Digital audio recording creates PCM data by measuring an analog (real world) audio signal’s magnitude at regular intervals (the sampling rate) and converting each sample to a numerical value. Most Core Audio services use and manipulate audio in linear pulse-code-modulated ( linear PCM) format, the most common uncompressed digital audio data format. Figure 1-2 iOS Core Audio architecture A Little About Digital Audio and Linear PCM Figure 1-2 provides a high-level view of the audio architecture in iOS. For example, Audio Session Services lets you manage the audio behavior of your application in the context of a device that functions as a mobile telephone and an iPod. However, there are additional services in iOS not present in OS X. There is no API for services that must be managed very tightly by the operating system-specifically, the HAL and the I/O Kit. Use System Sound Services (represented in the figure as “System sounds”) to play system sounds and user-interface sound effects.Ĭore Audio in iOS is optimized for the computing resources available in a battery-powered mobile platform. Use Core Audio Clock Services for audio and MIDI synchronization and time format management. Use Music Sequencing Services to play MIDI-based control and music data. In OS X you can also create custom audio units to use in your application or to provide for use in other applications. Use Audio Unit Services and Audio Processing Graph Services (represented in the figure as “Audio units”) to host audio units (audio plug-ins) in your application. In OS X you can also create custom codecs. Use Audio File, Converter, and Codec Services to read and write from disk and to perform audio data format transformations. Use Audio Queue Services to record, play back, pause, loop, and synchronize audio. You find Core Audio application-level services in the Audio Toolbox and Audio Unit frameworks. The Core MIDI (Musical Instrument Digital Interface) framework provides similar interfaces for working with MIDI data and devices.
You can access the HAL using Audio Hardware Services in the Core Audio framework when you require real-time audio.
Audio signals pass to and from hardware through the HAL. In OS X, the majority of Core Audio services are layered on top of the Hardware Abstraction Layer (HAL) as shown in Figure 1-1. Core Audio in iOS and OS XĬore Audio is tightly integrated into iOS and OS X for high performance and low latency. Read this chapter to learn what you can do with Core Audio.
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It includes a set of software frameworks designed to handle the audio needs in your applications. Core Audio is the digital audio infrastructure of iOS and OS X.
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