This discussion focuses on interactive audio/data streams. One-way streams are treated differently, since delay is less important.
Media transport typically uses IPv4/6 networks and can often be captured at one of the endpoints or somewhere in the network path. E.g., I captured a Skype call today on my PC using Wireshark. Whether this IP capture can be converted back to audio/video depends on whether the packets have been encrypted and requires detailed knowledge of the encoding used. It is common to digitize audio using a standard vocoder (for wireless, these include EVRC, AMR, AMR-WB) and then transport it using RTP headers and UDP/IP. I'll describe 4G voice (voice over LTE or VoLTE), as I'm most familiar with that technology. There are several delay contributors in the end-to-end path: - (source) Collecting digitized voice for an interval. In 2G/3G/4G wireless 20 msec is commonly used. - (source) Compressing the 20 msec of data . This data is stuffed into an RTP/UDP/IP packet - IP transport of the compressed voice to the receiver. This is unreliable: RTP packets can be dropped, duplicated, missequenced or corrupted. - (Receiver) Dejitter buffer (or jitter compensation buffer ...) - (Receiver) Error concealment The dejitter buffer adds variable delay dependent on the network and network conditions. I've seen it add 10's to 100's msec delay in VoLTE. One-way latency is sometimes called "mouth to ear delay" and can be measured in the analog domain with commercial gear; we used equipment from GL Communications. Time-stamped IP packet captures can also be helpful, but doesn't show what happens within the receiver. On Wed, May 6, 2020 at 2:44 PM jimlux <[email protected]> wrote: > 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. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
