[time-nuts] an interesting timing problem

[jimlux]

On 5/6/20 7:33 AM, Chris Howard wrote:
> 
> 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.
> 
>

OK, but this is sort of a manual measurement.
Ditto for looping back a tone and using an oscilloscope.

What I was wondering about whether there's a way to do it in an 
automated way - fire up N webex/zoom/skype "conferences" solely for the 
purpose of testing.
For instance, this morning was a "very bad day" for Cisco's webex, and 
it would be interesting to collect performance statistics over time.

And to explore the nature of the anomalies - are they "packet drops", 
"resends", random delays, etc.

Much like the folks did in the early days of NTP development.



Some others had suggested ways of doing it "during a call" which is 
interesting.  I wonder if there is a way to intercept the video and 
audio traffic within the OS and "tee" it off to a analyzer.

Last time I looked at doing that kind of thing, I was foiled at every 
turn with Windows, because of their sensitivity to digital rights 
management - you don't want someone "tee"-ing copyrighted material to 
storage and redistributing.  I would expect that Mac OS X is no 
different.  It may well be that the conferencing applications don't 
bother to use the "protected content" capability. I know that folks at 
work have found ways to chromakey their webcam feed before feeding it 
into the Webex video input.