of Microprocessors

LOW-COST SYNTHESIS TECHNIQUES
771
+12 V
FROM
INTERNAL
PARALLEL
PORT
5
6 87
86
7 85
8 84
9 83
10
II 82
12 81
8~
13
V+
COMP 16
V- "* 1000 pF
3
-12 V.
102°~~'~5"
Sti SPEAKER
0.05 _
}JF -
Fig. 20-6. Micro Technology Unlimited MTU·130 sound port
examined and the sound port toggled whenever the stream changes from 1 to
o or vice-versa. For proper reproduction, the resulting waveform should be
integrated (6 dB/octave low-pass filtered), but the sound is intelligible right
out of the built-in speaker. The quality is compatable to that of severely
distorted CB radio.
The Ohio Scientific C8P computer was probably the first to include a
D-to-A converter for producing sound. Unfortunately, the designers chose an
exponential rather than a linear DAC, which is very good for playback of
digitized speech but difficult to program for multipart music synthesis. The
machine also lacked an internal amplifier and speaker. The Micro Technology
Unlimited MTU-130 computer, which was introduced in 1981, used an 8-bit
DAC and had an unusually powerful built-in amplifier driving a 3 X 5 highcompliance
speaker. Figure 20-6 shows the audio circuit used in the
MTU-130 and subsequent models, which requires just two ICs for the DAC,
six-pole filter, and power amplifier. The DAC was driven by an internal 8-bit
parallel output port. Actually, the company manufactured several similar
add-on DAC boards for other 6502-based computers such as the Apple II and
PET well before the MTU-130 came out.
While there was nothing particularly innovative about the audio
circuit, the 6502 music software available to drive it and the add-on boards
was quite remarkable. The software was based on the routine listed in Fig.
20-2 but with an extension to allow, in effect, an independent envelope for
each harmonic during the course of a note. This was accomplished by
replacing the time wasting instructions at CHORD2 with code for
periodically changing the memory page number of the waveform table at
VxTAB, thus in effect dynamically switching through a sequence of waveform
tables. The actual program used a separate 256-entry table with pointers to