Multiphase Technical details

Foreword

In the late winter/spring of 2006 = I began=20 work on the MultiPhase Project. In June of that year I diverted = work to=20 some other pressing projects, and, at this writing, have yet to return = to the=20 MultiPhase Project. In the time between the beginning of this = project and=20 now (late May of 2010) several projects have come between me and the = completion=20 of this project. In that time, I've begun work on the MultiPhase's = sister=20 project, the HEADLine project. I fully intend to follow through = with this=20 project. I think of all the projects I've documented on this site = (and=20 those I have not or have yet to document), I find the set of samples = found on=20 the MultiPhase Diary page to be some of the most compelling of those = I've posted=20 here. I plan to return to this project (and the HEADLine) after = the summer=20 season has ended. Following is the original documentation I wrote = in 2006=20 concerning the technical aspects of the MultiPhase so far = completed.

When I return the project, I will continue to refne the=20 implementation. The concept is pretty much set in stone, but I've = learned=20 so much since those days in 2006. Working on the HEADLine has = revealed=20 finer technical details of how to accomplish the goals in mind.

Overview

The MultiPhase is designed to be a = highly=20 configurable rack mount opto-electric phase shifting device.

It contains two separate phase shifting sections (referred to as = Phase=20 Banks), with each bank modulated by its own control circuitry. The two = phase=20 banks can be operated individually, in parallel, or in cascade mode for = up to 32=20 stages of phase shifting.

Each phase bank has two separate tap selection switches for tapping = the wet=20 signal (referred to on this page simple as the 'phase tap') and for=20 regeneration. The tap selections for each phase bank are at the second, = fourth,=20 sixth, eighth, tenth, twelfth, fourteenth and sixteenth stage. Odd = numbers of=20 phase shift and regeneration are accomplished by injecting the input = signal=20 and/or the regeneration signal into either the first stage or the second = stage.

This configuration allows anywhere from 1 to 16 stages of phase shift = per=20 channel for stereo operation, or anywhere from 1 to 32 stages of phase = shift for=20 mono operation..

Modulation sources include three LFOs and an envelope follower. Two = of the=20 LFOs, based on Ray Wilson's "Cool New LFO" design, provide triangle, = pulse, and=20 sine waveforms each. Each LFO is adjustable between triangle and saw, = sine and=20 rampoid sine, and variable pulse width (the LFO's, except for the sine = shaping,=20 are taken directly from Ray Wilson's LFO design). A third LFO, based on = Thomas=20 Henry's Quadrature Function Generator, provides two different waveforms = -=20 Triangle and Hypertriangluar. This LFO is capable of offsetting the = selected=20 waveform, sent to Phase Bank B by 0, 90 or 180 degrees in relation to = the=20 waveform supplied to Phase Bank A.

The envelope follower is based on Ren=C3=A9 Schmitz's treatment of = the Elektor=20 envelope follower.

The modulation scheme is planned so that each phase bank can be = operated from=20 either an inverted or noninverted copy of each source so that anti-phase = modulation is possible between the two sections for any waveform. = Moreover, a=20 submixer is included in the modulation path of Phase Bank A so that the = level of=20 LFO1, LFO2, External CV and the Envelope follower controlling the two = phase=20 banks can be adjusted simultaneously, in phase or antiphase, by a single = set of=20 controls. A master intitial phase control is also provided so that the = initial=20 phase of the two phase banks can be adjusted simultaneously.

Each phase shift section will have a continuously variable mix of wet = to dry=20 signal. Voltage controlled crossfaders (voltage control normalled to the = selected LFO2 output) are planned so that the Phase Bank A and Phase = Bank B wet=20 signals can be crossfaded from channel to channel.

The MultiPhase project pays particular attention to the importance of = regeneration in the creation of dramatic phase shifting effects. Each = section=20 contains a soft-knee limiter/compressor as well as a high-cut control. = Each=20 section's regeneration can be either inverted or non-inverted, and, as = mentioned=20 before, an even or odd number of stages can be selected separately for=20 regeneration. Cross regeneration (the ability to send the regeneration = tap from=20 one section to the opposite section and vice versa) is implemented as = well.

