Specifications

CHAPTER 3

I/O Features

Keyboard

Routing of the Sound Signals 3

All audio sources are routed to the AWACS custom IC. The AWACS IC can enable the

input sources in two groups: the sound input jack (external microphone) or (CD-ROM,

TV/FM tuner, modem, DAV card, cross-platform card).

Digitizing Sound 3

The Macintosh Performa 6400 computer digitizes and records sound as 16-bit samples.

The computer can use anu one of three sampling rates: 11k samples per second, 22k

samples per second, or 44k samples per second.

The sound system plays samples at the sampling rate speciﬁed in the control panel for

sound.

Sound Modes 3

The sound mode is selected by a call to the Sound Manager. The sound circuitry

normally operates in one of three modes:

■

Sound playback: computer-generated sound is sent to the speaker and the sound

output jacks.

■

Sound playback with playthrough: computer sound and sound input are mixed and

sent to the speakers and the sound output jacks.

■

Sound record with playthrough: input sound is recorded and also sent to the speakers

and the sound output jacks.

When recording from a microphone, applications should reduce the playthrough volume

to prevent possible feedback from the speakers to the microphone.

The O’Hare IC provides separate sound buffers for input and for stereo output, so the

computer can record and send digitized sound to the sound outputs simultaneously.

Keyboard 3

The keyboard has a Power key, identiﬁed by the symbol

. When the user chooses Shut

Down from the Special menu, the computer either shuts down or a dialog box appears

asking if you really want to shut down. The user can also turn off the power by pressing

the Power key.

There are no programmer’s switches, so the user invokes the reset and nonmaskable

interrupt (NMI) function by pressing Command key combinations while holding down

the Power key, as shown in Table 3-8. The Command key is identiﬁed by the symbols

and .