Inside Macintosh: Sound /

Chapter 3 - Sound Input Manager / About the Sound Input Manager

Interaction With Sound Input Devices

The Sound Input Manager provides routines that allow your application to request information about a sound input device or to change a sound input device's settings. The types of information you can obtain about a sound input device include

• the name, icon, and icon mask of the device driver
• whether the device driver supports asynchronous recording
• the device's settings, such as the number of channels the device is to record, the compression type, the number of bytes per sample at the current compression setting, and the sample rate to be produced by the device
• the range of compression types, sample rates, and sample sizes that the device supports

You can also use the Sound Input Manager to change some of a sound input device's settings and to turn features on and off. For example, you can turn on and off automatic gain control on some device drivers. Automatic gain control moderates sound recording to give a consistent signal level. Second, you can turn on and off the playthrough feature, which allows the user to hear through the Macintosh speaker the sound being recorded. Third, you can turn on and off VOX recording, or voice-activated recording, which allows your application to record only when the amplitude of sound input exceeds a certain level. You can use VOX recording either to prevent recording from starting until sound is at least a certain amplitude or to automatically stop recording when sound falls below a certain amplitude. This latter capability is called VOX stopping.

An important feature of sound input devices is continuous recording. All sound input devices that support asynchronous recording should support continuous recording as well. Continuous recording allows your application to make several consecutive calls to the SPBRecord function without losing data between calls. For example, you might need to record a lengthy sound to disk but not be able to fit the entire sound into RAM. Thus, it's important to be able to save a buffer of data to disk while the sound input device driver continues to collect recorded data. The Sound Input Manager's SndRecordToFile function relies on continuous recording.

To get information about a device or to turn features on and off, you can use the SPBGetDeviceInfo and SPBSetDeviceInfo functions. These functions allow you to use sound input device information selectors to specify what type of information you need to know about the device or what settings you wish to change.