Simple difference equations in JSFX?

[Aesis]

I am trying to create simple difference equations of the form:

Y[t] = a*Y[t-1] + b*Y[t-2] + ... + X[t]

in JSFX.

My problem is that it's not very clear how the buffer works. Even looking at the very simplest example:

desc:Echo
slider1:120<0,1000,1>Length [ms]

@slider
echolength = slider1 / 1000 * srate ;

@sample
bufpos[0] = spl0 ;
bufpos = bufpos + 1 ;
bufpos > echolength ? bufpos = 0;
spl0 = spl0 + bufpos[0] ;
spl1 = spl0 ;

It seems like this echo is supposed to loop forever, but it only has one tap instead. Is there an explanation on what's going on with the code?

[junh1024]

May help http://forum.cockos.com/showpost.php...98&postcount=8

[SaulT]

btw if you want it to loop (repeat) then you need a feedback.

slider2:-6<-60,0,0.1>Feedback (dB)

...

@slider

feedback = 10^(slider2/20);

...

@sample

bufpos[0] = spl0 + bufpos[0] * feedback;
bufpos += 1;
bufpos >= echolength ? bufpos -= echolength;
spl0 += bufpos[0]; 
spl1 += bufpos[0];

I wrote a plugin for this, one that combined delay and saturation. It stores the delay at half srate to get maximum delay time. At 96k that's 80 seconds, much longer times are possible at lower samplerates.

http://forum.cockos.com/showthread.php?t=170874

[Aesis]

Originally Posted by SaulT

btw if you want it to loop (repeat) then you need a feedback.

slider2:-6<-60,0,0.1>Feedback (dB)

...

@slider

feedback = 10^(slider2/20);

...

@sample

bufpos[0] = spl0 + bufpos[0] * feedback;
bufpos += 1;
bufpos >= echolength ? bufpos -= echolength;
spl0 += bufpos[0]; 
spl1 += bufpos[0];

I wrote a plugin for this, one that combined delay and saturation. It stores the delay at half srate to get maximum delay time. At 96k that's 80 seconds, much longer times are possible at lower samplerates.

http://forum.cockos.com/showthread.php?t=170874

Feedback is no problem, however I don't know how to make the next sample value to be combination of past output values as the equation says. Any ideas on that?

[Aesis]

I finally understand it. The guide I have to say, is useless.

Here is a code for 2nd order:

desc:Echo
slider1:0<0,100000,1>Delay1 [sample]
slider2:0<0,100000,1>Delay2 [sample]
slider3:0<0,100,1>Feedback1 [sample]
slider4:0<0,100,1>Feedback2 [sample]

@slider
length0 = slider1-1;
length1 = slider2-1;
feedback0 = slider3 / 100;
feedback1 = slider4 / 100;

@init
buffer1 = length0+2

@sample

buffer0[0] = in0;
buffer0+=1;
buffer0 > length0 ? buffer0 = 0;

delay = -feedback0*buffer0[0];

buffer1[0] = in0;
buffer1+=1;
buffer1 > length0+2+length1 ? buffer1 = length0+2;

delay1 = -feedback1*buffer1[0];

spl0 = spl0 + delay + delay1;

spl1 = spl0 ;

in0 = spl0;

I will attempt to rewrite it in a way that only uses one buffer location. But in the meantime, there is a serious error. For the second delay to kick in, it takes time from the start of the playback. Is it the case that JSFX can't write to memory values that are outside of the number of played samples? Makes no sense...

[mschnell]

@slider can be executed after @init (e.g. whenever you move one), so you need to set "buffer1" in @slider as it depends on @length1, which is set in @slider (or just do "buffer1 = 100000;" in @init.

-Michael

[Aesis]

Originally Posted by mschnell

@slider can be executed after @init (e.g. whenever you move one), so you need to set "buffer1" in @slider as it depends on @length1, which is set in @slider (or just do "buffer1 = 100000;" in @init.

-Michael

Good catch. Here is a new improved code that writes to buffer only once.

desc:Eecho313
slider1:0<0,100000,1>Delay1 [sample]
slider2:0<0,100000,1>Delay2 [sample]
slider3:0<0,100000,1>Delay3 [sample]
slider4:0<0,100000,1>Delay4 [sample]
slider5:0<-100,100,1>Feedback1 [%]
slider6:0<-100,100,1>Feedback2 [%]
slider7:0<-100,100,1>Feedback3 [%]
slider8:0<-100,100,1>Feedback4 [%]

@slider
length0 = slider1;
length1 = slider2;
length2 = slider3;
length3 = slider4;

feedback0 = slider5 / 100;
feedback1 = slider6 / 100;
feedback2 = slider7 / 100;
feedback3 = slider8 / 100;

maximum = max(max(max(length0,length1),length2),length3);
buffer1 = -length0 - 1;
buffer2 = -length1 - 1;
buffer3 = -length2 - 1;
buffer4 = -length3 - 1;

@sample

buffer1+=1;
buffer1 > maximum ? buffer1 = 0;

buffer2 +=1;
buffer2 > maximum ? buffer2 = 0;

buffer3 +=1;
buffer3 > maximum ? buffer3 = 0;

buffer4 +=1;
buffer4 > maximum ? buffer4 = 0;

delay = feedback0*buffer1[0]+feedback1*buffer2[0]+feedback2*buffer3[0]+feedback3*buffer4[0];


spl0 = spl0 + delay;

spl1 = spl0;

in0 = spl0;


buffer0[0] = in0;
buffer0 +=1;
buffer0 > maximum ? buffer0 = 0;

Up to 4 taps now. Useful for making filters and cancelling delay taps. There is a bit of an issue with the controls, how would I stop the sliders refreshing delay time when I only touch the feedback?

[mschnell]

You can set a variable according to the slider value and check if they still are the same and only do the appropriate actions if they are not.

-Michael