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Author Topic: Realtime latency measurement  (Read 1774 times)

Andrew Broughton

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Realtime latency measurement
« on: December 11, 2020, 02:00:39 pm »

Anyone know mathematically how, given 2 audio sources containing the same signal, but one delayed from the other by a continuously changing amount (think streaming over WiFi or Internet), you would continuously calculate the exact latency in real time?
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David Sturzenbecher

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Re: Realtime latency measurement
« Reply #1 on: December 11, 2020, 03:59:11 pm »

Anyone know mathematically how, given 2 audio sources containing the same signal, but one delayed from the other by a continuously changing amount (think streaming over WiFi or Internet), you would continuously calculate the exact latency in real time?

Can you just run them into Smaart and have the tracker turned on?
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Dave Garoutte

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Re: Realtime latency measurement
« Reply #2 on: December 11, 2020, 05:21:29 pm »

Embed some timecode?
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Scott Holtzman

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Re: Realtime latency measurement
« Reply #3 on: December 11, 2020, 08:42:26 pm »

Anyone know mathematically how, given 2 audio sources containing the same signal, but one delayed from the other by a continuously changing amount (think streaming over WiFi or Internet), you would continuously calculate the exact latency in real time?


I would need to brush up a bit, however yes I thought that was what time code was for.  Stable local clock (GPS is easiest, we used to use TV color sync in old days to align simulcast transmitters) plesiochronous time sync is then very "doable" on asynchronous connections.  The buffering can then be synced to the nodal clocks at the source.
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Frank Koenig

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Re: Realtime latency measurement
« Reply #4 on: December 13, 2020, 11:54:40 am »

Anyone know mathematically how, given 2 audio sources containing the same signal, but one delayed from the other by a continuously changing amount (think streaming over WiFi or Internet), you would continuously calculate the exact latency in real time?

I can guess at what problem you're trying to solve and, as others have noted, timecode is likely the practical answer, if it's available.

More generally, a cross-correlation between two signals can provide an estimate of the time difference and will give an exact answer for identical, noise-free (and not otherwise pathological, like DC) signals with a constant time difference. If the time difference is variable, as you stated, this becomes a hard problem, the tractability of which depends on the spectrum of the signal in relation to the spectrum of the delay variation. Intuitively, the cross-correlation needs a long enough segment of the signal to "grab onto", but if the time difference varies appreciably during that segment then the cross-correlation is a poor estimate. It works if the time difference changes slowly compared with the frequency content of the signal.

If the spectra are well enough behaved for cross-correlation to be useful, then I believe that the computation can be done in real time in the formal sense, meaning that one can place a finite upper bound on the time taken by the computation. To be clear, real time does not mean instantaneous. It takes time (latency) to determine latency. Causality sucks...

--Frank
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Andrew Broughton

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Re: Realtime latency measurement
« Reply #5 on: December 13, 2020, 12:32:10 pm »

Can you just run them into Smaart and have the tracker turned on?
Been a very long time since I looked at SMAARY. Would that work continuously in real-time? I thought it only calculated the delay when you used the delay finder?
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-Andy

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Andrew Broughton

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Re: Realtime latency measurement
« Reply #6 on: December 13, 2020, 12:34:12 pm »

Embed some timecode?
Can timecode be imbedded inaudibly? Think audio going to and coming from a source that you have no control over, and it only accepts regular aac audio.
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-Andy

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Andrew Broughton

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Re: Realtime latency measurement
« Reply #7 on: December 13, 2020, 12:36:43 pm »

I can guess at what problem you're trying to solve and, as others have noted, timecode is likely the practical answer, if it's available.
It's not.

Quote
More generally, a cross-correlation between two signals can provide an estimate of the time difference and will give an exact answer for identical, noise-free (and not otherwise pathological, like DC) signals with a constant time difference. If the time difference is variable, as you stated, this becomes a hard problem, the tractability of which depends on the spectrum of the signal in relation to the spectrum of the delay variation. Intuitively, the cross-correlation needs a long enough segment of the signal to "grab onto", but if the time difference varies appreciably during that segment then the cross-correlation is a poor estimate. It works if the time difference changes slowly compared with the frequency content of the signal.

If the spectra are well enough behaved for cross-correlation to be useful, then I believe that the computation can be done in real time in the formal sense, meaning that one can place a finite upper bound on the time taken by the computation. To be clear, real time does not mean instantaneous. It takes time (latency) to determine latency. Causality sucks...

--Frank

This is what I'm talking about. Any links to the formulas or programming needed to do this sort of calculation? I think if it gets it right more often than not, it might be workable.
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Mac Kerr

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Re: Realtime latency measurement
« Reply #8 on: December 13, 2020, 12:53:44 pm »

Been a very long time since I looked at SMAARY. Would that work continuously in real-time? I thought it only calculated the delay when you used the delay finder?

Been a long time for me as well, but the last I looked, yes the SMARRT delay finder can run continuously, and displays the varying latency.

Mac
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David Sturzenbecher

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Re: Realtime latency measurement
« Reply #9 on: December 13, 2020, 02:03:00 pm »

Been a long time for me as well, but the last I looked, yes the SMARRT delay finder can run continuously, and displays the varying latency.

Mac

Yes, it can run continuously and will show the changing delay. However, it does need signal to work, so it may go wild with no signal...like in between songs...I havenít tested this though. There is a free demo if you wanted to try it out.  There is also an API available on request, so if you wanted to write some code to repackage just this portion you could.
« Last Edit: December 13, 2020, 02:39:13 pm by David Sturzenbecher »
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Re: Realtime latency measurement
¬ę Reply #9 on: December 13, 2020, 02:03:00 pm ¬Ľ


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