User guide

Important: To obtain meaningful values for hardware characteristics, ensure that the audio session is

initialized and active before you issue queries.

Two of the most useful audio session hardware properties are sampleRate and outputLatency. The

sampleRate property contains the hardware sample rate of the device. The outputLatency property contains

the playback latency of the device.

Specifying Preferred Hardware I/O Buffer Duration

Use the AVAudioSession class to specify preferred hardware sample rates and preferred hardware I/O buffer

durations, as shown in Listing 4-2. To set preferred sample rate you’d use similar code.

Listing 4-2 Specifying preferred I/O buffer duration using the AVAudioSession class

NSError *setPreferenceError = nil;

NSTimeInterval preferredBufferDuration = 0.005;

[[AVAudiosession sharedInstance]

setPreferredIOBufferDuration: preferredBufferDuration

error: &setPreferenceError];

After establishing a hardware preference, always ask the hardware for the actual value, because the system

may not be able to provide what you ask for.

Obtaining and Using the Hardware Sample Rate

As part of setting up for recording audio, you obtain the current audio hardware sample rate and apply it to

your audio data format. Listing 4-3 shows how. You typically place this code in the implementation file for a

recording class. Use similar code to obtain other hardware properties, including the input and output number

of channels.

Before you query the audio session for current hardware characteristics, ensure that the session is active.

Listing 4-3 Obtaining the current audio hardware sample rate using the AVAudioSession class

double sampleRate;

sampleRate = [[AVAudioSession sharedInstance] currentHardwareSampleRate];

Optimizing Your App for Device Hardware

Querying Hardware Characteristics