Native iPhone audio format

Question

I've currently got my output audio on the iPhone setup as this :

AudioStreamBasicDescription audioFormat;

audioFormat.mSampleRate = 48000;

audioFormat.mFormatID   = kAudioFormatLinearPCM;

audioFormat.mFormatFlags = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;

audioFormat.mFramesPerPacket = 1;

audioFormat.mChannelsPerFrame = 2;

audioFormat.mBitsPerChannel = 16;

audioFormat.mBytesPerPacket = 4;

audioFormat.mBytesPerFrame = 4;

However, when I examine my performance figures through shark I am seeing functions such as : SRC_Convert_table_i32_scalar_stereo

take a fair chunk of time.

This made me think - what is the ideal and suggested output format for the iPhone? The one that requires as little work for the device to play.

Answer 1

Shark does work with the iPhone. You can enable iPhone profiling by selecting "Sampling > Network/iPhone Profiling..." in the menu.

Definitely try using a 44100 Hz sampling rate. With 48000 I see the same function that you posted appearing in the callstacks -- no such function shows up when using 44100. The canonical audio format for Audio Units on the iPhone is non-interleaved 8.24 linear PCM:

streamFormat.mFormatID         = kAudioFormatLinearPCM;
streamFormat.mFormatFlags      = 
                  kAudioFormatFlagIsSignedInteger
                | kAudioFormatFlagsNativeEndian
                | kLinearPCMFormatFlagIsNonInterleaved
                | (24 << kLinearPCMFormatFlagsSampleFractionShift);
streamFormat.mSampleRate       = mixing_rate;
streamFormat.mBitsPerChannel   = 32;
streamFormat.mChannelsPerFrame = 2;
streamFormat.mFramesPerPacket  = 1;
streamFormat.mBytesPerFrame    = ( streamFormat.mBitsPerChannel / 8 );
streamFormat.mBytesPerPacket   = streamFormat.mBytesPerFrame *
                                 streamFormat.mFramesPerPacket;

Answer 2

From iphone dev centre (requires login) the hardware suppoorted codecs are

iPhone Audio Hardware Codecs

iPhone OS applications can use a wide range of audio data formats, as described in the next section. Some of these formats use software-based encoding and decoding. You can simultaneously play multiple sounds in these formats. Moreover, your application and a background application (notably, the iPod application) can simultaneously play sounds in these formats.

Other iPhone OS audio formats employ a hardware codec for playback. These formats are:

• AAC
• ALAC (Apple Lossless)
• MP3