This is the Apple Sound Chip. It contains 2K RAM that can be configured as either two 1K FIFOs or four 512-byte sound buffers. The ASC can operate in any of four functional modes:
• Monaural single voice mode. The on-board RAM is configured as two 1K FIFOs. The output of the channel A FIFO is converted to a PWM signal and fed to both channels. In this mode, the ASC can emulate the action of the free-form synthesizer in the sound driver.
• Stereo single voice mode. The on-board RAM is configured as two 1K FIFOs. The output of each FIFO is converted into a PWM signal; one is sent to the left channel and the other is sent to the right channel.
• Monaural four-voice synthesis mode. The on-board RAM is configured as four independent 512K-byte buffers. At each sample clock period, a sound value is taken from each of the four buffers, all four values are summed, and the result is converted to a PWM signal and output to both channels. In this mode, the ASC can emulate the action of the four-tone synthesizer in the sound driver.
• Stereo four-voice synthesis mode. The on-board RAM is configured as four independent 512-byte buffers. At each sample clock period, a sound value is taken from each of the four buffers, the values are summed in pairs, and the result of each summed pair is converted to a PWM signal. One of the PWM signals is output to the right channel and the other is sent to the left channel.
The useful bandwidth of the sound circuitry is approximately 7.5 KHz. The sample rate can be either the compact disk standard of 44.1 KHz or the rate used in the Macintosh SE and Macintosh Plus: 22.255 KHz.
