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Interfacing with Hardware Devices

Most audio applications have to connect with external hardware, either to output audio data (for example, to an amplifier/speaker) or to obtain it (such as through a microphone). These operations must go through the hardware abstraction layer (HAL). Fortunately, in most cases you do not need to write custom code to access the HAL. Apple provides three standard audio units that should address most hardware needs: the default output unit, the system output unit, and the AUHAL. Your application must call the Component Manager to discover and load these units before you can use them.

In this section:

Default and System Output Units
The AUHAL

Default and System Output Units

The default output unit and system output unit send audio data to the default output (as selected by the user) or system output (where alerts and other system sounds are played) respectively. If you connect an audio unit to one of these output devices (such as in an audio processing graph), your unit's callback function (sometimes called the render callback) is called when the output needs data. The output unit routes the data through the HAL to the appropriate output device, automatically handling the following tasks, as shown in Figure 3-3.

• Any required linear PCM data conversion. The output unit contains an audio converter that can translate your audio data to the linear PCM variant required by the hardware.

• Any required channel mapping. For example, if your unit is supplying two-channel data but the output device can handle five, you will probably want to map which channels go to which. You can do so by specifying a channel map using the kAudioOutputUnitProperty_ChannelMap property on the output unit. If you don't supply a channel map, the default is to map the first audio channel to the first device channel, the second to the second, and so on. The actual output heard is then determined by how the user has configured the device speakers in the Audio MIDI Setup application.

• Signal Start/Stop. Output units are the only audio units that can control the flow of audio data in the signal chain.

For an example of using the default output unit to play audio, see SimpleSDK/DefaultOutputUnit in the Core Audio SDK.

The AUHAL

If you need to connect to an input device, or a hardware device other than the default output device, you need to use the AUHAL. Although designated as an output device, you can configure the AUHAL to accept input as well by setting the kAudioOutputUnitProperty_EnableIO property on the input. For more specifics, see Technical Note TN2091: Device Input Using the HAL Output Audio Unit. When accepting input, the AUHAL supports input channel mapping and uses an audio converter ( if necessary) to translate incoming data to linear PCM format.

The AUHAL is a more generalized version of the default output unit. In addition to the audio converter and channel mapping capabilities, you can specify the device to connect to by setting the kAudioOutputUnitProperty_CurrentDevice property to the ID of an AudioDevice object in the HAL. Once connected, you can also manipulate properties associated with the AudioDevice object by addressing the AUHAL; the AUHAL automatically passes along any property calls meant for the audio device.

An AUHAL instance can connect to only one device at a time, so you can enable both input and output only if the device can accept both. For example, the built-in audio for PowerPC-based Macintosh computers is configured as a single device that can both accept input audio data (through the Mic in) and output audio (through the speaker).

For the purposes of signal flow, the AUHAL configured for both input and output behaves as two audio units. For example, when output is enabled, the AUHAL invokes the previous audio unit's render callback. If an audio unit needs input data from the device, it invokes the AUHAL’s render callback. Figure 3-4 shows the AUHAL used for both input and output.

An audio signal coming in through the external device is translated into an audio data stream and passed to the AUHAL, which can then send it on to another audio unit. After processing the data (for example, adding effects, or mixing with other audio data), the output is sent back to the AUHAL, which can then output the audio through the same external device.

For examples of using the AUHAL for input and output, see the SimplePlayThru and CAPlayThrough code samples in the ADC Reference Library. SimplePlayThru shows how to handle input and output through a single AUHAL instance. CAPlayThrough shows how to implement input and output using an AUHAL for input and the default output unit for output.

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