Sound

Introduction

The Einstein has a sound chip with sixteen registers, and HiSoft Pascal gives you one procedure to reach it: PSG(R,V) puts the value V into register R. There is nothing more - no "play a note" - so this section is about which registers to use for a note, and the one register you must be careful with.

Three Registers For A Note

The chip has three channels; channel A is enough for a tune. A note on channel A needs three things.

The pitch is a number called the period - the bigger it is, the lower the note - and the note's frequency is 2,000,000 ÷ (16 × period). Middle C, 261 Hz, is period 478. A period can be bigger than 255, so it goes in two registers: the remainder after dividing by 256 in register 0, the rest in register 1:

PSG(0,P MOD 256);
PSG(1,P DIV 256);

The volume is register 8: 0 is silent, 15 the loudest.

The switch is register 7, which turns the channels on. For channel A's tone, the value is 126:

PSG(7,126);

Register 7 Is Not Only For Sound

Register 7 does a second job: its top bits set up the part of the chip that reads the keyboard. 126 leaves them as they should be. Other values that also switch tone A on do not - 62, for instance - and then the keyboard stops working: not just in your program, but at the 0: prompt afterwards. The only way back is to restart the Einstein. Use 126.

Silence

A note does not stop when the program does. Whatever the chip was playing when your program ended, it goes on playing at the 0: prompt. End every program that makes a sound with PSG(8,0).

The Code

A scale, from middle C up:

10 PROGRAM SCALE;
20 VAR I,J:INTEGER;
30 N:ARRAY[1..8] OF INTEGER;
40 PROCEDURE TONE(P:INTEGER);
50 BEGIN
60 PSG(0,P MOD 256);
70 PSG(1,P DIV 256)
80 END;
90 BEGIN
100 N[1]:=478; N[2]:=426; N[3]:=379;
110 N[4]:=358; N[5]:=319; N[6]:=284;
120 N[7]:=253; N[8]:=239;
130 PSG(7,126);
140 PSG(8,12);
150 FOR I:=1 TO 8 DO
160 BEGIN
170 TONE(N[I]);
180 FOR J:=1 TO 400 DO
190 END;
200 PSG(8,0)
210 END.

Line 180 is an empty loop that holds each note for about a quarter of a second.

Starting from

SCALE.ASC saved from XBAS; HPEIN SCALE. In MAME: sound must be on - the command in S2 leaves it on.

What you should see

Nothing on the screen - but you hear eight notes going up, C D E F G A B C, about two seconds in all, and then silence.

Change One Thing

Take out line 200 (and the ; at the end of line 190). What do you hear after the program ends?
Change line 180's 400 to 1600. How long is each note now?
Change line 130 to PSG(7,62);. The scale still plays - but what happens when you then type DIR? (Save everything first: you will have to restart the Einstein.)

Exercises

18.1 Play the scale downwards.

18.2 Make a siren: a note that slides up from period 400 to 200 and back, over and over, until a key is pressed (S15).

Worked solutions are in Appendix II.

When It Goes Wrong

Symptom	Cause
No sound	Register 7 not set to 126, or register 8 left at 0.
A note goes on after the program	No `PSG(8,0)` at the end.
The keyboard is dead, even at `0:`	Register 7 was given a value that turns off the keyboard. Restart the Einstein, and use 126.
The note is wrong	Only register 0 was set: a period over 255 needs register 1 as well.

Summary

PSG(R,V) writes a register of the sound chip. For a note on channel A: the period in registers 0 and 1 (frequency 2,000,000 ÷ 16 ÷ period), the volume in register 8, and 126 in register 7 - no other value, or the keyboard goes dead. Finish with PSG(8,0).

S19, Reaching The Machine - PEEK, POKE, the I/O ports and machine code.

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