Summary

In this chapter, we have covered the following areas:

• Digital audio and Pulse Code Modulation (PCM), which is a technique for converting an analog audio signal into a digital representation. We have also looked at how PCM samples are encoded and interleaved channel by channel.
• The Core Audio architecture, which collectively provides sound/audio support to Mac OS X and iOS. The cornerstone of Core Audio is the HAL, which coordinates the use of audio hardware on behalf of clients and allows multiple clients to access audio hardware simultaneously.
• The Core Audio HAL, which always uses 32-bit floating-point format to represent audio samples. A driver is therefore responsible for converting the native format of the hardware to or from this format.
• IOAudioFamily, which provides the kernel-level side of the audio architecture. The key classes of the family are IOAudioDevice, IOAudioEngine, and IOAudioStream.
• The IOAudioDevice class, which represents a hardware audio device in the kernel.
• The IOAudioEngine class, which represents a single I/O engine for which an IOAudioDevice may have more than one. The class is abstract. The audio engine class may have one or more IOAudioStreams associated with it.
• An IOAudioStream is used to represent a single sample buffer.
• The operation of an audio engine is conceptually simple, the engine simply needs to tell the super class (which again communicates with Core Audio/Audio HAL) when the device has wrapped to the beginning of the sample buffer and how many times this event has occurred.