[sdiy] My latest project

[Antti Huovilainen]

On Wed, 17 Feb 2010, Tom Wiltshire wrote:

> LFO waveshape - which lookup table? (Sine, triangle, ramp, etc)

Pointer to interpolated LUT or pointer to function.

The function pointer is good if you use y = (x - 1/3*x^3)*3/2 to produce 
sine from triangle. It's almost perfect approximation for LFO purposes, 
but not quite good enough for audio as there are some harmonics starting 
at -40dB.

Rewriting the equation as y = (x - 1/2*x^3 + x/2) makes it more suitable 
for fixed point implementation:

; LFO sine approx
; y = (x - 1/2*x^3 + x/2)

mul.ss  w2, w2, w0      ; w1 = x^2 / 2
sl      w1, w1          ; w1 = x^2
mul.ss  w2, w1, w0      ; w1 = x^3 / 2
sub     w2, w1, w0      ; w0 = x - 1/2*x^3
asr     w2, w1          ; w1 = x/2
add     w0, w1          ; w0 = x - 1/2*x^3 + x/2

> Envelope - Which curve? (expo attack, expo decay, linear, cosine, etc)

Function pointer.

> Envelope - release instantly on gate low, or go through all remaining stages
> Envelope -  start from zero level on gate on, or start from current level
> Envelope - if loop mode is on, loop back to stage X.

These are easily handled by calling a function pointer when envelope 
finishes current stage or note on/off happens.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova