This thread so represents what I love about this community. A lil bit of arguing in the beginning about the actual intent / purposefulness of the OPs question, then a little „can’t be done that easily“ and then someone like @gc3 or @DaveVenom comes along and does a „I am going to science the sh*t out of this“ move and here we are. Problem solved, Mind blown, knowledge gained. Fantastic people!
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To be fair, we are just implementing what others were suggesting.
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Thanks, @aetrion-music! And shucks, I wasn’t even implementing (top marks to @DaveVenom for that), just throwing out some ideas… But I agree–when this forum is operating at its best (which I’d say is >95% of the time at the moment!), it can be a really incredible place. The blend between musicians, platform advocates/teachers, developers, and various mixes of all three is quite special.
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If you use the Pickup output of the Waveguide Delay, tuned to the same pitch as your oscillator, you can modulate the pickup position (basically the position in the waveguide buffer that the “pickup” reads) to phase from 0 to 360 degrees.
If it’s important to have your wet signal go smoothly through zero phase relative to your dry signal, you can get a relative range of -180 to +180 by using a second Wdelay to add 180 degrees to the phase of the “dry” signal. Use the Pickup output and keep the pickup position fixed at 0.5, like so:
-5v will give you -180 and +5v will give you +180.
waveguide_phase_180.vcv (1.4 KB)
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Oh my goodness, thank you! That makes this technique so simple 
If you subtract 1V from the VCO V/Oct and use that for the Delay CV, then the Pickup covers 720 degrees, with -5V = -360, 0V = 0, and 5V = +360
Subtract another volt (VCO V/Oct-2) then it covers 1440 degrees, etc.
Using a clean digital saw wave to modulate the pickup gives interesting results.
If the modulator frequency matches the VCO, then the output is “stalled”. If down 1 octave, then the output is the same wave but also down 1 octave. But if the modulator freq is double the VCO, then the output is the same frequency, but with the wave form in reverse!
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For those interested in “FM” in its more usual Phase Modulation implementation:
Using this concept, could we do Yamaha DX style Phase Modulation (phase shifting at audiorates) with any (audiorate) oscillator as a Carrier?
Since now we can have “external” phase control, we do no longer depend on an “integrated” phase modulation input for the Carrier. The Modulator could already be any audiorate oscillator.
In this setup, both would need the same V/Oct input to generate stable pitch/spectrum.
Adding to this…
Using some form of LFO (below audiorate) and/or of “oscillator” or waveshaper (at 0 Hz) we can also emulate LFO/0 Hz Modulator “FM style” Phase Modulation Wave Shaping (or Phase Distortion).
In that case, only the Modulator needs V/Oct input. The Spectrum will be controlled by the relative initial offset of the Modulator (+ audiorate output of the Modulator + any other control signal, e.g. ADSR/LFO).
Any Carrier (oscillator) frequency above 0 Hz will produce phaseshift cycling at that rate. This changes the generated wave shape and thus the spectrum cyclicly over time. You can compare this to slight detuning of Carrier and Modulator.
But, the LFO/0 Hz “Wave Shaping” does shave a different spectral cycle, as compared to carrier and modulator at audiorates.
The Yamaha DX7 could only go down to 1 Hz (and some detuning), so, no stable spectrum. The SY77/99 go all the way down to 0 Hz. The SY77/99 also has basic initial phase angle offset settings.
BTW, NYSTHI TZOP has most of this and can also do 0 Hz (but does not seem to save this in the patch). I guess it is currently the most versatile FM/PM oscillator in VCV Rack. But…does have “limited” internal waveshapes (but more then sine only).
This Waveguide Phase control solution might further open up Pandora’s box, for “FM” (PM) options. Since we might be able to use any oscillator as a Phase Modulated Carrier.
I guess I’m gonna set up some experiments on this subject…
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TZOP is great. Since you mentioned 0 Hz and waveforms, also be sure to check out Valley Terrorform. It has a 0 Hz mode and is also a wavetable oscillator.
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Hey synthi - I didn’t want you to think I was ignoring you. I had just begun to explore xatto and definitely saw great potential. I do like the context menu options. I was prepared to work out the math for the desired effects for both xatto and Waveguide Delay and compare the results.
But then cthonophonic posted the brilliantly trivial Waveguide Delay solution, and I didn’t see the point of continuing.
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Ah…to my shame, I never really explored Valley’s Terrorform. I mainly wrestle with basic VCO’s, to better understand how stuff actually works.
