Avalon: Routine at 58806

58806: Play music

Used by the routines at 49345, 51043 and 55944.

Input
A	Indicates which music to play (1 = intro tune, 2 = paused game tune)

"The Spectrum did not have much scope with its little beeper but I managed harmonies by counting down more than one loop at a time and reversing the pulse when each counter got to zero.
I got a phasing synthy sound by making the counters almost the same."

From interview with Steve Turner at retrovideogamer.co.uk/rvg-interviews-steve-turner

There are two tunes/melodies.

(1) The first tune is played at the end of the intro sequence when a new game is started, and also when the game is completed. Uses music data stored at 60400.

(2) The second tune is played then the game is paused. Uses music data at 60530.

58806

CP 1

Check which tune needs to be played

58808

JR NZ,58815

58810

LD HL,60400

Point HL at first note(s) of the intro/end tune data

58813

58815

LD HL,60530

Point HL at first note(s) of the paused game tune data.

Play notes

58818

LD A,(HL)

Get note value

58819

CP 255

255 is an end-of-music marker

58821

JR Z,58877

If so, RETurn to previous routine

58823

LD B,A

Get first note of pair

58824

INC HL

Get second note of pair

58825

LD A,(HL)

58826

LD C,A

...and store in C register

58827

INC HL

Move note data pointer on ready for next note

58828

LD (60244),HL

...and store

58831

LD H,B

H = first note of pair, e.g. 190

58832

LD A,C

58833

ADD A,32

+32 to create a phase-type effect

58835

LD L,A

L = second note of pair, plus 32, e.g. 222

58836

LD DE,7168

7168 = Note duration

58839

LD A,0

Reset A to 0

58841

DEC H

Decrement first note byte (for sound note loop)

58842

JR NZ,58847

Reached zero?

58844

OR 16

If zero, ensure speaker bit (4) is set to play sound

58846

LD H,B

...and restore first note of pair again

58847

DEC L

Decrement second note byte (for sound note loop)

58848

JR NZ,58853

Reached zero?

58850

OR 16

If zero, ensure speaker bit (4) is set

58852

LD L,C

...and restore second note of pair again

58853

ADD A,8

'Thickens out' the sound by ensuring speaker is toggled on a bit more

58855

AND 16

Filter out other bits, only keeping bit (4) speaker toggle bit

58857

OUT (254),A

...and play sound by toggling speaker on or off accordingly

58859

SRA A

Shift bits right

58861

LD (60243),A

..and store, as A register needed for next bit

58864

DEC DE

Decrement note counter

58865

LD A,D

And check if at zero

58866

OR E

58867

LD A,(60243)

Restore A register which holds the speaker bits

58870

JR NZ,58841

If not at zero yet, continue to play note

58872

LD HL,(60244)

Note played. Get data pointer

58875

...and go and play next note

58877

RET

Return to routine at 49345 or 51043.