Enumerating and Querying PortAudio Devices

Querying Devices

It is often fine to use the default device as we did previously in this tutorial, but there are times when you'll want to explicitly choose the device from a list of available devices on the system. To see a working example of this, check out pa_devs.c in the tests/ directory of the PortAudio source code. To do so, you'll need to first initialize PortAudio and Query for the number of Devices:

int numDevices;
 
numDevices = Pa_GetDeviceCount();
if( numDevices < 0 )
{
    printf( "ERROR: Pa_CountDevices returned 0x%x\n", numDevices );
    err = numDevices;
    goto error;
}

If you want to get information about each device, simply loop through as follows:

const   PaDeviceInfo *deviceInfo;
 
for( i=0; i<numDevices; i++ )
{
    deviceInfo = Pa_GetDeviceInfo( i );
    ...
}

The Pa_DeviceInfo structure contains a wealth of information such as the name of the devices, the default latency associated with the devices and more. The structure has the following fields:

int     structVersion
const char *    name
PaHostApiIndex  hostApi
int     maxInputChannels
int     maxOutputChannels
PaTime  defaultLowInputLatency
PaTime  defaultLowOutputLatency
PaTime  defaultHighInputLatency
PaTime  defaultHighOutputLatency
double  defaultSampleRate

You may notice that you can't determine, from this information alone, whether or not a particular sample rate is supported. This is because some devices support ranges of sample rates, others support, a list of sample rates, and still others support some sample rates and number of channels combinations but not others. To get around this, PortAudio offers a function for testing a particular device with a given format:

const PaStreamParameters *inputParameters;
const PaStreamParameters *outputParameters;
double desiredSampleRate;
...
PaError err;
 
err = Pa_IsFormatSupported( inputParameters, outputParameters, desiredSampleRate );
if( err == paFormatIsSupported )
{
   printf( "Hooray!\n");
}
else
{
   printf("Too Bad.\n");
}

Filling in the inputParameters and outputParameters fields is shown in a moment.

Once you've found a configuration you like, or one you'd like to go ahead and try, you can open the stream by filling in the PaStreamParameters structures, and calling Pa_OpenStream:

double srate = ... ;
PaStream *stream;
unsigned long framesPerBuffer = ... ; //could be paFramesPerBufferUnspecified, in which case PortAudio will do its best to manage it for you, but, on some platforms, the framesPerBuffer will change in each call to the callback
PaStreamParameters outputParameters;
PaStreamParameters inputParameters;
 
bzero( &inputParameters, sizeof( inputParameters ) ); //not necessary if you are filling in all the fields
inputParameters.channelCount = inChan;
inputParameters.device = inDevNum;
inputParameters.hostApiSpecificStreamInfo = NULL;
inputParameters.sampleFormat = paFloat32;
inputParameters.suggestedLatency = Pa_GetDeviceInfo(inDevNum)->defaultLowInputLatency ;
inputParameters.hostApiSpecificStreamInfo = NULL; //See you specific host's API docs for info on using this field
 
 
bzero( &outputParameters, sizeof( outputParameters ) ); //not necessary if you are filling in all the fields
outputParameters.channelCount = outChan;
outputParameters.device = outDevNum;
outputParameters.hostApiSpecificStreamInfo = NULL;
outputParameters.sampleFormat = paFloat32;
outputParameters.suggestedLatency = Pa_GetDeviceInfo(outDevNum)->defaultLowOutputLatency ;
outputParameters.hostApiSpecificStreamInfo = NULL; //See you specific host's API docs for info on using this field
 
err = Pa_OpenStream(
                &stream,
                &inputParameters,
                &outputParameters,
                srate,
                framesPerBuffer,
                paNoFlag, //flags that can be used to define dither, clip settings and more
                portAudioCallback, //your callback function
                (void *)this ); //data to be passed to callback. In C++, it is frequently (void *)this
//don't forget to check errors!

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