2.4.2. How to record multiple audio devices simultaneously (in order to measure latency)


I would like to measure the latency of some audio. I have the original audio and the delayed audio on different sound devices (background: I am measuring the signal time delay from the NIST time signal on the radio spectrum between a traditional receiver and an SDR receiver).

How can I setup Audacity to record 2 tracks, from 2 different sound devices at the same time?


Ideally you would use a mixing desk. Pan one signal all the way to the left and the other all the way to the right, then record in 2 channel stereo. When the recording is complete you can split the stereo track into two mono tracks: https://manual.audacityteam.org/man/splitting_and_joining_stereo_tracks.html

I have the original audio and the delayed audio on different sound devices

Already recorded? If you already have the work digitally recorded on two separate sound devices, you’re stuck. The sound mixing desk trick only works when you’re measuring the different between two live, real-time sound streams.

It doesn’t take much of a desk, either. A small Behringer 802 will do it.


If you’re good with fudging and you don’t need perfect, absolute accuracy, twist the two wires together (Y cable) and record the two as one signal. Use the Audacity magnifier and sort the difference between the two signals. If this is one of the services that periodically stops to announce the service, so much the better. You’ll get an easily discernible echo between the two voices.

Even more hardware-free possibility. If these are both radios with speakers, run them both at the same time and put your phone on recorder in the middle. Suck the sound file into Audacity and analyze the echo.


I can hear you objecting to the apparent sloppiness of the method.

By the time the radio signals get done bouncing around the Kennelly–Heaviside layer you’d be lucky to get “real” time to the nearest eighth second.

The SDR receivers have their own problems. The minute you transition between digital and analog (either direction) you can get a delay. You have to also regard the pathway. Any hint of internet connection can give you delays and they can change.

Time measurement can be entertaining. We got a complaint that our clocks in Washington were off time with New York. We responded our clocks were on perfect time with the Naval Observatory Master Clock roughly 2 miles away. We asked which clock they were using.


Or better, the 802USB.


Our favorite UCA-202 can do this.

If you can get the two signals into RCA connection form. Put one signal on the left and one on the right.