Setting preferred values prior to activation

Follow these guidelines:

• Set "preferred" values when the audio session is not active.

• Get "current" values once the audio session has been activated.

Listing 1  

// Configure the audio session for playback and recording

NSError *audioSessionError = nil;

AVAudioSession *session = [AVAudioSession sharedInstance];

[session setCategory:AVAudioSessionCategoryPlayback error:&audioSessionError];

if (audioSessionError) {

    NSLog(@"Error %ld, %@",

          (long)audioSessionError.code, audioSessionError.localizedDescription);

}

// Set some preferred values

NSTimeInterval bufferDuration = .005; // I would prefer a 5ms buffer duration

[session setPreferredIOBufferDuration:bufferDuration error:&audioSessionError];

if (audioSessionError) {

    NSLog(@"Error %ld, %@",

          (long)audioSessionError.code, audioSessionError.localizedDescription);

}

double sampleRate = 44100.0; // I would prefer a sample rate of 44.1kHz

[session setPreferredSampleRate:sampleRate error:&audioSessionError];

if (audioSessionError) {

    NSLog(@"Error %ld, %@",

          (long)audioSessionError.code, audioSessionError.localizedDescription);

}

// Register for Route Change notifications

[[NSNotificationCenter defaultCenter] addObserver: self

                                         selector: @selector(handleRouteChange:)

                                             name: AVAudioSessionRouteChangeNotification

                                           object: session];

// *** Activate the audio session before asking for the "Current" values ***

[session setActive:YES error:&audioSessionError];

if (audioSessionError) {

    NSLog(@"Error %ld, %@",

          (long)audioSessionError.code, audioSessionError.localizedDescription);

}

// Get current values

sampleRate = session.sampleRate;

bufferDuration = session.IOBufferDuration;

NSLog(@"Sample Rate:%0.0fHz I/O Buffer Duration:%f", sampleRate, bufferDuration);

...

In Listing 1 the AVAudioSession has been activated prior to asking for the current hardware sample rate and current hardware buffer duration. These returned values will accurately reflect what the hardware will present to the client. As previously stated, these values may be different then what was asked for using the "Preferred" APIs.

Changing preferred values after activation

When an application sets a preferred value, it will not take effect until the audio session has been activated. In most cases where setting a preferred value causes some sort of audio system reconfiguration with an active audio session, audio data I/O will be stopped and then restarted. Therefore, if an application plans to set multiple preferred values, it is generally advisable to deactivate the session first, set the preferences, reactivate the session and then check the actual values.

There are several cases however where an application must first activate the audio session (after setting the appropriate category, category options and mode), in order to lean about the capabilities of the current configuration before being able to set a "preferred" value.

For example:

• In order to call setPreferredInput:error:, an active audio session is required before querying the availableInputs property.

• Prior to calling setPreferredOutputNumberOfChannels:error:, an active audio session is required before asking for maximumOutputNumberOfChannels.

• If AVAudioSessionCategoryOptionDuckOthers has been set, going inactive will end ducking. Once your audio session reactivates, ducking of other audio will resume. When ducking has been set, your session is always mixable. This is because setting AVAudioSessionCategoryOptionDuckOthers to true will automatically also set AVAudioSessionCategoryOptionMixWithOthers to true.

• Using the AVAudioSessionSetActiveOptionNotifyOthersOnDeactivation option when deactivating will reactivate other non-mixable applications.

It is recommended to NOT use the AVAudioSessionSetActiveOptionNotifyOthersOnDeactivation option when going inactive for the purpose of changing some preferred values.

Reference

Audio Session Programming Guide