Technical Q&A QA1631

AVAudioSession - Requesting Audio Session Preferences

Q:  Can I set a preferred value for an AVAudioSession's buffer duration or sample rate at any time?

A: While it is safe to set the AVAudioSession audio category (setCategory:error:) or notification listeners like AVAudioSessionRouteChangeNotification for example, regardless of activation state, it is generally better to make preference requests such as preferred hardware buffer duration (setPreferredIOBufferDuration:error:) or preferred hardware sample rate (setPreferredSampleRate:error:) when the AVAudioSession is NOT active.

These preferred values are simply hints to the operating system, the actual buffer duration or sample rate may be different once the AVAudioSession has been activated. Therefore, asking for the current hardware buffer duration or sample rate before AVAudioSession activation could return incorrect values.

Setting preferred values prior to activation

Follow these guidelines:

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:

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


Audio Session Programming Guide

Document Revision History


Added note for iPhone 6s and 6s Plus


Updated for AVAudioSession


Added "Note" section


New document that describes when to request session preferences such as Preferred Hardware I/O Buffer Duration.