of Microprocessors

762 MUSICAL ApPLICATIONS OF MICROPROCESSORS
0001 0000 ; STRAGE SOUND EFFECT GENERATOR
0002 0000
0003 BFFO PORT $BFFO OUTPUT PORT BITS TO AMPLIFIER
0004 0000
0005 0000 *= 0 SIMULATED REGISTERS IN PAGE 0
0006 0000 0000 REGI •WORD 0 16-BIT OUTPUT REGISTER
0007 0002 0100 REG2 •WORD 1 16-BIT INCREMENT REGISTER
0008 0004
0009 0004 *= $700 PROGRAM ORIGIN
0010 0700 •ENTRY
0011 0700 A500 SSOUND LDA REG1 ADD REG2 TO REG1 WITH
0012 0702 18 CLC RESULT IN REG1
0013 0703 6502 ADC REG2 LOWER BYTE
0014 0705 8500 STA REG1
0015 0707 BDFOBF STA PORT MAKE LOWER BYTE AUDIBLE
0016 070A AS01 LOA REG1+l UPPER BYTE
0017 070C 6503 ADC REG2+l
0018 070E 8501 STA REGl+l
0019 0710 90EE BCC SSOUND REPEAT UNTIL REGI OVERFLOWS
0020 0712 E602 INC REG2 INCREMENT REG2 WHEN REGI OVERFLOWS
0021 0714 DOEA BNE SSOUND AND REPEAT INDEFINITELY
0022 0716 E603 INC REG2+l
0023 0718 DOE6 BNE SSOUND
0024 071A E602 INC REG2 ; DON'T LET REG2=0
0025 071C 4COO07 JMP SSOUND
Fig. 20-1. Sound effects generator
and enter the program into virtually any kind of computer. As written, the
sequence of sounds repeats every 20 sec on a I-MHz 6502-based computer.
For practical use, as a sound-effects generator, desirable points in the
sequence are determined, and the registers initialized to the corresponding
values. The loop is then run the desired number of iterations and terminated.
One difficulty with timed loops is that the microprocessor is fully
occupied while producing sound. Enough time for simple tasks, such as
looking at a control panel, can be taken between notes, however, with little
audible effect. A related problem is that the microprocessor must execute
instructions at a known, stable rate. Computers that stop or interrupt the
microprocessor to refresh memory, for example, would probably produce very
rough sounding tones.
Simplified Direct Synthesis
For much more controlled sound effects and listenable music, many of
the digital synthesis techniques described in Chapter 13 can be used, in
simplified form, for low-cost sound generation. A common requirement,
however, is a cheap D-to-A converter and a linear power amplifier (the singlebit
timed-loop techniques could use a one-transistor switch). In large
volumes, 8-bit D-to-A converter chips are well under $2, while half-watt
audio amplifier ICs are under a dollar. For really low cost, one can get by with
an 8-bit CMOS register, eight weighted 1% resistors, and a power
Darlington transistor emitter-follower amplifier. In either case, a two-stage