Multiphase Front Panel

This panel is currently around the = fourth=20 revision. Certain things are still a bit fluid in the design of the = MultiPhase,=20 and some minor aspects of the panel are bound to change a bit.

Click the Picture to Expand

In any event, it's a 3U high rack panel. I believe I have used the = proper=20 spacing in order to fit the front panel components on the board. Much as = I=20 dislike the idea, I plan to put all of the LFO outputs on the rear panel = -=20 there's just no room at the inn on the front panel. A patch bay can = bring them=20 round to the front if having them back there is too much of a pain.

MultiPhase Block = Diagrams


Audio Block Page = 1	Audio Block Page = 2	CV=20 Block

Above are the links to the basic block diagrams of the audio and CV = paths in=20 the MultiPhase. As soon as I reach the point in the project where they = become=20 available, I'll post actual board interconnections.

Phase Board A and Phase = Board=20 B

Phase Bank A and Phase Bank B operate = from two=20 electrically identical 'Phase Boards' - Phase Board A and Phase Board B. = Each=20 phase board contains the 16 all pass circuits associated with its = respective=20 phase bank. The actual current sink control transistors will be on these = boards=20 as well, though at this writing I do not have them installed - I want to = play=20 with that section a bit more, and will match the transistors for beta = and Vbe to=20 see if that makes much of a difference. The current sinks themselves = will be fed=20 from the Control Voltage Mixer Board, which is as yet undeveloped.


Phase Board A = Top	Phase Board A = Bottom	Phase Board B=20 Top

I used my favorite protoboard from Futurlec for these (same stuff I = used on=20 the Dim C main boards). It's great for keeping things organized, and = provides a=20 perfect opportunity to keep a good star power and grounding scheme = going.

The picture of Phase Board A has the Even/Odd switch connected to it. = This is=20 a C&K 7211 ON ON ON configured as follows:

Down Position - Signal input and resonance insertion is at = Stage=20 1.

Middle Position - Signal input is at Stage 1, resonance = insertion=20 is at Stage 2.

Up Position - Signal input and resonance = insertion are=20 both at stage 2.

The yellow wires along the top of the board are the tap points - = right now=20 I'm just clipping onto them with alligator clips.

Schematic of Phase Board = A

I have not posted one for Phase Board = B, but it=20 is identical in every way to Phase Board A, just the reference = designators have=20 been changed to accomodate it.

Phase Board A Schematic

One may notice the difference in the number of capacitors on the = schematic=20 and the number actually used on the boards. This is a result of my = 'matching'=20 capacitance with the Vactrols. I'm not sure if this is all that = effective, but I=20 went through the excercise anyway.

Basically, I measured each Vactrol at minimum resistance (IE, maximum = positive CV offset). I then used a measured value close to what all of = the=20 Vactrols measured and used that as my reference. I then calculated the = value all=20 of the other Vacrols actually deviated from that one value and converted = them to=20 a percentage of how far off they were. I then matched the capacitors to = that=20 same percentage, but in the opposite direction.

Say a Vactrol varied from the reference value by a positive 10%. I = would then=20 match capacitors to 2n7 minus 10% of 2n7 to match that Vactrol value. I = did this=20 by measuring 10% tolerance capacitors of 1n, 1n2, 1n8, 2n2, and 2n7 = values and=20 mixing and matching one or two in parallel to add them together to get = the=20 values that I needed.

To be honest, selecting Vactrols that match closely at that minimum=20 resistance value and just using 2n7 capacitors is most likely just as = effective.=20 In any event, I know the Vactrols and caps all agree at two points - = minimum=20 resistance and maximum resistance of the Vactrols. What happens in = between is=20 the magic of the MultiPhase. I *figured* that would gag you =3D-D.

