[time-nuts] an interesting timing problem

[Chris Howard]

At my current job we were looking into delay timings of video systems.

We were doing end-to-end measurement by putting a time display in front 
of a monitor
and have the camera show both the time display and the monitor.
It looks a bit like the old infinite mirror.
If you arrange things right it shows two images of the time display
one that lags the other.  And the difference is the round-trip delay.

When I'm on Skype and my co-worker shares his screen, I can see my own
camera image come back to me in a similar way.

So if I were to point my camera at a rolling time display, he shares
that image back to me, I could take video (with a second camera)
of both the live time display and the delayed.

The particular system we were measuring ended up having a multi-frame delay
due to the video codec having an optional frame re-ordering feature which
required buffering a group of frames at startup and carrying that delay 
forward.
We found out how to turn that off.


Chris



On 5/6/20 9:00 AM, jimlux wrote:
> Given that there's a lot more people spending time zooming, webexing, 
> teaming, skype, facetime, etc. these days, I'm curious if anyone has 
> figured out to *quantify* the issues of lag, desynchronization, etc.
>
> How would one go about instrumenting it (without access to the source 
> code or servers involved)?
>
> There's two areas of some interest to me:
> 1) there's several studies that say that when voice and image aren't 
> perfectly synchronized, particularly if it's not a consistent delay, 
> or if there are gaps and jumps, that it is more stressful and creates 
> a cognitive workload that does not exist with actual in-person 
> meetings (the "why am I more tired after a day of telework than the 
> real thing")
>
> 2) If you wanted to do group music playing or singing, relative timing 
> among the streams is critical.  Is there a threshold where it all 
> breaks down?  For instance, in an orchestra or choir, one has visual 
> cues from the conductor, but most people do not sing or play using the 
> conductor as a metronome triggering the next measure's notes. They 
> also listen to the players around them (or perhaps on the other side 
> of the stage, some 30-40 milliseconds late)
>
>
> I can think of ways to "test" a given teleconferencing system 
> (blinking LEDs in a pattern, tone bursts on audio), but I think 
> there's some challenges in things like compression algorithms (do they 
> have constant latency?) and highly structured test signals might not 
> measure the same as actual video and audio.
>
> I will note that there are subjective difference among the various 
> tools, and there's differing effects from compression artifacts and 
> bandwidth/packet transport.
>
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