If I use a vd~ reading from a delay line (as in the rotating tape head pitch shifter example, but with added recirculating feedback) I can get pitch change effects (pretty great pitch change effects tbh) but they're unstable - they keep rising or falling in pitch with each circulation as they get pitch shifted each time they go around the loop.

I figured one way to implement the effect would be to change the write speed and the read speed by the same amount (I'd guess this is how the pedal is doing it)- But I have no idea how to change the delay write speed in PD.
I thought of changing sample rate using block~ in a subpatch with the delay line but that doesn't seem to give the desired results (or in fact any sound at all - so I'm guessing you can't upsample a delay line).

Any ideas???

The pitch keeps rising because the altered pitched output goes back into the delay line to be altered again?

I'm talking about a delay line with feedback. I was mucking about and I stuck a feedback loop into example G09. With that set-up the pitch rises with each repeat because each time it's going back into the delay line and being read back at a rate higher than it was written. It sounds cool (very cool to my ears) but I now want to get a different effect -

On many old delay pedals, when you twist the knob to change the delay time, you just raise the pitch of the repeats by a constant interval (and shorten the delay time by the appropriate amount). The pitch doesn't rise or fall with each repeat - presumably this is because the write and read "heads" on the delay line are speeded up or slowed down by the same amount. I'm trying to replicate that effect in PD. Not neccessarily by the same means - by any means really.

http://www.pdpatchrepo.info/hurleur/pitch-delay_2.pd

Maelstorm - thanks also. I understand why the pitch changes on a delay pedal. The pitchshifter patch was a bit of a red herring - though of course it's the same principle. The difference between what you're (both, I think) talking about and what I'm talking about is the way that the pitch changes.

Assuming a stable C tone playing into the delay:
With the standard simple PD delay set up, if you move the read point of a vd~ then you get a glissando as it accellerates, a constant pitch change as it moves at constant speed. So if you turn the knob to change the delay time in the middle of a tone you start with a constant pitch (C), then get a rise of pitch, then it levels out at a new pitch (as you turn then stop turning the knob),
_
___/

If you feed back into the delay, the glissando is repeated as the read speed changed while the write speed was constant:
_ _ _ _
/ |/ |/ |/ |

The effect I'm looking to emulate on the other hand is more akin to changing the speed of a phasor~ reading an array - the pitch change is not a blip, but a stable interval's transposition - eg: you turn the knob, the pitch of the repeats rise by a given interval and stays at that pitch as it repeats (now more quickly):
______
___/

If you play a constant C tone, then speed up the delay until it is a major third higher, you get a major third diad (until the delay dies away), rather than a C tone with a repeating squiggle overlaid.
The effect is the same as you get by speeding up a tape loop delay (though the pedal I'm trying to imitate is a digital delay) which is why I think the rate of the write and read heads are being increased by the same amount.

[edit, just tried to make this clearer and removed a couple of errors]

I mentioned tape loop delays (though I'm not trying to emulate tape echo) because if you speed up the tape you speed up the read and write speed simultaneously - so you don't get pitch wiggles on the regenerations, you get constant pitch.

In most cases it'd be a pretty subtle difference, but I'm trying to get a particular effect.

I think, as I said earlier, what I need to do is to change the speed of the write "head" by the same amount as the read "head" - I'm guessing it can't be done with delay objects, but I wonder if the same thing is possible with other objects.

delayline for pitch effect (without feedback loop)
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delayline with feedbackloop for delay effect
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dac~

have a look at the attachment.

http://www.pdpatchrepo.info/hurleur/pitch-delay_3.pd

It's very analog-sounding for a digital delay, so I threw together something based on my very rudimentary knowledge of how bucket brigade analog delays work. I used 5 different delwrite~ and vd~ pairs and modulate the delay times of all vd~s simultaneously. That basically does the simultaneously shifting reading and writing heads thing you talked about (I think?) I also put a filter in the feedback loop to emulate the signal decay in an analog delay. I still hear the ramp-up between frequencies with each echo but it seems to be blurred somewhat. This is as close as I can get to the DE7 right now.

Edit: changed it to 10 delwrite~/vd~ pairs though it's late and I can't actually tell if it sounds any better.

http://www.pdpatchrepo.info/hurleur/bbd-test.pd

i think the effect you want can be acheived by using a array-based aproach rather than a delayline, by writing and reading from the array at changing speed, but I don't know how to do this in Pd. For Max there is a 3rd party object called ipoke~
http://pierrealexandretremblay.com/no-tv//MaxMSP/
that does the trick (the standard max poke~ object has no interpolation, so it can't write at speeds slower/faster than 1). I attached a screenshot from the help file. IMHO this does exactly what you describe.
There is a thread on the list a few weeks ago suggesting such an object for pd vanilla ("tabwriteat~"), but I don't know about interpolation. Also sorce was posted but no ready to use object, maybe someone else can suggest an existing object for pd that does this.