Using CMSampleTimingInfo, CMSampleBuffer and AudioBufferList from raw PCM stream - ios

Question

I'm receiving a raw PCM stream from Google's WebRTC C++ reference implementation (a hook inserted into VoEBaseImpl::GetPlayoutData). The audio appears to be linear PCM, signed int16, but when recording this using an AssetWriter it saves to the audio file highly distorted and higher pitch.
I am assuming this is an error somewhere with the input parameters, most probably with respect to the conversion of the stereo-int16 to an AudioBufferList and then on to a CMSampleBuffer. Is there any issue with the below code?
void RecorderImpl::RenderAudioFrame(void* audio_data, size_t number_of_frames, int sample_rate, int64_t elapsed_time_ms, int64_t ntp_time_ms) {
OSStatus status;
AudioChannelLayout acl;
bzero(&acl, sizeof(acl));
acl.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;
AudioStreamBasicDescription audioFormat;
audioFormat.mSampleRate = sample_rate;
audioFormat.mFormatID = kAudioFormatLinearPCM;
audioFormat.mFormatFlags = kLinearPCMFormatFlagIsSignedInteger | kLinearPCMFormatFlagIsPacked;
audioFormat.mFramesPerPacket = 1;
audioFormat.mChannelsPerFrame = 2;
audioFormat.mBitsPerChannel = 16;
audioFormat.mBytesPerPacket = audioFormat.mFramesPerPacket * audioFormat.mChannelsPerFrame * audioFormat.mBitsPerChannel / 8;
audioFormat.mBytesPerFrame = audioFormat.mBytesPerPacket / audioFormat.mFramesPerPacket;
CMSampleTimingInfo timing = { CMTimeMake(1, sample_rate), CMTimeMake(elapsed_time_ms, 1000), kCMTimeInvalid };
CMFormatDescriptionRef format = NULL;
status = CMAudioFormatDescriptionCreate(kCFAllocatorDefault, &audioFormat, sizeof(acl), &acl, 0, NULL, NULL, &format);
if(status != 0) {
NSLog(#"Failed to create audio format description");
return;
}
CMSampleBufferRef buffer;
status = CMSampleBufferCreate(kCFAllocatorDefault, NULL, false, NULL, NULL, format, (CMItemCount)number_of_frames, 1, &timing, 0, NULL, &buffer);
if(status != 0) {
NSLog(#"Failed to allocate sample buffer");
return;
}
AudioBufferList bufferList;
bufferList.mNumberBuffers = 1;
bufferList.mBuffers[0].mNumberChannels = audioFormat.mChannelsPerFrame;
bufferList.mBuffers[0].mDataByteSize = (UInt32)(number_of_frames * audioFormat.mBytesPerFrame);
bufferList.mBuffers[0].mData = audio_data;
status = CMSampleBufferSetDataBufferFromAudioBufferList(buffer, kCFAllocatorDefault, kCFAllocatorDefault, 0, &bufferList);
if(status != 0) {
NSLog(#"Failed to convert audio buffer list into sample buffer");
return;
}
[recorder writeAudioFrames:buffer];
CFRelease(buffer);
}
For reference, the sample rate I'm receiving from WebRTC on an iPhone 6S+ / iOS 9.2 is 48kHz with 480 samples per invocation of this hook and I'm receiving data every 10 ms.

Answer 1

First of all, congratulations on having the temerity to create an audio CMSampleBuffer from scratch. For most, they are neither created nor destroyed, but handed down immaculate and mysterious from CoreMedia and AVFoundation.
The presentationTimeStamps in your timing info are in integral milliseconds, which cannot represent your 48kHz samples' positions in time.
Instead of CMTimeMake(elapsed_time_ms, 1000), try CMTimeMake(elapsed_frames, sample_rate), where elapsed_frames are the number of frames that you have previously written.
That would explain the distortion, but not the pitch, so make sure that the AudioStreamBasicDescription matches your AVAssetWriterInput setup. It's hard to say without seeing your AVAssetWriter code.
p.s Look out for writeAudioFrames - if it's asynchronous, you'll have problems with ownership of the audio_data.
p.p.s. it looks like you're leaking the CMFormatDescriptionRef.

Answer 2

I ended up opening up the audio file that was generated in Audacity and saw that every frame had half of it dropped, as shown in this rather bizarre looking waveform:
Changing acl.mChannelLayoutTag to kAudioChannelLayoutTag_Mono and changing audioFormat.mChannelsPerFrame to 1 solved the issue and now the audio quality is perfect. Hooray!