Method StartPortAudioStream

libraries/lib-audio-io/AudioIO.cpp:473–752

}

bool AudioIO::StartPortAudioStream(const AudioIOStartStreamOptions &options,
   unsigned int numPlaybackChannels, unsigned int numCaptureChannels)
{
   auto sampleRate = options.rate;
   mNumPauseFrames = 0;
   SetOwningProject( options.pProject );
   bool success = false;
   auto cleanup = finally([&]{
      if (!success)
         ResetOwningProject();
   });

   // PRL:  Protection from crash reported by David Bailes, involving starting
   // and stopping with frequent changes of active window, hard to reproduce
   if (mOwningProject.expired())
      return false;

   mInputMeter.reset();
   mOutputMeter.reset();

   mLastPaError = paNoError;
   // pick a rate to do the audio I/O at, from those available. The project
   // rate is suggested, but we may get something else if it isn't supported
   mRate = GetBestRate(numCaptureChannels > 0, numPlaybackChannels > 0, sampleRate);

   // GetBestRate() will return 0.0 for bidirectional streams when there is no
   // common sample rate supported by both the input and output devices.
   // Bidirectional streams are used when recording while overdub or
   // software playthrough options are enabled.

   // Pa_OpenStream() will return paInvalidSampleRate when trying to create the
   // bidirectional stream with a sampleRate of 0.0
   bool isStreamBidirectional = (numCaptureChannels > 0) && (numPlaybackChannels > 0);
   bool isUnsupportedSampleRate = isStreamBidirectional && (mRate == 0.0);

   // July 2016 (Carsten and Uwe)
   // BUG 193: Tell PortAudio sound card will handle 24 bit (under DirectSound) using
   // userData.
   auto captureFormat = mCaptureFormat;
   auto captureFormat_saved = captureFormat;
   // Special case: Our 24-bit sample format is different from PortAudio's
   // 3-byte packed format. So just make PortAudio return float samples,
   // since we need float values anyway to apply the volume.
   // ANSWER-ME: So we *never* actually handle 24-bit?! This causes mCapture to
   // be set to floatSample below.
   // JKC: YES that's right.  Internally Audacity uses float, and float has space for
   // 24 bits as well as exponent.  Actual 24 bit would require packing and
   // unpacking unaligned bytes and would be inefficient.
   // ANSWER ME: is floatSample 64 bit on 64 bit machines?
   if (captureFormat == int24Sample)
   {
      captureFormat = floatSample;
      // mCaptureFormat is overwritten before the call to
      // StartPortAudioStream. So the hack with captureFormat_saved is
      // still working (assuming it worked before).
      mCaptureFormat = captureFormat;
   }