Sound System
The 16-bit stereo audio circuitry provides high-quality sound input and output through the built-in microphone and speaker. The user can also connect external input and output devices by way of the sound input and output jacks.
The sound system is based on the Singer codec IC along with input and output amplifiers and signal conditioners. In the PowerBook 1400, the Singer codec supports two channels of digital sound with sample sizes up to 16 bits and sample rates of 11.025 kHz, 22.05 kHz, and 44.1 kHz.
The frequency response of the sound circuits, not including the microphone and speaker, is within plus or minus 2 dB from 20 Hz to 20 kHz. Total harmonic distortion and noise is less than 0.05 percent with a 1V rms sine wave input. The signal-to-noise ratio (SNR) is 85 dB, with no audible discrete tones.
Note: All sound level specifications in this section are rms values.
Sound Inputs
The sound system accepts inputs from several sources:
- built-in microphone
- external sound input jack
- sound from the expansion bay
- 1-bit sound from the PC card slot
The sound signal from the built-in microphone goes through a dedicated preamplifier that raises its nominal 30 mV level to the 1 V level of the codec circuits in the Singer IC.
Stereo sound signals from the external sound input jack go through an analog multiplexer that selects either the external signals or the sound signals from the expansion bay. The multiplexer also lowers the levels of the maximum 2 V signal at the input jack to match the 1 V level of the codec circuits in the Singer IC.
The sound input jack has the following electrical characteristics:
- input impedance: 6.8kW
- 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 sound output level. You can do that using the Sound Manager as described in Inside Macintosh in the Sound chapter.
Stereo sound signals from the expansion bay go through an analog multiplexer that selects either those signals or the line signals from the external input jack. The multiplexer also raises the nominal 0.5 V level of the expansion bay sound to the 1 V input level of the codec circuits.
The sound input from the expansion bay has the following electrical characteristics:
- input impedance: 3.2kW
- maximum level: 0.5 V rms
Each PC card has one sound output pin (SPKR_OUT), and the computer accepts either one or two cards. The 1-bit digital signals from the sound output pins are exclusive-ORed together and routed to the built-in speaker and the external sound output jack.
Sound Outputs
The sound system sends computer-generated sounds or sounds from an expansion bay device or PC card to a built-in speaker and to an external sound output jack. The sound output jack is located on the back of the computer.
The sound output jack provides enough current to drive a pair of low-impedance headphones. The sound output jack has the following electrical characteristics:
- output impedance: 33 W
- minimum recommended load impedance: 32 W
- maximum level: 1 V rms
- maximum current: 32 mA peak
The computer turns off the sound signals to the internal speaker when an external device is connected to the sound output jack and during power cycling.