title: Using RtAudio with Core-Audio, the probeDeviceOpen function fails
body: <p>I am trying to write a C++ program which plays real-time processed audio. I have decided to use RtAudio but cannot make the test programs play audio.</p>
  
  <p>I am using Mac OS X, so I compiled with <code>./configure --with-core</code> and <code>make</code>. After compiling the files in the test directory, I ran <code>./audioprobe</code> to return this list.</p>
  
  <pre><code>Compiled APIs:
    OS-X Core Audio
  
  Current API: OS-X Core Audio
  
  Found 3 device(s) ...
  
  Device Name = Apple Inc.: Built-in Microphone
  Probe Status = Successful
  Output Channels = 0
  Input Channels = 2
  Duplex Channels = 0
  This is NOT the default output device.
  This is NOT the default input device.
  Natively supported data formats:
    32-bit float
  Supported sample rates = 44100 48000 88200 96000 
  
  Device Name = Apple Inc.: Built-in Input
  Probe Status = Successful
  Output Channels = 0
  Input Channels = 2
  Duplex Channels = 0
  This is NOT the default output device.
  This is the default input device.
  Natively supported data formats:
    32-bit float
  Supported sample rates = 44100 48000 88200 96000 
  
  Device Name = Apple Inc.: Built-in Output
  Probe Status = Successful
  Output Channels = 2
  Input Channels = 0
  Duplex Channels = 0
  This is the default output device.
  This is NOT the default input device.
  Natively supported data formats:
    32-bit float
  Supported sample rates = 44100 48000 88200 96000 
  </code></pre>
  
  <p>Since the Built-in Output device is index 2, I then ran <code>./playsaw 2 44100 2</code>, but an error occurred:</p>
  
  <pre><code>RtApiCore::probeDeviceOpen: system error (kAudioHardwareUnknownPropertyError)
      getting stream format for device (2).
  </code></pre>
  
  <p>How can this be fixed?</p>