Can't get AudioStreamBasicDescription from capture output

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)}
}

@Volker sorry what do you mean by "does format exist"? and i would assume the samplebuffer is valid, since everything else works when i comment out the method call, is there something i should be checking for specifically? — justin shores, Jul 29 '15 at 07:42
have you checked if sampleBuffer and format are not nil / hold useful values? You are forcing format to exist by using ! ... — Volker, Jul 29 '15 at 08:36
@Volker i added the values help in format to the question, and samplebuffer is definitely not nil — justin shores, Jul 29 '15 at 09:04
@Volker thats what i thought too, is there another way to go about this? — justin shores, Jul 29 '15 at 18:37

score 1 · Answer 1 · answered Jan 20 '18 at 16:22

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;

Can't get AudioStreamBasicDescription from capture output

1 Answers1