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The sound system accepts inputs from six possible sources:
The microphone preamp and the sound input jack have dedicated analog input channels on the Screamer IC; the other inputs send digital data to the IC. The analog inputs are switched on and off by the hardware; they can be selected one at a time for play-through or recording. The digital inputs can be selected or mixed by the Screamer IC.
In addition to the signal sources connected to the sound system, the computer also accepts digital sound data from a device in the expansion bay (such as the DVD drive) or from devices connected to the USB or FireWire ports. Sound data from those sources can be sent to the sound system to be converted to analog form for output to the speakers and the output jack.
The sound signal from the built-in microphone goes through a dedicated preamplifier that raises its nominal 30-mV level to a nominal 150 mV (peak-to-peak) signal to the Screamer IC. That signal level assures good quality digitizing without driving the analog input into clipping.
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:
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.
Modem activity sound signals from the communications slot are sent to the Screamer IC as 8-bit digital data.
The CardBus socket has a pin (SPKR_OUT) that carries a one-bit digital sound signal output from the PC Card and input to the computer's sound system. The one-bit digital signal from the sound output pin is routed to the Screamer IC, which in turn sends it to the built-in speaker and the external sound output jack.
Sound that accompanies zoomed video signals from the CardBus slot is routed as digital data by way of the I2S bus to the Screamer IC. When an external clock is used, the sound data are 8 bits wide; with the internal clock to the DAC, the data are 16 bits wide.