Can't get AudioStreamBasicDescription from capture output - video-capture

Question

I'm trying to access mSampleRate and mChannelsPerFrame and assign the values to global variables.
Method:
func setAudioFormat(format: CMFormatDescriptionRef) {
let asbd: UnsafePointer<AudioStreamBasicDescription> = CMAudioFormatDescriptionGetStreamBasicDescription(format)
sampleRate = asbd.memory.mSampleRate // breakpoint
channels = asbd.memory.mChannelsPerFrame
}
Method Call:
func captureOutput(captureOutput: AVCaptureOutput!, var didOutputSampleBuffer sampleBuffer: CMSampleBuffer!, fromConnection connection: AVCaptureConnection!) {
...
let format: CMFormatDescriptionRef = CMSampleBufferGetFormatDescription(sampleBuffer)!
self.setAudioFormat(format)
...
}
am I doing something wrong? is there a better way to get AudioStreamBasicDescription from capture output samplebuffer
Edit:
format is holding these values:
<CMAudioFormatDescription 0x14516150 [0x346c08a0]> {
mediaType:'soun'
mediaSubType:'lpcm'
mediaSpecific: {
ASBD: {
mSampleRate: 44100.000000
mFormatID: 'lpcm'
mFormatFlags: 0xc
mBytesPerPacket: 2
mFramesPerPacket: 1
mBytesPerFrame: 2
mChannelsPerFrame: 1
mBitsPerChannel: 16 }
cookie: {(null)}
ACL: {(null)}
}
extensions: {(null)}
}

Answer 1

Once you have a CMFormatDescriptionRef instance, you can use this code (in Objective-C, sorry) to retrieve the ASBD data:
const AudioFormatListItem *audioFormatListItem = CMAudioFormatDescriptionGetFormatList(formatDescription, nil);
AudioStreamBasicDescription asbd = audioFormatListItem->mASBD;
float sampleRate = asbd.mSampleRate;