Scott Stites Synth DIY


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VTL5C2 Phase Shifter

I've breadboarded this phase shifter using the standard op amp all pass filter stage using the VTL5C2 phase shifter with a 10M resistor in parallel as the variable resistive element. There are currently 10 all pass stages in the phase shifter, and I'm using Rene Schmitz's exponential current sink from his VCO4069 as the current control to the Vactrol LED's.

The response time of the VTL5C2 is fairly slow, so fast transients on the CV line get slurred, which is a *feature* of this phase shifter =-D. Actually, I do like the response - for example, as the modulating frequency gets higher, the depth of modulation drops commensurately. Square waves and fast EG's get mangled by the Vactrols, and if one is using the keyboard CV to control the phase shift, there is a pleasing slew from note to note. Another plus is that the Vactrols are naturally quiet. The down side is that if you want it to *not* slew on fast transients (for example, if you're controlling it with a sample and hold), this is not the phase shifter to use.

In these samples, no filter other than the phase shifter itself is used. The signal (a sawtooth and a pulse wave) go directly to the phase shifter and on through to the VCA. The phase shifter is modulated mostly with a triangle wave LFO, and occasionally by keyboard voltage. Spring reverb is heard in all of the samples, except for sample 3. My handy Ray Wilson AR EG is controlling the VCA.

If you're interested in learning more about LDR control of phase shifters, and phase shifters in general, check out my friend Motohiko Takeda's page about his phase shifter - it's way cool and there are some excellent sound samples by him. His approach is different in that he uses LDR's grouped around a single LED for the control - it's brilliant!

Motohiko Takeda's Phase Shifter Page

High Resonance

In this sample, the resonance (feedback) is set fairly high.

High Resonance (412 KB)

Low Resonance

This is an example of the phase shifter at low resonance.

Low Resonance (363 KB)

In-Phase/Antiphase Resonance

In this example, only the 'wet' signal from the phase shifter is used, so there really is no phase cancellation going on - it's more of a series of band pass filters. I've attached a switch to shift the phase of the feedback 180 degrees. When using the phase shifter in this mode, the difference in feedback phase is quite apparent. In this mode, you can get some funky, formant like timbres going. Halfway through the sample, I flip the switch, so you can hear the difference. When using it as a 'normal' phase shifter (IE mixing wet and dry signals), the effect really doesn't work. In this sample, the LFO and Keyboard CV are controlling the phase.

In-Phase/Antiphase (283 KB)

Effect of Vactrols on Sweep Frequency

In this sample, at high resonance, the phase shifter is modulated by a slow triangle wave. As the sample progresses, you can hear the modulating frequency increase and the depth of modulation decrease as the LFO begins moving too fast for the Vactrols to respond fully.

Slow/Fast Sweep (455 KB)

Keyboard Voltage Modulating the Phase

In this sample, the phase shifter is modulated slighty by the LFO, and also is modulated by the keyboard CV. The keyboard CV is also running through Scott Bernardi's lag generator, with the + changes slewed slightly, while the - changes are not slewed at all.

Over the Rainbow (228 KB)

The All Pass Filter's Role in a Modular Synthesizer

Phase shifters are basically all pass filters set up in a chain of usually four or more stages. An all pass filter passes all frequencies, but will alter the phase of a particular frequency. When mixed with the dry signal, the out of phase frequencies will combine with the original frequencies and produce a series of notches and peaks, depending on the number of all pass stages. Two all pass stages will create one notch and one peak, four all pass stages will produce 2, etc.

Most people, (though not all) generally view a phase shifter as a sort of post-synthesis processer, and usually this consists of running the signal through a phase shifter that is swept with either a sine or triangle wave LFO. My philosophy on phase shifters is that they are not only a great post-processor, but also a useful tool in the initial synthesis of a signal. In other words, the phase shifter to me is just another type of filter, which is just as important as a low pass, high pass, bandpass or notch filter. All pass filters can be simulated to a certain degree by mixing all of the outputs of a state variable filter together, but normally this isn't as intense as a full fledged resonant all pass filter. All pass filters can be controlled by not only LFO's, but by any other control voltage source such as keyboard voltage, sample and hold, envelope generators, sequencers, etc. For an example of an all pass filter controlled by a sample and hold, check out my Small Stone sample and hold example on the home page. The all pass filter in a 'static' state (IE, not modulated) is wonderful for simulating resonant cavities.

And with this particular phase shifter, the Vactrols do play a role in imparting some acoustic properties as well. Remember when I said this wouldn't be terribly good for a sample and hold? I guess I'll have to eat those words. As long as the modulation isn't too intense, you get a nice plucking envelope. Listen to this sample (same setup as sample and hold #3 on my 291 page, only the S&H is fed a triangle wave instead of noise).

Controlled by Sample and Hold (486 KB)

Another S&H Controlled Sample 1

Another S&H Controlled Sample 2

Of course this isn't the only filter I see normally 'left out' - there's also the flanger (the comb filter), but that's another story.

Following are a couple of samples of the VTL5C2 phase shifter used more in the capacity of an 'all pass filter'.

The same patch as in the preceding samples is used - sawtooth and square wave signals mixed together through the phase shifter and through the VCA (no other filter is used). The difference is that the LFO sweep is attenuated to a very small factor. The EG is used as the main modulating signal.

Phaser Filter 1 (359 KB)

Phaser Filter 2 (440 KB)

As with any module in a modular synth, ideally the phase shifter can be used to process control as well as audio. In this example, one VCO's triangle output is modulating a second VCO exponentially through the phase shifter. The triangle of the modulated VCO is connected directly to the VCA and then on out through the amplifier. The phase shifter is set to high resonance, and there are a number of peaks that are added to the modulation as the phase shifter itself is modulated by an LFO. Note that the phase shifter is not part of the audio chain, but rather the control chain.

Phase Shifter Processing Modulation (584 KB)