The AVFoundation framework ( amework) provides the AVAudioPlayer class, a streamlined and simple Objective-C interface for audio playback. The Audio Unit framework ( amework) lets applications work with audio plug-ins, including audio units and codecs. In iOS, this framework includes Audio Session Services, the interface for managing your application’s audio behavior in the context of a device that functions as a mobile phone and iPod. The Audio Toolbox framework ( amework) provides interfaces for the mid- and high-level services in Core Audio. Take note that the Core Audio framework is not an umbrella to the other frameworks here, but rather a peer. This section quickly lists them to give you a sense of where to find the pieces that make up the Core Audio layers. You get another view of Core Audio by considering its API frameworks, located in /System/Library/Frameworks/. All applications can use OpenAL, although it is best suited for games development. It is built on top of the system-supplied 3D Mixer audio unit. OpenAL is the Core Audio implementation of the open-source OpenAL standard for positional audio. It gives you a unified interface for reading and writing uncompressed and compressed sound files. The AVAudioPlayer class provides a simple Objective-C interface for playing and looping audio as well as implementing rewind and fast-forward.Įxtended Audio File Services combines features from Audio File Services and Audio Converter services. It employs codecs as necessary to deal with compressed audio formats. The highest layer in Core Audio includes streamlined interfaces that combine features from lower layers.Īudio Queue Services lets you record, play, pause, loop, and synchronize audio. The middle layer in Core Audio includes services for data format conversion, reading and writing to disk, parsing streams, and working with plug-ins.Īudio Converter Services lets applications work with audio data format converters.Īudio File Services supports reading and writing audio data to and from disk-based files.Īudio Unit Services and Audio Processing Graph Services let applications work with digital signal processing (DSP) plug-ins such as equalizers and mixers.Īudio File Stream Services lets you build applications that can parse streams, such as for playing files streamed over a network connection.Ĭore Audio Clock Services supports audio and MIDI synchronization as well as time-base conversions.Īudio Format Services (a small API, not shown in the figure) assists with managing audio data formats in your application.Ĭore Audio in iOS supports most of these services, as shown in Figure 1-2. The result is a significantly smaller, tuned API set appropriate for a mobile platform. OpenAL, for example, employs direct I/O for real-time audio in games. Indeed, Core Audio in iOS provides ways to achieve real-time audio using higher level interfaces. Many audio applications, however, don’t access this layer. Mac apps can be written to use these technologies directly when they require the highest possible, real-time performance. Host Time Services, which provides access to the computer’s clock
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The audio hardware abstraction layer (audio HAL), which provides a device-independent, driver-independent interface to hardwareĬore MIDI, which provides software abstractions for working with MIDI streams and devices
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The I/O Kit, which interacts with drivers Figure 2-1 The three API layers of Core Audio The programming interfaces for Core Audio are arranged into three layers, as illustrated in Figure 2-1. The later sections in this chapter introduce you to how Core Audio works with files, streams, recording and playback, and plug-ins.
Continue reading to understand the design principles, use patterns, and programming idioms that pervade Core Audio. Read the first two sections in this chapter for a brief introduction to these interfaces and how they work together. Apple has designed the software interfaces to Core Audio using a layered, cooperative, task-focused approach.
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