to watch. Especially with live news or something like that.
Click to expand...

No, it's not a propagation thing -- because all that's being transmitted is a series of data packets, each of which is affected equally by any atmospheric delays.
 
w9wi said:
No, it's not a propagation thing -- because all that's being transmitted is a series of data packets, each of which is affected equally by any atmospheric delays.
Click to expand...

In that case then, I'd have a pixelization issue to go along with the sound issue?

When I do have dropouts with the digital TV, the sound sort of goes into a skipping CD mode and the picture becomes pixelized.
 
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