Pictured above is my test setup in a makeshift sound booth on the extreme opposites side of my house. (I have no idea why I own a 65 foot xlr cable)

One thing that my Bluetooth wireless audio experiments taught me is that latency can be a real pain. Thankfully audio for video is much more forgiving in terms of latency, then the needs of musicians. I’ve found that about 36 to 45 milliseconds is when people start to notice audio being out of sync with the talent in a given peace of footage. Where as some musicians start to complain if the latency is more then 20 milliseconds. (skip down to the next photo if you don’t want to read the technical stuff)

The reason I bring this up is that I’ve been taking measurements on the latency produced by the Jangus Wi wireless transmitter. When I first started looking into this unit I found forum post from musicians complaining about the latency issues when using the Jangus Wi as a wireless audio monitoring system. Most of these complaints seem to center around the Jangus Wi being plugged into a mixer at the back of the venue. Audio was then transmitted back to the musician’s headphones on stage.

There is a glaring problem to this sort of scenario. The musician is only a few feet from the audio source and wearing headphones from another audio source. Unless the headphones completely remove all outside sound from their ears, he or she will be able to hear both the speakers audio output and the headphones audio output at the same time. Lets call this “relative latency”, the time between receiving one audio signal and another.

The reason I use the term relative is because you need something to compare or relate the audio to in order to perceive a delay. If you have no point of reference, you have no way to tell if the audio is out of sync. In the musicians case, the point of reference is the speaker he stands next to on stage. Any audio that comes to him after the audio received by the speaker will sound delayed.

In the case of film, the viewers only point of reference is the actors mouth movements and body language. These cues are much more forgiving because your point of reference is more ambiguous then a direct audio reference.

There is also the matter of resolution. One frame of 24fps video (24p) lasts for about 41.6 milliseconds, and 0ne frame of 29.97fps (30p) lasts about 33.4 milliseconds. That means if your audio is delayed by less then 33 milliseconds you’ll still be within a single frame of your video playback.

Alright that’s enough about latency, lets look at how the Jangus Wi preformed. For this test I used a 65 foot XLR cable plugged into a field recorder on the bottom channel (above) and the Jangus Wi wireless system fed by the Rode VideoMic Pro on the top channel (above). I used the expanded waveform feature in Sound Forge to measure from the trough of one waveform to the identical trough of the other. As you can see above in red you have a delay of 25 milliseconds.

In the tests I’ve run so far, it seem that the latency decreases as you decrease the distance between units which makes sense. At about 4 feet I’ve been getting around 8 to 13 milliseconds with 25 milliseconds (at 65 feet) being the max I could measure without making a new cable. I’ve provided the 65 foot test for download so you can get an idea of what the relative latency sounds like in, what I would consider to be the the worse case scenario.

Jangus Wi relative latency test at 65 feet download (mp3)

Take a listen to the test and see what you think. I could see this being a problem for musicians but for video work I think the slight delay is more then acceptable. Also note that I don’t normally spend a lot of time measuring latency, so if anyone has a more precise method of measurement let me know, I’d be more then happy to give it a try.

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