Apple Lisa - CVSD and Speaker

Confidential
Lisa Hardware Reference Manual
THE CVSD AND THE SPEAKER
The Lisa has a built-in speaker, and a circuit that performs
digital-to-analog and analog-to-digital conversion. On the back of
the Lisa is a microphone input jack, and a remote speaker jack.
The 6522 which controls the keyboard interface also controls the
circuit which drives the speaker. The circuit uses a Continuously
Variable Slope Deltamodulator (CVSD) to provide both audio input and
output. The central part of the circuit is a Motorola 3417 CVSD
chip.
The CVSD chip records and plays back audio data at a programmable
rate. CVSD input can come from a microphone plugged into the back of
the Lisa mainframe. The length of the recording depends on "the
amount of
memory available for it. Since the normal CVSD speed is
16,000 bits/second, even a short recording can require a substantial
amount of memory. CVSD output can be sent to the internal speaker,
or picked up at the remote speaker jack.
To record a signal, the ENC bit (PBO of the 6522) is set high. To
play back a signal, the ENC bit is set low. The T2 timer (one of the
6522 internal registers) is then set to produce the desired clock
speed, and data is read or written (played or recorded) to the 6522
shift register. The 6522 can be programmed to generate an interrupt
whenever the shift register is full or empty. CB1 clocks the data
transfers between the 6522 shift .register and the CVSD.
The volume of both the CVSD and the speaker output is controlled by
the three VC bits (PBl, PB2, and PB3 in the 6522). The volume is
proportional to the magnitude of the 3-bit unsigned integer formed by
the VC bits.
In addition to handling voice I/O, the CVSD can be programmed to
produce tones from the speaker. The CVSD represents a waveform as a
series of l's and O's where a 1 indicates a rising signal, and a 0
indicates a descending signal. A series of alternating l's and a's
therefore produces no sound at all. When its comparator indicates to
the CVSD that the signal has been rising faster than the rate at
which the CVSD has been rising for several samples, the CVSD
increases the slope of its signal. This continuously variable slope
enables it to produce a closer approximation of the desired waveform
than a simple slope modulator can. Of course, the quality of the
approximation depends on the rate at which the waveform is sampled.
The
higher the sampling rate, the better. See the Motorola MC3417
data sheet for full details. To send a pattern to the CVSD, load the
shift register and program the T2 timer to produce the desired
frequency. The resultant waveform need not be a square wave.
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7-Jul-81