AVAssetReader and Audio Queue streaming problem

问题:

I have a problem with the AVAssetReader here to get samples from the iPod library and stream it via Audio Queue. I have not been able to find any such example so I try to implement my own but it seems that somehow the AssetReader is "screwed up" at the callback function of audio queue. Specifically it fails when it does the copyNextSampleBuffer ie it returns null when it is not finished yet. I have made sure the pointer exists and such so it will be great if anyone can help.

Below is the callback function code that I have used. This callback function 'works' when it is not called by the AudioQueue callback.

    static void HandleOutputBuffer (
        void                *playerStateH,
        AudioQueueRef       inAQ,
        AudioQueueBufferRef inBuffer
        ) {

 AQPlayerState *pplayerState = (AQPlayerState *) playerStateH;
 //if (pplayerState->mIsRunning == 0) return;    



 UInt32 bytesToRead = pplayerState->bufferByteSize;
 [[NSNotificationCenter defaultCenter] postNotificationName:NOTIF_callsample object:nil];


 float * inData =(float *) inBuffer->mAudioData;
 int offsetSample = 0;
 //Loop until finish reading from the music data
 while (bytesToRead) {

  /*THIS IS THE PROBLEMATIC LINE*/
  CMSampleBufferRef sampBuffer = [pplayerState->assetWrapper getNextSampleBuffer]; //the assetreader getting nextsample with copyNextSampleBuffer

  if (sampBuffer == nil) {
   NSLog(@"No more data to read from");
 //  NSLog(@"aro status after null %d",[pplayerState->ar status]);
   AudioQueueStop (
       pplayerState->mQueue,
       false
       );
   pplayerState->mIsRunning = NO;

   return;
  }

  AudioBufferList  audioBufferList;
  CMBlockBufferRef blockBuffer;
  CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(sampBuffer, NULL, &audioBufferList, sizeof(audioBufferList), NULL, NULL, 0, &blockBuffer);
  AudioBuffer audioBuffer = audioBufferList.mBuffers[0];
  memcpy(inData + (2*offsetSample),audioBuffer.mData,audioBuffer.mDataByteSize);
  bytesToRead = bytesToRead - audioBuffer.mDataByteSize;
  offsetSample = offsetSample + audioBuffer.mDataByteSize/8;



 }
 inBuffer->mAudioDataByteSize = offsetSample*8;

 AudioQueueEnqueueBuffer ( 
        pplayerState->mQueue,
        inBuffer,
        0,
        0
        );


}

回答1:

I was getting this same mystifying error. Sure enough, "setting up" an audio session made the error go away. This is how I set up my audio session.

- (void)setupAudio {
    [[AVAudioSession sharedInstance] setDelegate:self];
    [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryAmbient error:nil];
    NSError *activationError = nil;
    [[AVAudioSession sharedInstance] setActive: YES error:&activationError];

    NSLog(@"setupAudio ACTIVATION ERROR IS %@", activationError);
    [[AVAudioSession sharedInstance] setPreferredIOBufferDuration:0.1 error:&activationError];
    NSLog(@"setupAudio BUFFER DURATION ERROR IS %@", activationError);
}

回答2:

From the Audio Session Programming Guide, under AVAudioSessionCategoryAmbient:

This category allows audio from the iPod, Safari, and other built-in applications to play while your application is playing audio.

Using an AVAssetReader probably uses iOS' hardware decoder, which blocks the use of the AudioQueue. Setting AVAudioSessionCategoryAmbient means the audio is rendered in software, allowing both to work at the same time - however, this would have an impact on performance/battery life. (See Audio Session Programming Guide under "How Categories Affect Encoding and Decoding").

回答3:

Ok I have somehow solved this weird error... Apparently it is because of the audio session not properly set. Talk about lack of documentation on this one...