---

Audio File and Converter Services

Audio File and Converter Services lets you read or write audio data, either to a file or to a buffer, and allows you to convert the data between any number of different formats. This service consists of the functions, data types, and constants defined in the following header files in AudioToolbox.framework and AudioUnit.framework:

• ExtendedAudioFile.h

• AudioFile.h

• AudioFormat.h

• AudioConverter.h

• AudioCodec.h (located in AudioUnit.framework).

• CAFFile.h

In many cases, you use the Extended Audio File API, which provides the simplest interface for reading and writing audio data. Files read using this API are automatically uncompressed and/or converted into linear PCM format, which is the native format for audio units. Similarly, you can use one function call to write linear PCM audio data to a file in a compressed or converted format. “Supported Audio File and Data Formats” lists the file formats that Core Audio supports by default. Some formats have restrictions; for example, by default, Core Audio can read, but not write, MP3 files, and an AC-3 file can be decoded only to a stereo data stream (not 5.1 surround).

If you need more control over the file reading, file writing, or data conversion process, you can access the Audio File and Audio Converter APIs directly (in AudioFile.h and AudioConverter.h). When using the Audio File API, the audio data source (as represented by an audio file object) can be either an actual file or a buffer in memory. In addition, if your application reads and writes proprietary file formats, you can handle the format translation using custom Component Manager components that the Audio File API can discover and load. For example, if your file format incorporates DRM, you would want to create a custom component to handle that process.

In this section:

Audio Converters and Codecs
File Format Information
Audio Metadata
Core Audio File Format

Audio Converters and Codecs

An audio converter lets you convert audio data from one format to another. For example, you can make simple conversions such as changing the sample rate and interleaving or deinterleaving audio data streams, to more complex operations such as compressing or decompressing audio. Three types of conversions are possible:

• Decoding an audio format (such as AAC (Advanced Audio Coding)) to linear PCM format.

• Encoding linear PCM data into a different audio format.

• Translating between different variants of linear PCM (for example, converting 16-bit signed integer linear PCM to 32-bit floating point linear PCM).

The Audio Converter API lets you create and manipulate audio converters. You can use the API with many built-in converters to handle most common audio formats. You can instantiate more than one converter at a time, and specify the converter to use when calling a conversion function. Each audio converter has properties that describe characteristics of the converter. For example, a channel mapping property also allows you to specify how the input channels should map to the output channels.

You convert data by calling a conversion function with a particular converter instance, specifying where to find the input data and where to write the output. Most conversions require a callback function to periodically supply input data to the converter.

An audio codec is a Component Manager component loaded by an audio converter to encode or decode a specific audio format. Typically a codec would decode to or encode from linear PCM. The Audio Codec API provides the Component Manager interface necessary for implementing an audio codec. After you create a custom codec, then you can use an audio converter to access it. “Supported Audio File and Data Formats” lists standard Core Audio codecs for translating between compressed formats and Linear PCM.

For examples of how to use audio converters, see SimpleSDK/ConvertFile and the AFConvert command-line tool in Services/AudioFileTools in the Core Audio SDK.

File Format Information

In addition to reading, writing, and conversion, Audio File and Converter Services can obtain useful information about file types and the audio data a file contains. For example, you can obtain data such as the following using the Audio File API:

• File types that Core Audio can read or write

• Data formats that the Audio File API can read or write

• The name of a given file type

• The file extension(s) for a given file type

The Audio File API also allows you to set or read properties associated with a file. Examples of properties include the data format stored in the file and a CFDictionary containing metadata such as the genre, artist, and copyright information.

Audio Metadata

When handling audio data, you often need specific information about the data so you know how to best process it. The Audio Format API (in AudioFormat.h) allows you to query information stored in various audio structures. For example, you might want to know some of the following characteristics:

• Information associated with a particular channel layout (number of channels, channel names, input to output mapping).

• Panning matrix information, which you can use for mapping between channel layouts.

• Sampling rate, bit rate, and other basic information.

In addition to this information, you can also use the Audio Format API to obtain specific information about the system related to Core Audio, such as the audio codecs that are currently available.

Core Audio File Format

Although technically not a part of the Core Audio programming interface, the Core Audio File format (CAF) is a powerful and flexible file format , defined by Apple, for storing audio data. CAF files have no size restrictions (unlike AIFF, AIFF-C, and WAVE files) and can support a wide range of metadata, such as channel information and text annotations. The CAF format is flexible enough to contain any audio data format, even formats that do not exist yet. For detailed information about the Core Audio File format, see Apple Core Audio Format Specification 1.0.

---