Spin Semiconductor

Hey all.

Folks who have played with the pitch-shift routine for octave-up and octave-down know about the "crossfade phase cancellation" issue. For octave-down, you get a tremolo effect due to the slower ramp speed (-8192). But for octave-up, the fast ramp (16384) results in an unpleasant warble. We can't use a longer delay than 4096, so we are stuck with it, right?

Here is a trick I just came up with: Do two pitch shifts in a row, each shifting up by the square root of two. The magic ramp speed is 6783. Voila, an octave up with a pleasing tremolo instead of the warble. It requires another 4096 memory and 8 instructions, but still only 1 ramp (so you can still use the other for octave down, etc).

Try this code:
;
delay1 mem 4096 ; longest possible
delay2 mem 4096 ; so use two of them
temp mem 1
;
skp run, START
wldr RMP0, 6783, 4096 ; pitch up sqrt(2) ramp
START:
;
rdax ADCL, 1 ; read inputs
rdax ADCR, 1
wra delay1+512, 0 ; write to first delay, clear ACC
;
cho rda,RMP0,REG|COMPC,delay1 ; std pitch shift code
cho rda,RMP0,,delay1+1
wra temp,0
cho rda,RMP0,RPTR2|COMPC,delay1
cho rda,RMP0,RPTR2,delay1+1
cho sof,RMP0,NA|COMPC,0
cho rda,RMP0,NA,temp
;
wra delay2+512, 0 ; do it again in second delay
;
cho rda,RMP0,REG|COMPC,delay2
cho rda,RMP0,,delay2+1
wra temp,0
cho rda,RMP0,RPTR2|COMPC,delay2
cho rda,RMP0,RPTR2,delay2+1
cho sof,RMP0,NA|COMPC,0
cho rda,RMP0,NA,temp
;
wrax DACL, 1 ; write to outputs
wrax DACR, 0

I'll give it a try, I'm working on a way to cancel the tremolo effect for the octave, still no results :S

Sweetalk, a year ago I tried for weeks to reduce the tremolo without much success. For example, I tried to smear things out with some allpass filters, but it sounds "reverby" and only makes the latency worse. Maybe adding some "wobble" to the ramp LFO rate, i.e. a little bit of vibrato. That would cause the cancellations to move around, rather than happen at a regular interval. Whether this sounds better is a matter of taste, I think.

I'm thinking in some sort of limiter to chop the amplitud modulation of the tremolo

Your improved octave up algorithm works quite nicely donstavely, it definitely sounds more pleasant than the standard algorithm, too bad the latency is doubled. I tried halving the delay length to 2048, but the sound quality degrades to the point that the it sounds worse than the standard algorithm with 4096 delay length

Yes, the tradeoff is latency. In the application I am working on I am going for more of a chorus effect - i.e. multiple instruments rather than a single instrument like a 12-string.

Notice that I write 512 words into the 4096-word delay. This reduces the latency by that amount. The reason you can do this is that the crossfade waveform is zero for the first 1/8th of the delay. If you try a 2048 word delay you will have to change the "wra delay1+512,0" to "wra delay1+256, 0" in both places. This is probably why it sounded worse than the standard single-step 4096 algorithm.

Frank commented in an older thread that this trick of writing 1/8th of the way into the buffer shouldn't work. I assure you (and Frank) that does. It is totally glitchless, and reduces the average latency by 1/4 for free. I don't think Frank considered it carefully.

Yeah I did noticed that you wrote 512 samples into the delay line, and I did changed that to 256 when I reduce the delay length to to 2048, with this setup the tremolo effect becomes much more pronounced, to my ear it sounds worse than the warble effect of the single step algorithm with 4096 length