3. How is the sound
Unfortunately, the ZX-Spectrum to control the sound allotted
only one bit. This bit D4 port 254 (# FE). But despite
this, the programmers manage to create
very beautiful melodies and effects.
When this bit is set, the speaker (amplifier) is
energized, and its membrane is located in one position. When he
is cleared, the dynamics of the voltage No, and the membrane is
in a different position. Thus, if you change the state of this
bit with a fairly high frequency, the membrane will vibrate and
you hear a sound.
Use this method can even be of
BASIC:
10 FOR A = 1 TO 300: OUT 254,23
20 OUT 254,7: NEXT A
And as easy on the assembler, but this
case will prohibit the interruption, if
You want to get high-quality sound:
1415.
10 DI; ban interrupt
20 LD BC, 2560; BC = length
30 LD A, 7; A = Border color
40 BEGIN XOR 16; inverting bits D4
50 OUT (254), A; output to port A 254
60 LD D, 100; D = delay (frequency)
70 PAUSE DEC D; D = D-1
80 JR NZ, PAUSE; if D <> 0 then loop
90 DEC BC; BC = BC-1
100 LD D, A; preservation A
110 LD A, B; BC =
120 OR C; 0?
130 LD A, D; A recovery
140 JR NZ, BEGIN; if BC <> 0 then loop
150 EI; permission to interrupt
160 RET; return to BASIC
2
You've probably noticed that in these programs to the drop
port is not 16 and 0, would be consistent set and reset bits
D4, and 23 and 7. The fact that this port The speaker controls
the border and even the color output on a tape recorder.
Acquainted with its ability to close:
Bits D0 ... D2 determine the color of the border:
000 (0) - Black 100 (4) - Green
001 (1) - Blue 101 (5) - blue
010 (2) - red 110 (6) - yellow
011 (3) - Purple 111 (7) - white
Bit D3 control the output on tape.
Bit D4 controls the sound. Bits D5 ... D7
not used.
When you enter a byte from a port bit D6 controls the tape
input.
I think now it is clear where it came from
numbers 23 and 7: to set and reset bits
D4 is necessary to consistently bring in
port value of 16 and 0. But it must
retain color border is 7 (white).
Consequently, the first value will be 16 +7 = 23, and the
second - 0 +7 = 7. In fact, You can set any color border.
Moreover, we can create new different color effects. It's
enough when inverting the sound bits use a different color
border.
You've probably noticed that in the above program in
assembly language prohibits interruption. I think we should
clarify this. The fact is that the ZX-Spectrum is designed so
that every fiftieth of a second routine is called ROM, which is
located at address 56 (# 38). And if you want a quality (not
popping) sound, then it must be disabled, and that achieved by
the prohibition of interrupt (command DI). When you return to
the BASIC interrupt need to re-authorize (command EI).
