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Cat5/6 or Coax for extended video?

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Brad Weber:

--- Quote from: Jonathan Johnson on February 28, 2011, 08:18:12 PM ---This has me wondering about something that I haven't taken the time to research (yet): with digital broadcast, what is the latency?

That is, let's say I'm attempting to run live video to an overflow area using HD, and doing it using modulators, splitters, and what have you. What is the best way of delivering the audio: via the digital signal, or a direct feed from the sound board?

Since the audio from the sound board is essentially a pure analog signal and any latency is introduced solely by the speed of light and length of the cable (read: immeasurably low), and digital tends to have measurable delay due to buffering, will the video be noticeably asynchronous with the audio?

OR, should the audio in the overflow room come from the video feed, ensuring synchronicity at the possible expense of degraded audio? (Is the audio quality even a concern?)

Just some thoughts. Yours?

--- End quote ---
Any added latency in the transmission aspect might also depend on the signal formats involved, for example if there is a D/A conversion out of the video system and then an A/D conversion into the transmission system versus a straight digital path.  However, chances are pretty good that any live video or video through a production system may already have some noticeable latency, so that could be an issue even without consdering any transmission latency.

I can see that it could be desirable to add some delay to the remote audio signal either prior to transmission or at the receiving end.  I often include such signals in the system processing so that delay, limiting, etc. can be applied.  Having a digital board in the overflow room with delay on each input would also be a nice solution!  ;D

A bit off topic, but the same issues can apply to other sends such as ALS and interpretation.  It is often desirrable to add some delay to those signals to both better sync with the video images and to reflect the natural delay that would occur for someone sitting out in the middle of the listener space.

Brian Ehlers:
Just to expand on Brad's comments a bit:

The human brain is quite tolerant of audio lagging video, because that's what we're used to seeing in real life.  But the brain is extremely intolerant of the video lagging the audio even a little, because it's so unnatural.

Even if you use the audio which is transported with the video, there's no guarantee that it will be synchronized.  Every piece of equipment is different.  And since audio codecs and processors typically use smaller, shorter frame sizes than video codecs and processors, there's a good chance the final video will be later than the final audio -- exactly what you don't want.  For this reason I'd always design a system so that I can add more delay to the audio somewhere.

Since overflow seating is often within earshot of the main room, I like to delay the audio to the overflow seating even if there is no video.  That way the bleed from the main room is perceived as a little extra ambience, not a late echo.

Kevin Hoober:

--- Quote from: Jonathan Johnson on February 28, 2011, 08:18:12 PM ---This has me wondering about something that I haven't taken the time to research (yet): with digital broadcast, what is the latency?

That is, let's say I'm attempting to run live video to an overflow area using HD, and doing it using modulators, splitters, and what have you. What is the best way of delivering the audio: via the digital signal, or a direct feed from the sound board?

Since the audio from the sound board is essentially a pure analog signal and any latency is introduced solely by the speed of light and length of the cable (read: immeasurably low), and digital tends to have measurable delay due to buffering, will the video be noticeably asynchronous with the audio?

OR, should the audio in the overflow room come from the video feed, ensuring synchronicity at the possible expense of degraded audio? (Is the audio quality even a concern?)

Just some thoughts. Yours?

--- End quote ---

For simplicity's sake, I'd use the audio from the modulator (it is quite good as long as you watch your levels).  There is a bit of latency in the digital modulators (I only have experience w/ CR's digital modulators)--150-200mS, maybe--but they do a good job of keeping audio and video synced together.  (that's not to say your tuner will do the same--as Brian alluded to; adjustable audio delay is a good thing)

If you ran audio separately, you'd definitely need to delay the audio back to the video.

Kevin H.

Jonathan Johnson:

--- Quote from: Kevin Hoober on March 03, 2011, 05:20:30 PM ---For simplicity's sake, I'd use the audio from the modulator (it is quite good as long as you watch your levels).  There is a bit of latency in the digital modulators (I only have experience w/ CR's digital modulators)--150-200mS, maybe--but they do a good job of keeping audio and video synced together.  (that's not to say your tuner will do the same--as Brian alluded to; adjustable audio delay is a good thing)

If you ran audio separately, you'd definitely need to delay the audio back to the video.

Kevin H.

--- End quote ---

It seems to me that it would be wise to have the delay box at the location of the remote video monitor so you could visually sync it.

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