Technical information

CHAPTER 3

Devices and Ports

56 Sound System

The microphone and the sound input jack have dedicated input channels on the

Screamer IC; the sound input from the PC Card slot has its own input, and the

other three inputs share an input on the IC. Those three inputs are switched on

and off by the hardware; they can be selected one at a time for play-through or

recording.

Built-in Microphone 3

The sound signal from the built-in microphone goes through a dedicated

preampliﬁer that raises its nominal 30-mV level to the 0.6-V level of the codec

circuits in the Screamer IC.

External Sound Input 3

The external sound input jack is located on the back of the computer. The sound

input jack accepts line-level stereo signals or an Apple PlainTalk microphone.

When a connector is plugged into the external sound input jack, the computer

turns off the sound input from the built-in microphone. The input jack has the

following electrical characteristics:

■ input impedance: 6.8k ohms

■ maximum level: 2.0 V rms

Note

The sound input jack accepts the maximum sound output

of an audio CD without clipping. When working with

sound sources that have signiﬁcantly lower levels, you may

wish to increase the signal gain of the sound input circuit.

You can do that using the Sound Manager as described in

Inside Macintosh: Sound. ◆

Expansion Bay Sound Input 3

The sound input from the expansion bay has the following electrical

characteristics:

■ input impedance: 3.2k ohms

■ maximum level: 0.5 V rms