EDIT: Just scanned Terrorform’s manual for FM/PM https://valleyaudio.github.io/rack/terrorform/
There are many worlds hidden within this beast…including 0 Hz DX Style “Wave shaping”
Zero Frequency
“Zero Frequency? What is the purpose of that?!” I hear you say
The eagle eyed amongst you may have noticed one of the benefits of including the DX style phase mod flavour of FM. Remember, in phase modulation, the phase of the of the read phasor is shifted, regardless of frequency. Therefore, if the frequency s zero and you shift the phase of the read phasor, you will still read the contents of the wavetable but with an external source. When this feature is enabled, the read phasor is “parked” at the beginning of the wavetable so that no hidden phase offset is introduced. The following two sub-sections discuss example uses for this feature.
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I just created this basic Proof of Concept video, showing we can use ANY oscillator as an “FM” Operator, using cthonophonic’s solution using the Waveguide Delay to do the Phase Modulation bit.
Phase Modulation (any Oscillator) using Waveguide Delay
Hoping it will trigger more creativity…
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Fascinating thread and very creative use of the waveguide delay. I am happy that this module is of interest to this community. Concerning its use as a general-purpose phase modulation tool, I think that it could be useful for low pitched signals… Now, for high pitched signals, a lot of aliasing is probably going to be present. I am not sure whether something can be done to prevent it….
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Cool! We used to have some really nice phase modulation oscillators from Animated Circuits in Rack. Sadly they were left by the wayside…
https://library.vcvrack.com/plugins?query=animated+circuits&license=&sort=name
https://github.com/AnimatedCircuits/ACRackModulesPreview
https://www.facebook.com/Animated-Circuits-324990851279344
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Wow - that Cozmic VCO sounds amazing. I’m not understanding why it is called phase distortion, but I love the sound and the layout. I see opportunity for some simple changes to make the layout even better. Like making the wave form lights function as radio buttons so you can quickly select any specific form. Make the CV control be addressable rather than sequential, and add another selection mode that would enable/disable waveforms for when doing CV control. It is a shame the collection is no longer available.
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Aliasing is indeed an issue in many FM/PM implementations. Especially in the higher note range. As I know from my yesteryear Yamaha SY77. But just like the original Supersaw implementation, aliasing can also add “character”.
FM works on the basis of SUM and DIFFERENCE of carrier/modulator frequencies, just like AM (RM/multiplication). So, higher frequencies in Modulator or Carrier can soon generate frequencies that reach Nyquist and can then bounce back into the audible spectrum (aliasing).
Using just sines can restrict the amount of generated very high frequencies. But other shapes, e.g. “jagged” ones that may already contain many high amplitude high frequencies, might soon generate frequencies above Nyquist.
Eventually it will all end up in “FM noise” territory. Usefull for metallic/percussive stuff…
On the bottom end, AM generated subsonic frequencies “disappear” below 0 Hz, but FM generated frequencies will bounce back into the spectrum, but with inverted phase. Another kind of aliasing, you could say, and an integral part of FM/PM synthesis. But it will take quite some high pitch and/or modulation to get those reflected ones all the way above Nyquist too.
Anyway, I was pleasantly surprised by the “smooth” results using Waveguide Delay. It definitely opens up a whole world of new FM/PM options.
E.g. using something like Squinky labs Chebyshev to generate harmonic series frequencies (thus providing/replacing multiple integer-multiple tuned phase synced oscillators). Maybe place a Wave Shaper / transfer function with moduation in the operator signal path. Or use Wavetable Oscillators.
It’s fantastic, you can try it out in v1, the buillds exist.
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Mockba Modular offers some Casio CZ like Phase Distortion modules.
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You’re right, not enough time to check out modules 
They look nice, nowhere near as full-featured as Cozmic though, that was very unique.
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Phase Distortion opens up another chapter. It is basically sort of “special case of” FM/PM, in the direction of more generic “Waveshaping”.
Phase Distortion Synthesis
Comparison to other types of synthesis[edit]
(…), phase distortion broadly applies similar mathematical concepts to phase modulation synthesis, but their implementation and results are not equivalent. Whereas PM - pioneered by John Chowning and commercially used by Yamaha - uses an oscillating modulator that can have its own period, PD applies an angular modulator of straight-line segments hard-synchronised to the same period as its corresponding carrier, i.e. modulating each cycle identically. PM/FM produces Bessel function-derived spectra unless linearised by the application of feedback, whereas PD produces more linear spectra. This manifests in PD synths’ reputation for being easier to produce traditional subtractive sounds, such as those typically associated with analogue synths, which are characterised by linear spectra. These facts demonstrate how although the broad concept - alteration of phase - is the same, implementation and results differ greatly.
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Fascinating stuff! I really like the sounds coming out of that Cozmic (PD) a lot, more than FM.