Sound Out Your Spectrum



If snowdrops and Spectrums are among your favourite things,

 these valuable machine-code routines by Jeremy Hall will

    help improve your micro's sound-producing ability.

Impressive noises can be produced to rival the machine-code

             routines used by software houses.



With a little knowledge of machine code and of how the

Spectrum produces sound, some quite impressive noises can

be produced, despite its limited sound facility.

  The Spectrum produces sound by sending a series of clicks

to its internal loudspeaker. The time interval between each

clock, and hence the pitch of the note, is dependent on the

value held in the HL register pair of the Z80. The higher

the value stored in HL, the londer the interval between

clicks, hence the lower the pitch of the note produced,

that is, the number of clicks, is controlled by the value

stored in the DE register pair; the larger the number, the

longer the note.

  Having set these registers to the required values, it is

then simply a matter of calling the sound-producing routine

in the Basic ROM. This starts at address 03 B5 hex, 949

decimal. Program 1 demonstrates this idea very simply, and

figure 1 shows the machine-code mnemonics of this program.

Try changing some of the values of HL and DE in this pro-

gram by altering the DATA statements, but before you do

this, save the program on cassette in case you crash the

system.

  It probably will not take you very long to tire of

program 1 and you will want to move on to some more inter-

esting sounds. This is where program 2 comes into it;

figure 2 shows the machine-code mnemonics for this program.

Register B is loaded with the number of times that the

whole sound is to be repeated. Try loading it with 1, that

is change the second number in the data statement from 10

to 1.

  HL and DE are set to the required value and the sound

routine called. On returning from the routine, DE is loaded

with 16, which is then added to HL to increase its value,

and lower the pitch of the next note. The sound routine is

then called again, and this process repeated 255 times.

Register B is then decremented and if it is zero the pro-

gram will end and return to Basic, otherwise the whole

process will be repeated. Note that registers HL and BC

must be saved by it.

  In the final program, program 3, the machine code held in

each data statement is based on the previous program, but

with different values of HL and DE in each case. Enter the

program exactly as shown, with the correct number of zeros

after each DATA statement. These zeros are used as padding

to make each routine 30 bytes long and this make each USR

address easier to remember - 32400 to 32430 and so on. Try

experimenting with the values of HL and DE again; you might

be surprised at the results.



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 Mnemonic     Hex         Decimal      Comment

 LD DE,128    11 7F 00     17 128 0    Note length

 LD HL,768    21 00 03     33   0 3    Pitch

 CALL 949     CD B5 03    205 181 3    Call sound

 RET          C9          201          Return to Basic

 

 Figure 1.

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 Mnemonic     Hex         Decimal      Comment

 LD B,10      06 0A         6  10      Repeat sound 10 times

 PUSH BC      C5          197

 LD HL,15     21 0F 00     33  15 0    Initial pitch

 LD DE,20     11 14 00     17  20 0    Note duration

 PUSH HL      E5          229

 CALL 949     CD B5 03    205 181 3    Sound routine

 POP HL       E1          225

 LD DE,16     11 10 00     17  16 0    Decrease

 AND A        A7          167           the

 ADC HL,DE    ED 5A       237  90       pitch

 LD A,L       7D          125          Repeat

 CP 255       FE FF       254 255       256 times

 JRNZ -18     20 ED        32 237

 POP BC       C1          193

 DJNZ -25     10 E6        16 230      Dec B, repeat if not zero

 RET          C9          201

 

 Figure 2.

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