Another difference that may be noted between the schematic and the = board pics=20 is that capacitors 135 through 149 are not present on the board pics. I = took the=20 board pictures before I put in these caps.

When I breadboarded the phase banks, I would have problems with high=20 frequency oscillation when chaining greater than sixteen stages. To = assuage=20 this, I put in low value (47 pF) feedback caps to kill the oscillations. = I=20 chalked this up to all of the flying wires of the breadboard, but kept = it in=20 mind when laying out the Phase A and Phase B boards - I went through = special=20 pains to leave these pads open should I actually need the capacitors on = the=20 finished boards.

I'm glad I did. Each board individually worked fine, but when = chaining them -=20 hoo boy - let's just say I built a very exotic combination white noise=20 generator/radio receiver. From stage to stage (above 16 stages) the = noise got=20 progressively worse, with stage 32 ending up as a roaring font of noise = and=20 Mexican radio. I put in the 47pF caps, and really got jiggy with the = last two=20 stages, using 100 pF caps. This killed the problem completely. My = concern was=20 having all of those caps in there would dull the high end, but I've = noticed no=20 difference in that regard.

I have the current sink on the schematic, though without any values. = As=20 mentioned above, I'm still experimenting with that.

(Mis)Using the = Thomas=20 Henry Sine Shaper

Thomas Henry's books are an endless = source of=20 wonderful designs, understandable theory and inspirational ideas. One of = the=20 bits I've culled from his works is an outstanding sine shaping circuit = that=20 ***does not use an OTA***, taken from the Thomas Henry Book "Making = Music with=20 the 566" (Copyright 2003, Midwest Analog Products).

Especially with the demise of the CA3080, and even before that, I was = loathe=20 to use an OTA for sine shaping after discovering this gem of a circuit. = Instead=20 of an OTA, Thomas resorts to the discrete differential pair method of = deriving a=20 sine wave from a triangle waveform. It is an exquisite circuit that can = yield=20 distortion figures under 1%. Thomas does a bit of magic with the circuit = that=20 I'll reserve for Thomas' words himself - IOW, buy the book, which = is =20 available through Magic Smoke Electronics:

Go to Magic Smoke

I use this circuit for all of my sine shaping. In the case of the = MultiPhase,=20 I'm using Ray Wilson LFO's, which put out the requisite 10Vp-p triangle = wave,=20 centered around 0V (ground). In fact, I am using the circuit for the = normal sine=20 outputs of Ray's LFO, eschewing Ray's use of the LM13700. However, I am = also=20 using Thomas' circuit for forming the hypertriangular waveform. Previous = to=20 this, I was using a rather fussy method of FET distortion to get the=20 hypertriangle wave. Ren=C3=A9 Schmitz mentioned the idea of = mis-adjusting a sine=20 shaper to get the same effect. Bingo! I put Thomas' circuit to work and = below is=20 a rather blurry picture of the result:

Hypertriangular Waveform

When misadjusted for hypertriangularity, the output is roughly = 10Vp-p, with a=20 very slight negative offset - close enough for the girls I go out with, = or, in=20 this case, the MultiPhase. I may or may not balance it out a bit = downstream -=20 right now it works perfectly well, and sounds great modulating the = MultiPhase.=20

Thomas Henry Sine Shaper

Thomas Henry was kind enough to give me his permission to post this = circuit=20 fragment. Below is a schematic of the the essence of Thomas' circuit = itself.=20 I've left out a portion dealing with bumping up the 566 output to = 10Vp-p, and=20 I've left off the 1K output resistor because this portion of the circuit = will be=20 going to the CV mixer of the MultiPhase. I've used a TL072, and I have = one half=20 of it sitting at rest, waiting to be used for something - I usually have = no=20 problem finding work for an unused op amp section.

LFO3 - Wave Select = and Phase=20 B Offset

The concept of LFO3 has changed a bit. = Previously the idea was to be able to select either a single triangle = wave sent=20 to both phase bank CV mixers, a single hypertriangle wave sent to both = phase=20 bank mixers, or a hypertriangle wave sent to Phase Bank A and a 180 = degree=20 offset hypertriangle wave sent to Phase Bank B.

