Android Getting distance using sound between two devices

Question

The idea is Phone A sends a sound signal and bluetooth signal at the same time and Phone B will calculate the delay between the two signals.

In practice I am getting inconsistent results with delays from 90ms-160ms. I tried optimizing both ends as much as possible.

On the output end:
Tone is generated once Bluetooth and audio output each have their own thread
Bluetooth only outputs after AudioTrack.write and AudioTrack is in streaming mode so it
should start outputting before the write is even completed.

On the receiving end:
Again two separate threads
System time is recorded before each AudioRecord.read

Sampling specs:
44.1khz
Reading entire buffer
Sampling 100 samples at a time using fft
Taking into account how many samples transformed since initial read()

Answer 1

Your method relies on basically zero latency throughout the whole pipeline, which is realistically impossible. You just can't synchronize it with that degree of accuracy. If you could get the delays down to 5-6ms, it might be possible, but you'll beat your head into your keyboard before that happens. Even then, it could only possibly be accurate to 1.5 meters or so.

Consider the lower end of the delays you're receiving. In 90ms, sound can travel slightly over 30m . That's the very end of the marketed bluetooth range, without even considering that you'll likely be in non-ideal transmitting conditions.

Here's a thread discussing low latency audio in Android. TL;DR is that it sucks, but is getting better. With the latest APIs and recent devices, you may be able to get it down to 30ms or so, assuming you run some hand-tuned audio functions. No simple AudioTrack here. Even then, that's still a good 10-meter circular error probability.

Edit:

A better approach, assuming you can synchronize the devices' clocks, would be to embed a timestamp into the audio signal, using a simple am/fm modulation or pulse train. Then you could decode it at the other end and know when it was sent. You still have to deal with the latency problem, but it simplifies the whole thing nicely. There's no need for bluetooth at all, since it isn't really a reliable clock anyway, since it can be regarded as having latency problems of its own.

Answer 2

This gives you a pretty good approach http://netscale.cse.nd.edu/twiki/pub/Main/Projects/Analyze_the_frequency_and_strength_of_sound_in_Android.pdf

You have to create an 1 kHz sound with some amplitude (measure in dB) and try to measure the amplitude of the sound arrived to the other device. From the sedation you might be able to measure the distance.

As I remember: a0 = 20*log (4*pi*distance/lambda) where a0 is the sedation and lambda is given (you can count it from the 1kHz) But in such a sensitive environment, the noise might spoil the whole thing, just an idea, how I would do if I were you.