BBD + LFO Chorus?

[apehead]

OK, this may require some background info...

Can't find a keyboard player, but tons of guitar players. Want to make guitar sound similar to organ. EH POG and HOG too expensive, found affordable Micro Synthesizer instead. Problem is, Sub-Octave, Octave, and Square generators on Micro Synthesizer have no ability to de-tune relative to dry input signal. Was interested in using BBD clocked by LFO to variably, independently, de-tune all four (octave, dry, sub-octave, square) signals pre-VCF, via pedal mod. Does this circuit have precedence? Are there any existing pedals out there I can just clone for the de-tune module? I am not an engineer by any means, and would need some serious help if required to design the circuit, rather than "plug and play". Any help would be much appreciated.

Alternately, perhaps a PLL circuit with a "de-tune" feature would be a better alternative? Again, I'm no engineer, so a existing circuit to reference would be most helpful.

[Mark Hammer]

Looking at the schematic, that would seem to be a pretty big hack.

Your instincts about needing to do it via an external de-tune are correct though. You do not need to do it with BBD-based time delay, however. Note that a phase-shifted signal, properly modulated, without a straight signal to combine it with, will produce a vibrato. The more phase shift stages are used, the greater the pitch shift.

I suggest this because, quite frankly, you really don't want all those clock signals floating around in there. Just too many ways to create unnecessary noise. Far better to be content with a simple LFO married to 4 phase shift stages.

Because the vibrato will be applied to sub-octave, you may want to consider using lag stages, rather than lead (i.e., cap goes to ground, instead of resistance to ground).

But boy oh boy, that's gonna be one monster hack.

[apehead]

Yeah, I've considered just building a pedal from scratch, but again, being ignorant of the finer points of circuit design has pigeon holed me into relying on combining "tried and true" circuit architectures to achieve my sonic goals.

Would phase shift translate into a pitch shift? Essentially, I am attempting to re-create the subtle beating effect of slightly out of tune notes being played in unison (12 string guitar, piano, or the myriad tonewheels in a Hammond organ), which seems to be integral to the tombre of an organ. I don't understand how phase shift would accomplish this, although I can see how it would be useful in simulating a Leslie-type sound, which I haven't even begun pondering.

I'm also a bit in the dark about the basic electronic functionality of the effect. Fortunately, the schematic incorporates some "block function" text, but I am confused why they label what appears to be a PLL in the sub-octave section as a "track and hold modulator". Also, I am confused as to where/what the octave signal is generated (by). I was hoping that pehaps I could just manually alter the tuning of the oscillators, but it seems by your comment that wouldn't be possible.

And by the way, this is only the beginning of what I've been cooking up for this effect.

Edit: Sorry, I was confusing "vibrato" with "tremolo". How does a phaser accomplish a pitch shift? That is very interesting.

[Mark Hammer]

I wish I understood how phase shifting accomplishes pitch shifting. All I know is that if you lift the dry signal on a phase shifter or Uni-Vibe, so that you only have the phase-shifted signal, you get a clear pitch wobble. Not as dramatic a pitch wobble as the detuning of a chorus or flanger, but a clearly audible one. And, as noted, if you have more phase shift stages, you get more pronounced wobble. I build it into pretty much every phase shifter I make.

The Microsynth uses full-wave rectification to produce an octave up, and uses the classic method of dividing down with a 4013 flip-flop to produce the octave and sub-octave. Well, let me correct that. There are two methods of using the 4013. One is to use the square output of the flip-flop directly, and the other is to use the flip-flop's output to control something else. In this case, the flip flop is being used to control that 4302 FET, via the 3094. This gives a smoother, warmer-sounding octave down.

[apehead]

Awesome, thanks for the help. It really helps me to understand sort of the "theory of operation" of a circuit before I take a look at the schematic. I'm sure it will make a whole lot more sense to me now. Are there any extant phase shift circuits that you would recommend?

Edit: How would one control the amount of vibrato? The "rate" (phase shift amount) control? Do standalone vibrato pedals operate on the same principle as this?

[Mark Hammer]

Some standalone vibrato pedals use a phase-shift-only approach, although it would seem that most are essentially de-chorused chorus pedals.