That idea has now changed - instead of a third Ray Wilson LFO, LFO3 = will now=20 be the Quadrature Function Generator designed by Thomas Henry, published = both in=20 Polyphony magazine and his book "Build a Better Music Synthesizer". The = speed=20 range control will be dropped in favor of a Wave Select switch. The = function of=20 this two position switch is to determine whether LFO3 will send triangle = or=20 hypertriangular waves to the phase banks. The three position Offset = switch will=20 select which offset of the wave selected by the LFO3 Wave Select Switch = to send=20 to Phase Bank B. The down position will be the 0 degree wave (same as = sent to=20 Phase Bank A), the middle position will send the 90 degree offset = quadrature=20 wave to Phase Bank B, and the up position will send the 180 degree = offset wave=20 to Phase Bank B.

Anyone who's read any of Thomas Henry's book will recognize his = theorem of=20 "Never open a can of beans with a stick of dynamite." Rarely do I manage = to meet=20 that criteria - not that it's not very good advice, it is, it's just = that=20 usually I'm not clever enough to operate the can opener.

When the idea struck me, I sat down to sketch out a switching scheme. = The=20 first design I came up was classic "stick of dynamite".

Stick of Dynamite LFO3 Switch Diagram

The Stick of Dynamite method uses three waveshapers to convert the 0, = 90 and=20 180 degree triangle waves to hypertriangular waves. Each of the triangle = outputs=20 and hypertriangular outputs are connected to a three pole, double pole = ON-ON=20 switch. The 3PDT switch feeds a double pole double throw ON-ON-ON = switch, which=20 sends the wave selected by the first switch to Phase Bank B.

The Stick of Dynamite aspect of the design is this: it requires three = wave=20 shapers, and a 3PDT switch, which is rather large and possibly too bulky = to fit=20 into my current panel design.

Dissatisfied with this method, I went back to my scratch pad and = designed the=20 Can Opener method.

Can Opener LFO3 Switch Diagram

To my brain, this method is more complex, but yet accomplishes the = same task=20 with only two waveshapers and a DPDT ON-ON switch instead of the 3PDT = switch.=20

The first section of the Wave Select Switch selects between the 0 = degree=20 triangle wave and 0 degree hypertriangular wave.

The second section of the Wave Select Switch selects between the = either the=20 90 degree or 180 degree triangle wave OR the 90 degree or 180 degree=20 hypertriangular wave. Whether it's the 90 degree version or 180 degree = version=20 of the selected wave is determined by one section of the LFO3 Offset = switch.=20

The first section of the LFO3 Offset switch selects between either = the 0=20 degree triangle or hypertriangular wave, as selected by the the first = section of=20 the Wave Select Switch, OR the wave selected by the second section of = the Wave=20 Select Switch, described above.

The second section of the LFO3 Offset Switch directs either the 90 = degree or=20 the 180 degree triangle wave to the second waveshaper. This in effect = chooses=20 either 90 degree or 180 degree offset for the triangle or = hypertriangular wave,=20 depending on which wave is selected by the second section of the first = switch.=20 Whew!

The method of switching at this point is academic - I may just go = with a=20 switching IC and the switches themselves will just control the on/off = stages of=20 the IC switch sections, but the same general design will be followed. =

Wet Signal Crossfaders

The wet signal crossfaders come in = handy for a=20 couple of things - mixing in a portion of the opposite phase bank's wet = signal=20 to the wet signal of the current phase bank makes for some great = variation in=20 timbre/sweep, and modulating the cross-faders with a single CV adds = greatly to=20 the pallette of stereo panning sweeps (even without modulating the = crossfaders,=20 the stereo field can be affeted by the crossfaders as well).

