Question

In iPhone SDK 4.3 I would like to record what is being played out through speaker via Remote IO and also record the mic input. I was wondering if the best way is to record each separately to a different channel in an audio file. If so which apis allow me to do this and what audio format should I use. I was planning on using ExtAudioFileWrite to do the actual writing to the file.

Thanks

Answer 1

If both tracks that you have is mono, 16bit integer with the same sample rate:

format->mFormatFlags = kLinearPCMFormatFlagIsSignedInteger | kLinearPCMFormatFlagIsPacked;
format->mBitsPerChannel = 16;

you can combine those tracks to the 2 channels PCM by just alternating sample from one track with sample from another.

[short1_track1][short1_track2][short2_track1][short2_track2] and so on.

After that you can write this samples to the output file using ExtAudioFileWrite. That file should be 2 channel kLinearPCMFormatFlagIsSignedInteger | kLinearPCMFormatFlagIsPacked of course.

If one of tracks is stereo (I don’t think that it is reasonable to record stereo from iphone mic), you can convert it to the mono by taking average from 2 channels or by skipping every second sample of it.