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Pitch Shifter - how it works Multi-Effects Processors (HW)
Old 5th September 2011
Gear Head

Pitch Shifter - how it works

Hi.. I just want to know in which way the effect (pitch shifter) is works in order to transpose the signal's pitch.

I know that part of the process involve in identification of constant (complex) wavecycles within the incoming signal. then it doubles or removes these constant wavecycles in order to shrink or lengthen the processed signal (similar to granular synthesis).

this theory has not yet clarified the fact that the effect is changes the pitch of the signal

So, does the effect brings down/up its interior sample rate in order to change the pitch of the signal and only then it doubles or removes constant wavecycles so as to keeping the duration of the signal as it been ?

Old 8th September 2011
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Rogue Ai's Avatar
Well, pitch shifting on most samplers simply play the sample faster. While granular synths chop up the sample, cycles the waveforms and plays it at different rates.
Old 8th September 2011
Gear Maniac

Originally Posted by Agurvitz View Post

I know that part of the process involve in identification of constant (complex) wavecycles within the incoming signal. then it doubles or removes these constant wavecycles in order to shrink or lengthen the processed signal (similar to granular synthesis).
you've answered you're own question... a simple pitch shift of an octave would mean identifying a periodic waveform and doubling the amount of instances in the same given timeframe; you're doubling the frequency of the waveform which pushes the pitch up an octave. different intervals are achieved using different functions of multiplication or division.
Old 8th September 2011
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acreil's Avatar

There are a lot of different ways to do it. Early models (like the Eventide H910, and most cheap/simple algorithms) use a two tap modulated delay. The data is written at a fixed rate and read out at a variable rate that determines the amount of pitch shift. Since the delay length is finite, at some point the read and write pointers will cross, producing a discontinuity. To prevent this, there are two read pointers and a modulator that smoothly crossfades between them. The actual idea dates to the 60s, I think. Previously it was done using a tape recorder with multiple heads on a rotating drum (the Universal Phonogene might be the earliest) or with sounds recorded on optical film. The implementation is of course different, but the result is the same. The pitch shifted sound has a sort of tremolo effect. The rate of modulation depends on the length of delay as well as the amount of pitch shift. Longer delays give a smoother sound, and work better for more extreme pitch shifts, but they also have an indeterminate delay time over a larger time interval. You can also easily get reverse delay effects this way.

Later ones, starting with an optional board for the H949, use some period detection to permit splicing at a zero crossing. Since there's no lengthy crossfade, there's no tremolo, and well behaved periodic signals can be cleanly pitch shifted with minimal artifacts and less latency. But if the signal is complex, there's no regular period, and you'll hear a sort of granularity as the splices are made at irregular intervals. The behavior depends on the intended input. Guitar processors (the Digitech Whammy being a classic example) prioritize fast response and are intended to work on a clean monophonic signal, but sound nasty if you attempt to play chords. More general purpose algorithms may change their behavior to adapt to the input signal, splicing rapidly for cleanly periodic signals but using longer delays and crossfades if no regular period can be detected.

It can be done in the frequency domain too, using a phase vocoder. These have the disadvantage of possible "phasiness", but can shift the pitch without changing the formant. This is useful in vocal processors.
Old 9th September 2011
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Eventide H969

The heart of the Eventide harmonizing effect is the phase locked loop (PLL) configured as a voltage controlled frequency converter which is capable of dividing (pitch shift down) or multiplying (pitch shift up) the frequency of input signals. Division using a PLL is a simple, but designing a reliable PLL multiplying circuit is very tricky. This not only requires expert circuit design but expert system design and PC board design in that there are plenty of evils that can trip the best PLL system - power buss noise, EMI from external high frequency sources, you name it. Careful PC Board design is critical to reliable PLL operation - local power regulators, power supply bypass, low impedance ground busses, guard traces around critical paths - the works! Take a look at the PLL system comprising three circuit boards. An excellent PLL system separates the men from the boys. The same is true for pro audio digital converters - a flawed PLL design will create jitter in the clock signal which will cause irreversible distortion in the digital conversion of an analog signal (not clipping but conversion error which results in loss of stereo imaging and depth)
Old 9th September 2011
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acreil's Avatar

You wouldn't use a PLL in an all digital, fixed sample rate system. The Eventide implementation is overcomplicated compared to what you'd do now in DSP. An interpolated delay line is much more convenient, though not without its own artifacts. It doesn't involve FIFO buffers or VCAs, anyway...
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