AS3. Duration of microphone recording less then original sound

Question

In my as3 application i capture sound input using Microphone class. But i faced with problem.

Duration of my recording less then original sound. The problem is easily reproduced if put a microphone device near the sound speakers.

I begin to play local mp3 using Sound class and put microphone samples into byte array. After that i save byte array with microphone samples into local file (raw data) and load this file to Audacity. Also i load to Audacity original mp3 file.

The picture shows two sound waves as it shows in the Audacity.

I expect to see in the Audacity two synchronized sound waves - record and original sound. But i see by the end of the recording out of sync increase. And at the end i see different durations.

And one more thing. I have 2 computers for my tests. This problem occurs on first computer if i initialize microphone with rate 44100, but if i set rate 22050 problem there is no. On the second computer is no problem at all - neither with a rate of 44100, nor with the rate 22050.

Does anyone had this problem? Why this occur?

Hope on your help, thanks!

Answer 1

This is an extremely tough thing to get right. There are a lot of variables at play - when the microphone starts recording, when playback begins, when they stop, etc. There's very little you can get back from the AS3 API that reveals start and stop times.

In general, you can usually count on the delay from when you ask playback to start and when sound actually comes out of your speakers to be greater than the delay of when you start listening for recorded samples. In your example above, you can see this. The problem isn't that the microphone recording is late, even though it appears that way. Instead, the problem is that the audio playback began later than the microphone, so the microphone recorded some relative silence at the beginning. Therefore, when you put the mic recording back up against the playback track, you see that silence first, then the recorded playback, giving the false impression that the mic was late.

On the other end, you have to contend with a very substantial delay between when the microphone picks up sound and when the SampleDataEvent arrives with those samples in Flash. When you get that event, you might be getting the samples that were recorded quite a while before - maybe a half second or more even. What this means is that if you stop the recording and the playback at the same time, you're going to chop off any samples from the microphone that haven't arrived yet, even though they were from the actual time that the audio was playing. Your best bet here is to NOT cut the microphone off at the same time as playback, but rather keep checking the length of your total recorded bytes in that SampleDataEvent handler, and when it is finally as long as the sound file (possibly plus an amount allowing for the play delay discussed before), stop it.