Note that, because small pitch changes are hard for the human ear to detect unless they are fast enough, vibrato devices will usually oscillate at faster rates than chorus, flanger, or phase-shift; i.e., generally not slower than 1hz. It also seems to be the case that the preferred LFO waveform for vibrato is somewhat different than that for slow filtering (phaser, flanger) sweeps. That's one of the reasons why so many folks loved their Boss CE-1; it used a different LFO waveform for vibrato than it used for chorus. Very slow sweeps seem to be more musical if they are "hypertriangular" (triangular at the topmost part of the sweep and sinusoidal at the lower end), while quick sweeps for vibrato seem to be more pleasing if they are sinusoidal at top and bottom.

The "turnaround" (I like to use an Olympic swimming metaphor) in the sweep is a critical aspect of what makes modulation feel more musical. When FETs are used for modulated phase-shift stages, it is often important that they be matched so that they are all changing resistance concurrently during the sweep. If they are unmatched, then one or more can crap out on you and stop changing resistance shy of the extremes of the sweep, and the "turnaround" becomes less graceful. LDRs are relatively immune to that problem, since few light sources ever reach a point where they hit the maximum and minimum resistance of the LDR is reached. They (FETs and LDRs) do NOT have to be matched for their resistance, only for their ability to keep changing resistance in response to a shared control voltage from the LFO.

OTAs have the advantage that they do not need to be matched, however many OTAs do not handle larger amplitude signals very well, necessitating attenuation beforehand and gain recovery afterwards, which risks poorer S/N rations. The ideal is LED/LDR since that does not require matching, and handles higher signal amplitudes nicely.

Look for Google images of phase-shifter schematics, and I'm sure you'll come across scads of LDR-based designs. They're pretty common.

[apehead]

So, an LFO is a requirement to the design? If you manually controlled the phase shift via a slider/pot, would the pitch shift still occur, or is it more an artifact of the constant phase shift occurring when the all pass is modulated by the LFO? That would stand to reason, as that is a requirement for a BBD circuit to change pitch (I'm thinking specifically of manually stroking the "rate/delay" pot on a delay pedal, which causes some wild pitch shifts in the delayed signal). Of course, I could be completely off the mark, too.

[Mark Hammer]

Well if you want a constant detune, such that the octave down is always an octave (and a weeeeeeee bit more) below, that would require a different technology. If you want the chorus-ey kind of detune, then you will need an LFO, and probably separate LFOs for the individual channels.

I'm assuming, perhaps naively so, that you want the result to have the sort of "animated" sound that one gets from an old piano where the 3 strings per note are not precisely in tune?

[apehead]

Crap, I double posted, went to edit my double post, and it deleted my initial post as well!!!

Anyways, I think the pitch shift is caused by Phase Modulation, which is a result of the operational priciples of an All-Pass Filter combined with its modulation by the LFO.

Basically, the LFO continuously changes the cut-off frequency of the high-pass section of the all-pass. This alters the phase shift of the carrier (in this case, the audio) as the filter elbow is shifted higher and lower along the frequency spectrum. This continuous phase shifting incidentally alters the frequency of the carrier as well as phase, which would explain why a vibrato is produced. It also explains why more filters in serial can increase the effect, as the phase lags more and more through each successive filter.

I think that the effect can be controlled via adjustable LFO amplitude and frequency.

I think I just found the exact circuit for my needs! I'll probably replace the FET's with opto-couplers, though.

I want to simulate the slight beating caused by slightly out of tune tones heard in unison. Manually detuning isn't all that important if I can achieve this sonic goal some other way.

[apehead]

I suppose I was initially looking for a manually variable detune for each voice, so that a natural "chorus" effect would result from each voice simultaneously being heard, yet ever so slightly out of tune with one another. I would say the piano analogy is appropriate, and I've learned that there seems to be simpler methods to achieve that effect, without the need to manually detune each voice.

I also talked to an engineer at my work, and we have a hypothesis as to why phasers can cause vibrato when not mixed with the dry signal. Apparently, an all-pass filter's length of phase delay is dependent on the frequency of the input signal. Only frequencies close to the elbow of the high-pass section of the all-pass filter are phase shifted 90 deg. The lower or higher the frequency in relation to the filter elbow, the longer or shorter the phase delay (as the impedance increases or decreases). Since the LFO is constantly shifting the elbow of the filter (it acts upon the resistance of the high-pass section of the all-pass) the amount of phase shift is also constantly being altered. We believe that this effect causes some sort of Phase Modulation with the audio acting as the carrier, and the LFO acting as the intelligence (how's that for role reversal?).

I would be interested in how I could take advantage of this effect. Perhaps LFO amplitude and frequency controls as the sliders/knobs for each voice (would these correspond to the "depth" and "rate" functions on a regular phaser, or is depth usually part of the comparator)?