# 2 TVs, not always in sync



## puzzley (Aug 25, 2006)

We have a large flat acreen in the den, and a small flat screen on a wall mount in the kitchen. HD recorder HR-22 in the den, H-23 receiver in the kitchen. Whole home dvr service. 

You can see and hear the smaller tv in the kitchen from the den, and on some channels they are not in sync. Very distracting when the small one in the kitchen is running ahead of the larger one in the den. As an example, I had headline news on both, and they were perfectly in sync. I changed both to USA and the small one in the kitchen was immediately about 1-2 seconds ahead of the one in the den. 

Anyone have any insight on the issue?


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## trh (Nov 3, 2007)

I believe the DVR is recording to a buffer which puts it slightly behind the non-DVR receiver.


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## veryoldschool (Dec 10, 2006)

puzzley said:


> We have a large flat acreen in the den, and a small flat screen on a wall mount in the kitchen. HD recorder HR-22 in the den, H-23 receiver in the kitchen. Whole home dvr service.
> 
> You can see and hear the smaller tv in the kitchen from the den, and on some channels they are not in sync. Very distracting when the small one in the kitchen is running ahead of the larger one in the den. As an example, I had headline news on both, and they were perfectly in sync. I changed both to USA and the small one in the kitchen was immediately about 1-2 seconds ahead of the one in the den.
> 
> Anyone have any insight on the issue?


There is a group delay that may differ with every device.
The DVRs do have more than the receivers, "but" even when you have the same model DVR, tuned to the same channel, if the TVs are different, you'll still hear a slight delay on one of them.
What can sometimes be very funny is when I'm using the same TV and start with the TV OTA local HD, then switch to my H25, and then to my HR24. I hear the same part of the program THREE times. :lol:


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## puzzley (Aug 25, 2006)

Thanks for the replies. Strange, though, that some channels, (Headline News, for example), are perfectly syncronized...


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## TomCat (Aug 31, 2002)

Typically, there is no buffer delay on DTV receivers. It only comes into play if you use trick play; if you have not done that once tuning the channel, the buffer is not in play.

What causes this discrepancy between STBs is the fact that MPEG is somewhat elastic. It speeds up and slows down imperceptibly, but over time this can cause a significant delay. And the MPEG decoder is also somewhat elastic; it is sophisticated enough so that it can make intelligent decisions about how it decodes, sometimes even differently than an identical decoder in an identical STB. It is not uncommon for two receivers to decode the same stream one slightly ahead of the other, especially since one box may be just decoding one easy to decode channel while another may be decoding two channels and doing background tasks at the same time, all (except for the decoding itself) on one CPU.

This is also why audio gets out of sync with video, and why a channel up/down will fix (reclock) it. In broadcasting we use bit-bucket server delays for delaying live programming to air at another time (usually an hour or two later due to time zone issues). We found that if we left the delay running all night, that the audio would be as much as 7 seconds ahead of the video the next day. The reason is that when the sat feed we were delaying went off line, the analog sat went to snow, which is something extremely difficult for MPEG to encode due to its random quick changes. So the encoder slowed down, dropped a frame or so every so often. The audio, which IS NOT elastic like MPEG is, marched right along at the normal pace it always does. The end result is audio ahead of video, specifically due to the nature of MPEG encoding/decoding.


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## TomCat (Aug 31, 2002)

puzzley said:


> Thanks for the replies. Strange, though, that some channels, (Headline News, for example), are perfectly syncronized...


That depends on whether you mean HLN is perfectly synchronized video to audio, or from STB to STB. If STB to STB, that is simply coincidental. It is vulnerable to the same issues all DBS channels are vulnerable to.

Audio to video, well, that is another story. Folks here might remember the dark days of early HD broadcasts from cable/sat channels (which DTV gets credit for starting, actually). Many channels had permanent audio delay issues. Being a relatively new medium, the delays were not anticipated. And to correct them, they each had to budget for delay modules to correct the delay. And there usually is a "delay" between the time you budget for equipment and the point where you get to put it on line, so there was 3 to 9 months of horrible lipsync issues on many of the new DBS HD channels at first.


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## veryoldschool (Dec 10, 2006)

TomCat said:


> Typically, there is no buffer delay on DTV receivers. It only comes into play if you use trick play; if you have not done that once tuning the channel, the buffer is not in play.
> 
> What causes this discrepancy between STBs is the fact that MPEG is somewhat elastic. It speeds up and slows down imperceptibly, but over time this can cause a significant delay. And the MPEG decoder is also somewhat elastic; it is sophisticated enough so that it can make intelligent decisions about how it decodes, sometimes even differently than an identical decoder in an identical STB. It is not uncommon for two receivers to decode the same stream one slightly ahead of the other, especially since one box may be just decoding one easy to decode channel while another may be decoding two channels and doing background tasks at the same time, all (except for the decoding itself) on one CPU.
> 
> This is also why audio gets out of sync with video, and why a channel up/down will fix (reclock) it. In broadcasting we use bit-bucket server delays for delaying live programming to air at another time (usually an hour or two later due to time zone issues). We found that if we left the delay running all night, that the audio would be as much as 7 seconds ahead of the video the next day. The reason is that when the sat feed we were delaying went off line, the analog sat went to snow, which is something extremely difficult for MPEG to encode due to its random quick changes. So the encoder slowed down, dropped a frame or so every so often. The audio, which IS NOT elastic like MPEG is, marched right along at the normal pace it always does. The end result is audio ahead of video, specifically due to the nature of MPEG encoding/decoding.


I know you know "a fair amount", but this post suggests/points to things that may not be as significant as the mere difference between the H and HR, which do have fairly significant delay difference.
Anytime you're in the area, please stop by and I'll let you see for yourself what these are and let you run any kind of setups/tests so you see it for yourself.


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## puzzley (Aug 25, 2006)

I'd like to clear somthing up, here. I'm NOT talking about audio and video being out if sync at all on either tv. What is happening is that while audio and video do not have a sync problem on either tv, at times the tv in the kitchen is running everything, audio and video, about two seconds ahead of the tv in the den. It seems to be different on different channels. Again, I can switch both TVs back and fourth between Headline News, (both TVs exactly in sync WITH EACH OTHER) and the USA network, (TV in the kitchen runs the entire program about two seconds ahead of the one in the den) to see the difference.

Thanks....


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