Varispeed Overdubbing in Looper - Delay-line approach to rate > 1

Hi all,

Another newbie question. I appreciate the support.

Based a suggestion from Graham W., I am trying to apply his delay-line approach to overdubbing at rates > 1 in a varispeed looper I am building. I am adopting this approach from the looper example in Jeff K's/Gregory T's tutorials seen here: https://technophony.com/building-live-loopers-in-max/chapter-4-varispeed-overdubbing-looper-gen/

I have to say that the conceptual jump from the previous patch to this patch is quite a challenge for me.

In any case, my question is regarding the operation to "fill in skipped samples" when overdubbing at sample counter rates > 1. Graham's example is the following:

for (i=0; i<min(32, abs(t)); i+=1) {
	// get the corresponding sound from our past input:
	s = indelay.read((abs(t)-i)/aspd);
	// write it to tape at the writer position
	// using crossfading with existing data
	poke(tape, s*xb, wb, 0, xa, boundmode="wrap");
	// move the writer on one sample in the desired direction
	wb += sign(t);	
}

t = how many samples are skipped, i.e., how many samples we need to fill in (if I am interpreting the other code correctly), and
aspd = (basically) the rate of playback
xb/xa = a crossfade on the input/overdub

I have a couple of questions:

• I don't really follow the math in the delay read() function. Why divide by the speed?

• how does this account for fractional sample indices, such as when the playback rate is a non-integer (e.g., 1.75)?

• I'm having a lot of trouble wrapping my head around how the delay read function is working. For delay.read(x) - are you fetching the data at sample x in the delay buffer? For the code example above, wouldn't the value for read() need to be relative to where we are "in time" i.e., fetch the data from 3 samples ago?

Thanks in advance for helping me wrap my head around this stuff!