After getting some dates for gigs with Betablocker DS, I am spending some time looking into audio algorithms, and implementing them on the Gameboy DS. Last Thursday I spent some time at the TAI studio/bunker with Till Bovermann investigating these PD patches by Claudius Maximus:
The algorithm uses feedback to create sounds that take some time to play through a wide range of frequencies. It works by writing into the same buffer it’s playing, but at a different rate/position – the resulting ugliness suits the style of BBDS very much. As an aid to our understanding Till converted the algorithm to Supercollider, then over the next couple of days I managed to get it running with an inner loop of 9 instructions on the DS (could probably be optimised further, but I’m still a beginner):
@ ----------------------------------------------------
@ qrz_maximus: *dst, length, readpos, writepos [freq, *tab]
@ ----------------------------------------------------
.global qrz_maximus
.type qrz_maximus, %function
qrz_maximus:
push {r4,r5,r6,r7} @ push the registers we need
ldr r4, [r13,#20] @ load freq from stack into r4
ldr r5, [r13,#24] @ load *tab from stack into r5
ldr r6, .tablength @ load the tablen into r6
.maximus_loop:
ldrh r7, [r5,r2] @ load the sample into r7
strh r7, [r0], #2 @ write output: *dst=r7 dst++
strh r7, [r5,r3] @ feedback into tab[writepos]=r7
add r2, r2, r4 @ readpos+=freq
and r2, r2, r6 @ mask readpos into range
add r3, r3, #2 @ writepos++
and r3, r3, r6 @ mask writepos into range
subs r1, r1, #1 @ length--
bne .maximus_loop @ repeat until length zero
mov r0, r2 @ return readpos
pop {r4,r5,r6,r7}
bx lr @ exit
.tablength:
.word 0x00003FF
And here it is running on autopilot with a test program in Betablocker:
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