// Register for changes to a connection's format
  [[NSNotificationCenter defaultCenter] addObserver:self 
    selector:@selector(connectionFormatDidChange:) 
    name:QTCaptureConnectionFormatDescriptionDidChangeNotification 
    object:nil];

 // This method called when the connection format changes
  - (void)connectionFormatDidChange:(NSNotification *)notification
  {
    // You may now use the QTCaptureConnectionEnabledAudioChannelsAttribute
    // attribute to enable audio channels for the connection
    [self enableAudioChannelsForConnection:[self myAudioCaptureConnection]];
  }

  // Specify the audio channels that should be used on the connection
  -(void) enableAudioChannelsForConnection:(QTCaptureConnection *)captureConnection
  {
    // Make sure attribute is writeable before we use it
    if ([captureConnection attributeIsReadOnly:
      QTCaptureConnectionEnabledAudioChannelsAttribute] == NO)
    {
      NSRange channelIndexes;
      channelIndexes.location = 0;
      channelIndexes.length = 2;
      NSIndexSet *channelIndexSet = 
        [NSIndexSet indexSetWithIndexesInRange:channelIndexes];

      // Example: enable audio channels 1,2 (only) for the connection
      [captureConnection setAttribute:channelIndexSet 
        forKey:QTCaptureConnectionEnabledAudioChannelsAttribute];
    }
  }