In review, a phase bank's wet signal, before it is mixed with the = original=20 dry signal feeding the phase bank, can be mixed with the wet signal from = the=20 opposite phase bank in any ratio, as set by the XFade controls. The = crossfaders=20 can be modulated so that a constantly varying amount of crossfading can = be=20 accomplished. The control input to the phase banks is normalled to the = selected=20 wave of LFO2, the level of which is controlled by the XFade CV Level = control. An=20 external CV can control the crossfade panning as well by inserting the = CV into=20 the crossfade CV input - this disconnects LFO2 and allows the control = voltage to=20 control the crossfading.

In the case of voltage controlled crossfading, either by external CV = or=20 internal LFO2, the XFade controls act as the starting point of the = crossfade.=20 Setting them in center feeds an equal amount of each phase bank's wet = signal=20 through the crossfader. The expected CV input is a 10Vp-p signal = centered around=20 0V. With the XFade control centered, -5V will allow full signal from the = associated phase bank to pass, with no signal from the opposite phase = bank. +5V=20 will allow the opposite phase bank's signal to fully pass, with the = associated=20 phase bank's signal fully attenuated.

I've changed things a bit since I drew the panel diagram and defined = the=20 crossfade function. Originally, I had a bipolar level control for each=20 crossfader. I figured out that this was fairly redundant - the idea was, = one=20 could modulate the crossfader's opposite of what LFO2 or the external CV = was=20 doing, in varying amounts. Well, that can be accomplished by the = inverting=20 attenuators in the modulation control section. And, I found that the = need to=20 modulate the crossfaders in different amounts was rare to non-existent. =

So, I dropped the extra pot and added an 'XFade Invert' switch. This = switch=20 will allow inversion of the wet signal from the opposite phase bank = before it's=20 applied to the crossfader. Now *that* I found is very useful - it opens = up=20 another pallete of timbral and panning effects.

MultiPhase Crossfader Schematics

I used Mark Verbos' crossfader (adapted from a Buchla design) as a = starting=20 point for the MultiPhase crossfader. It's Vactrol based and is quite = effective.=20

I never had much luck with the original circuit as drawn - the = Vactrols drop=20 very low in resistance, and they are the input resistors to an op amp = that has a=20 100K feedback resistor. IIRC, the Vactrols drop to around 4.7K, or = thereabouts,=20 so that gives the circuit a gain of around 21. IE, the circuit would = shriek and=20 distort.

I dropped the 100k feedback resistor to 10k, and added 2k2 resistors = in=20 series with the Vactrol LDR's. This cut the gain down considerably, and = when I=20 re-invert the signal in the second stage, I get a little of that back = with a=20 100k input/82k feedback resistor. The signals exactly match in level = from input=20 to output, and, as with the original circuit, there is no amplitude = 'bump' in=20 the middle of the crossfade - it works quite well.

The other task was balancing out the XFade and CV control inputs to = render a=20 smooth crossfade. A range of 0 to 6.5V will give a smooth crossfade from = one=20 side to the other. I set the center trim to between 3.25 and 4 volts. I = then=20 created a +5V and a -5V source for the XFade controls to operate = between. I=20 scale that voltage down in the first stage of the control section with = 33k input=20 and 22k feedback resistors. I allow the external CV to have a slight bit = more=20 effect by using a 30K input resistor (to account for possible variations = in=20 control voltage sources). Instead of a bipolar attenuator, I use a = simple=20 unipolar attenuator (read pot set up as a voltage divider) to allow the = user to=20 easily cut out all possible modulation by simply setting it to full CCW. = The=20 second stage inverts the control signals back to original polarity after = going=20 through the first stage.

The LEDs (D1 and D2) are panel mount - the brighter they glow, the = more of=20 the opposite wet signal is being crossmixed into the associated phase = bank's wet=20 signal.

The XFade Invert switch just selects between the direct wet signal, = or an=20 inverted copy of it before it is fed to the crossfader op amp.

This circuit, BTW, will work great for crossfading any signals in a = modular=20 synth. Obviously the XFade Invert Switch and op amps wouldn't be = necessary in=20 that case.

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