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Sequence Grabber - Setting the sound channel play through state

Q: When recording sound using the Sequence Grabber, how can my application duplicate the sound source "Speaker" play through settings as seen when SGSettingsDialog is used to configure a Sound Channel? What API is used to turn play through on and off?

A: To change the sound channel play through state, use the Sequence Grabber SGGetChannelUsage and SGSetChannelUsage API pair.

Listing 1 demonstrates how you can set the play through state to one of the following:

  • Off
  • On during record and preview
  • Off during record but On during preview

Listing 1. SetSoundChannelPlayThruState.

// SetSoundChannelPlayThruState
// Duplicates the functionality of the menu popup control
// in the Source tab of the Sequence Grabber Sound Settings dialog
//  inSoundChannel - a sequence grabber sound channel
//  inControl - a three entry pop-up menu control
//
ComponentResult SetSoundChannelPlayThruState(const SGChannel inSoundChannel,
                                             const ControlRef inControl)
{
  SInt32 value, usage;
  ComonentResult err;

  value = GetControl32BitValue(inControl);
  switch (value) {
   // always off
  case 1:
    value = 0;
    break;
   // on during preview and record
  case 2:
    value = seqGrabPreview | seqGrabPlayDuringRecord;
    break;
   // on during preview only
  case 3:
    value = seqGrabPreview;
    break;
  }

  err = SGGetChannelUsage(inSoundChannel, &usage);
  if (noErr == err) {
    usage &= ~(seqGrabPreview | seqGrabPlayDuringRecord);
    usage |= value;
    err = SGSetChannelUsage(inSoundChannel, usage);
  }
    
  return err;
}

References:

[Sep 23, 2003]