[sdiy] MXR Pitch Transposer Schem/Circuit
martin at martinfay.com
Mon Mar 11 10:55:19 CET 2013
Forgot to copy the list...
> Hi Richie,
> Sean's blog has a whole series of posts on pitchshifting covering
> tape-based onward. My description more or less matches the H910
> which used triangle waves for the crossfade.
> I think something like this could be done with BBDs but the
> artifacts of the modulated clock would surely be "interesting"!
> Sent from my iPhone
> On 10 Mar 2013, at 20:21, Richie Burnett
> <rburnett at richieburnett.co.uk> wrote:
>> The two read-pointers with cross-fading to minimise the glitches is
>> really the bare minimum for pitch shifting. It still sounds quite
>> rough though in practice. Even though you fade the contribution of
>> each read pointer down to zero when it crosses the write pointer
>> theres still a problem unless a whole number of audio cycles fit
>> perfectly into the ring buffer's memory. Essentially the waveforms
>> from the two read pointers are rarely coherent for arbitrary audio
>> inputs. So when you cross-fade between them you get constructive/
>> destructive interference that causes wobbles in the amplitude and
>> phase of the pitch shifted audio output. This sounds rough.
>> Any decent pro-audio pitch shifter these days also performs an auto-
>> correlation on the incoming audio to determine any periodicity.
>> Then each of the two read pointers are moved by whole numbers of
>> audio periods when they would cross the write pointer and otherwise
>> cause a glitch. If you nudge the read pointers by whole numbers of
>> audio cycles then there should be no glitches in theory. However
>> cross-fading is still used to minimise any glitches caused by less
>> than perfect period detection or poor auto-correlation when pitch-
>> shifting complex polyphonic material. At least now the waveforms
>> from the two playback pointers are as coherent as they can be.
>> Almost identical and in-phase. So the cross-fading now causes
>> minimal destructive interference and amplitude wobble.
>> That's how pitch shifting has been done since Eventide's H949
>> Harmonizer in 1977. Sean Costello provides a good explanation here:
>> Sent from my Sony Ericsson Xperia ray
>> Martin Fay <martin at martinfay.com> wrote:
>>> Hi Gordon,
>>> I believe the trick is to have 2 read pointers into the delay buffer
>>> and crossfade between those outputs to avoid reading close to the
>>> Sent from my iPhone
>>> On 10 Mar 2013, at 10:00, Gordon JC Pearce <gordonjcp at gjcp.net>
>>>> On 08/03/13 16:13, Tom Wiltshire wrote:
>>>>> That's a damn interesting gizmo. Does anyone have any idea how it
>>>> Someone on the SQ80 mailing list - where we were discussing time-
>>>> based effects the other day, as it happens - mentioned a pitch
>>>> shifter that modulated the delay clock with a sawtooth. I can see
>>>> how that would give a steady-ish pitch shift but surely it would
>>>> "hiccup" as the saw reset?
>>>> Gordonjcp MM0YEQ
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