IOS Audio -AudioUnit (official documentation)

4. Use specific Audio Units

Every iOS audio unit has some things in common and some things unique. The first few chapters of this document describe common areas, including the need to find an audio unit at run time, instantiate it, and ensure that its stream format is properly set. This chapter explains the differences between audio units and provides details on how to use them.

In a later section, Identifier Keys for Audio Units lists the code you need to find a dynamic link library for each Audio unit at runtime.

Using I/O Units

IOS provides three I/O (input/output) units. The vast majority of audio unit applications use the Remote I/O Unit, which connects to input and output audio hardware and provides low-latency access to individual incoming and outgoing audio sample values. For VoIP applications, the voice-processing I/O Unit extends the Remote I/O unit by adding echo cancellation and other features. To send audio back to the application rather than Output audio hardware, use the Generic Output Unit

Remote I/O Unit

The RemoteI /O unit (subtype kAudioUnitSubType_RemoteIO) is connected to the device hardware for input, output, or both. Use it for playback, recording, or low-latency synchronous input and output without the need for echo cancellation.

The device’s audio hardware forces its audio Stream Formats outside of the Reteme I/O unit, as described in Understanding Where and How to Set Stream Formats. The audio unit provides format conversion between hardware audio formats and application audio formats through the included format converter audio unit.

For sample code that demonstrates how to use this audio unit, see the sample code project aurioTouch

Table 3-1 describes the usage of the audio unit.

Table 3-1 Using the Remote I/O unit

Voice-Processing I/O Unit

The voice-processing I/O unit (subtype kAudioUnitSubType_VoiceProcessingIO) has the features of Remote I/O units and adds echo suppression for two-way duplex communication. It also adds automatic gain correction, voice processing quality adjustment and mute function. This is the correct I/O unit for Voice over Internet Protocol (VoIP) applications.

All of the considerations listed in Table 3-1 also apply to the Voice-Processing I/O unit. In addition, the specific Properties available for this Audio Unit are described in the Voice-Processing I/O Audio Unit Properties

Generic Output Unit

When sending the output of the Audio Processing graph to an application instead of the audio hardware, use an audio unit of subtype kAudioUnitSubType_GenericOutput. You typically use Generic Output units for offline audio processing. Like other I/O units, the generic output unit integrates a format converter unit. This allows the Generic Output unit to perform format conversions between the stream format used in the Audio Processing Graph and the format you want.

Using Mixer Units

IOS provides two Mixer units. In most cases, you should use a Multichannel Mixer unit, which provides mixing for any number of mono or stereo streams. If you need the functionality of a 3D Mixer unit, you’ll most likely use OpenAL instead. OpenAL is built on top of the 3D Mixer unit, providing the same performance through a simpler API ideal for game application development.

Multichannel Mixer Unit

The Multichannel Mixer Unit (subtype kAudioUnitSubType_MultiChannelMixer) accepts any number of mono or stereo streams and combines them into a single stereo output. It controls the audio gain for each input and output, and lets you turn each input on or off individually. Starting with iOS 4.0, The Multichannel Mixer supports stereo panning for every input.

For sample code that demonstrates how to use this audio unit, see the Sample code project Audio Mixer (MixerHost).

Table 3-2 describes the usage of the audio unit.

Table 3-2 Using the Multichannel Mixer unit

Using Effect Units

The iPodEQ unit (subtype kAudioUnitSubType_AUiPodEQ) is the only effect unit available in iOS 4. This is the same equalizer that uses the built-in iPod app. To see the iPod app’s user interface for this audio unit, go to Settings > iPod > EQ. This audio unit provides a set of preset equalization curves such as Bass Booster, Pop, and Spoken Word.

You must provide your own user interface iPod EQ unit, as you must do for any audio unit. The Mixer iPodEQ AUGraph Test sample code project demonstrates how to use the iPodEQ unit and shows a way to provide a user interface for it.

Table 3-4 describes the usage of the audio unit.

Table 3-4 Using the iPod EQ unit

Identifier Keys for Audio Units

This table provides the identification keys you need to access the dynamic link library for each iOS audio unit, along with a brief description of the audio unit.

Table 3-5 Identifier keys for accessing the dynamically-linkable libraries for each iOS audio unit