[PD] PD Pitch shifter

[richard.bowers]

Many thanks again, Pix. I'll revisit the patch. I've posted this to the list
as you suggest in case people may find it useful.

Richard Bowers.

Subject: Re: [PD] PD Pitch shifter


| On Wed, 16 May 2001 09:49:15 +0100
| "richard.bowers" <richard.bowers at ntlworld.com> wrote:
|
| > Thanks very much, Pix. That helps a lot. I understand the principle now.
| > My
| > remaining problems are to do with the pd objects themselves. Maybe you
| > could
| > help me out? I've enclosed the patch so that we are looking at the same
| > thing.
| >
| > I think I pretty much understand the left hand column - for setting the
| > transposition via tape head speed. However...
| >
| > 1) Am I right in thinking the delay time (5000 samples or ms?) is
| > immaterial, other than it has to be larger than the window size and that
| > it
| > will actually delay the response.
|
| yes.. it is the maximum size of the delay line. it has to be larger than
| the window size. just like csound (i used to use csound) you have to set
| it at init time and you can't change it afterwards, so you set it to some
| arbitrarily large value (but not so large as to soak up all your ram). the
| delay in the output will only be as big as your window, which you can
| change live.
|
| > 2) I am unsure about the roles of moses
|
| moses "splits a stream" of numbers. so here, all the numbers >= 1.5 will
| appear on the right output, and all the numbers less than that will appear
| on the left. you can see that numbers on the left immediately get used to
| trigger a "1" signal. so this just means that any numbers smaller that 1.5
| will be turned into 1's. this is to stop reading from the very beginning
| of the delay line where bad things can happen.
|
| > 3) I don't see what wrap and t b f are doing.
|
| wrap is almost just returning the fractional part of a floating point
| number. eg, 1.5 -> 0.5, 2.3 -> 0.3... it's called wrap because of the way
| it deals with negative values, -1.1 -> 0.9 (and not 0.1), -2.7 -> 0.3 etc.
|
| "t b f" is a trigger object. it takes it's input, and first, sends it out
| as a float to the right most output, then sends it out as a bang on the
| left input. the reason this is done, is because the "/" object that it is
| being sent to will 'trigger' when it receives a bang or a value in it's
| left most inlet (like most objects in pd). so the trigger makes sure that
| the value is sent to the right inlet, and then after it has done that, it
| bangs the left inlet to make it recalculate the division.
|
| the trigger object is used almost exclusively to get around the facts that
| objects only trigger when hit on certain inputs. it's also used to make
| sure that things get done in the right order.
|
| > Sorry, this is very much a newbie question. I would be grateful if you
| > could
| > help me with this. I haven't sent this to the list. I'm using a pitch
| > shifter based on a similar model in CSound but it is simply not as
| > smooth.
| > I'd like to be able to write the pd version in CSound.
|
| maybe you should send this to the list incase it is useful for other ppl.
| i initially didnt post to the list because the traffic has been picking up
| recently, but now i'm thinking it probably would have been useful info.
|
| > Many thanks,
| >
| > Richard.
| >
| > | i'm pretty sure the one you are talking about is the rotating tape
| > head
| > | pitch shifter, which i agree is very cool. actually the idea crops up
| > all
| > | over the place in my main patch. anyhow...
| > |
| > | it is modelled on an actual physical device. you have a tape loop,
| > running
| > | at constant speed (eg, a delay line). then you have a number of read
| > heads
| > | mounted on the edge of a disc that rotates, passing the heads over the
| > | tape (a number of delay taps, which can move). the gain on the
| > different
| > | read heads is oscilated so that the signal is only used from the read
| > | heads that are in contact with the tape. you also have to be careful
| > to
| > | have the gain completely off when the head first touches the tape, or
| > when
| > | it leaves the tape (eg, dont mix in the signal from the delay reader
| > as
| > | you change it's position).
| > |
| > | in the digital world you only really need two delay taps (because you
| > dont
| > | hav eto worry about the mechanics of rotating them around a disc, in
| > which
| > | case you would probably need about three). the delay taps sweep (ala,
| > | phasor) across the delay line (at a speed which determines the pitch
| > | shift). you oscilate the gain of the two taps with a cosine
| > (oscillating
| > | between 0 and 1), in such a way that just as the tap is about to leap
| > from
| > | one end of the tap to the other, it's gain is 0.
| > |
| > | there's my possibly incoherent description...
| > |
| > | pix.
| > |
| > | On Wed, 16 May 2001 00:31:22 +0100
| > | "richard.bowers" <richard.bowers at ntlworld.com> wrote:
| > |
| > | > Hi all.
| > | >
| > | > I was exploring the supplied patches in pd and was delighted at the
| > | > quality
| > | > of the delay pitch-shifter. Would anyone be able to describe it's
| > | > implementation to me as I would like to be able to converted it into
| > a
| > | > CSound instrument (sorry ;) ). I've had a go at understanding it but
| > I'm
| > | > not
| > | > very confident with the PD language so I need some help.
| > | >
| > | > Many thanks,
| > | >
| > | > Richard Bowers.
| > | >
| > | > ============================
| > | > Richard Bowers at
| > | > http://www.kakutopia.fsnet.co.uk
| > | > ============================
| > | >
| > |
| >