4.2. Volume Control
In fact, ZX-Spectrum absolutely
adjusted for volume control, but
it still can be changed within a small range due to the
relatively high clock rate microprocessor Z80 (About 3.5 MHz).
As mentioned in Chapter 2, when you
sends a signal to the speaker, his diaphragm
changes its position. If, during its
motion to send a return signal, it
returns to its original position without going through the
desired path. Thus, the amplitude (And consequently the volume)
is less than normal. For example, compare the sound of two
nearly identical programs: 1415.
10 PROG1 DI; ban interrupt
20 LD B, 255; B = frequency
30 LD C, 255; C = duration
40 XOR A; A = border color (0)
50 LOOP1 XOR 16; inverting bits D4
60 OUT (254), A; output to port A 254
1970 PUSH BC; preservation BC
80 LOOP2 DJNZ LOOP2; delay
1990 POP BC; restore BC
100 DEC C; C = C-1
110 JR NZ, LOOP1; if C <> 0 then loop
120 EI; permission to interrupt
130 RET; return to BASIC
10 PROG2 DI;
20 LD B, 255;
30 LD C, 255;
40 XOR A;
50 LOOP1 XOR 16, all
60 OUT (254), A;
70 XOR 16; (except these two lines,
80 OUT (254), A; repeating previous)
1990 PUSH BC;
100 LOOP2 DJNZ LOOP2; similarly
110 POP BC;
120 DEC C; previous
130 JR NZ, LOOP1;
140 EI; example!
150 RET;
2
When you have collected in assembly and
run, you were surprised. However,
against the facts do not trample - the second subprogram will
be heard several times quieter first!
Bring the text more universal routines:
1415.
10 DI; ban interrupt
20 LD B, 50; B = volume
30 LD C, 50; C = frequency
40 LD D, 15; D = duration
50 LD E, 50; E = number of notes
60 XOR A; A = border color (0)
70 LOOP1 PUSH DE; conservation DE
80 LOOP2 PUSH BC; preservation BC
90 XOR 16; inverting bits D4
100 OUT (254), A; output to port A 254
110 LOOP3 DJNZ LOOP3; delay (volume)
120 XOR 16; inverting bits D4
130 OUT (254), A; output to port A 254
140 LD B, C; B = C
150 LOOP4 DJNZ LOOP4; delay (frequency)
160 POP BC; restore BC
170 DEC D; D = D-1
180 JR NZ, LOOP2; if D <> 0 then loop
190 POP DE; restoration DE
200 NOP; reserve
210 NOP; reserve
220 NOP; reserve
230 DEC E; E = E-1
240 JR NZ, LOOP1; if E <> 0 then loop
250 EI; permission to interrupt
260 RET; return to BASIC
2
In this effect, you can choose the desired volume. But do
not get carried away: at values greater than 40-50, the
volume has not increased. It must be noted that a change in
volume during playing time and frequency changes.
Therefore, it is necessary to provide compensation.
Now the three-byte transfer (command
NOP). They are designed to change
volume (case B), frequency (case C)
and duration (case D). If you replace these three NORa to
commands DEC B (volume down), INC C (compensation rate), and
INC D (prolongation) then you get an interesting effect with
decaying volume.
