Measuring a ’12 Nexus 7’s microphone to speaker latency

The recent release of patchfield for Android made me wonder whether Android’s audio system has developed sufficiently over the last three years to be suitable for real time signal processing. There have been a couple of developers at google working hard to reduce the output latency but their work does not yet encompass input latency.

However since patchfield allows the speaker to be digitally connected to the microphone it is easy to use patchfield itself to measure input to output latency of an Android system.

The following picture shows the test set up I used to measure it.

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On the left is my 2012 Nexus 7. This is connected to the earphones sitting in top of the (red) sound card. Using earphones prevents feedback between the mic and speaker. Finally a microphone rests on top of the earphones. Critically the microphone picks up both the environment and the output of the earphones.

If you are particularly eagle eyed you might have notices a few missing cables in the above picture. Specifically the earphones are not actually plugged into the Nexus and the microphone is not plugged into the soundcard! I haven”t faked the photo however I only decided to blog about it after I had starting taking things apart again. In may haste to get a picture of it I forgot to plug it all back together.

Latency is measured simply by knocking in the table surface. This creates a short burst of noise that is captured both by the microphone in the picture and by the Nexus’ internal microphone. Patchfield captures the sound and replays it to the earphone which the microphone also picks up. The latency can then by calculated by looking at the waveform of the signals picked up by the microphone.

And the results are… 132ms from input to output. Just to put this number into perspective a piano, which is one of the highest latency musical instruments, takes about 25ms from the start of a keypress to the hammer striking a string. Another way to express it is that it is way too much for any musical application. The brain doesn’t just hear it as an echo… it hears it as a LOOONNNGG echo!

I repeated the same test on my Nexus 4 and that was better but not by enough to make much difference. That “only” took 124ms.

I guess there remains lots of very good reasons by iRig is not available for Android. There is one remaining ray of hope: the latest Samsung Mobile SDK includes a professional audio API based on jack and dedicated to handling low latency. It looks like Samsung really do want to take the fight to apple.

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