Analog Vocoder Project  [verified]

Original effect designs complete with verified schematics, layouts and instructions. All project were freely contributed by members and cannot be used for commercial purposes without consent of the respective owners of the copyright.
lithiumdeuteride
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Post by lithiumdeuteride »

Outline
It's time to embark on an epic journey into the realms of analog electronics. It's time to talk like a robot! Will you join me on my quest? C'mon, you know you always wanted to shout "Domo arigato, Mr. Roboto" into one of these things while mashing the synthesizer keys. OK, it's not exactly a guitar pedal, but it would work just fine with a distorted guitar (or any harmonically-rich source) and a vocal mic or another instrument, so I think it's well within the scope of this forum. It's also a really cool machine. Just ask Kraftwerk: . Or these guys:

Purpose
The purpose of this thread is to make the best analog vocoder possible, while keeping cost reasonable. I've read this forum for a few months, so I know there are a lot of knowledgeable and creative people here. If I've done something stupid, or you have an idea for an improvement, please let me know.

What is it?
A vocoder is a device which reads a frequency envelope from a control signal (usually a vocal mic), then applies that envelope to a carrier signal (usually a synthesizer). This is accomplished by splitting both control and carrier signals into many frequency bands, reading the envelope of each control band, then amplifying the corresponding carrier band according to that envelope. The end result is that you get to sound like a classic sci-fi robot. I think David Bowie's guitarist used something similar on 'Let's Dance', with the guitar controlling, and a brass section as the carrier. I'm sure there are other interesting combinations.

Channels
This sucker is going to have up to 14 channels, with adjacent channels' center frequencies being about half an octave apart, or 10^(1/6), to be precise. This allows for a bunch of identical PCBs to be used, with only the capacitor values changed from one channel to another, greatly saving on cost. This also means that convenient E6 capacitor values can be used, hence the 10^(1/6) frequency spacing.

Manufacturing
I'm planning to have Express PCB make boards for this thing, and since the recurring cost for the boards is reasonably small compared to the up-front cost, I can could order enough boards to make several vocoders, in case anyone else wants to build one. I would be happy to sell a set of boards to anyone who wants one at only the recurring cost, plus shipping. I expect one full set of boards will be between $100 and $150, and will need about $200 in parts to fully populate. I also plan to use one or more main boards, with sockets for each channel (1x14socket, 2x7socket, or 3x5socket). The individual channel boards will have plated tabs which press into the sockets, just like a video game cartridge. This will prevent the main board from becoming a tangled mess, and will keep soldering and wiring stripping work to a minimum.

Power
I'm anticipating it will be powered with a +/-12V split supply. A single 9V supply doesn't give it enough headroom, and it has so many op amps and draws so much current that a floating ground seems annoying to implement. It seems much easier to simply run it off a computer's ATX power supply. If someone knows of a simple way to do a floating ground for a large number of op amps, and has a convenient way to power it with a single supply, I'm all ears.

Idea and Improvements
I got the idea to make this from a project on the Instructables website: http://www.instructables.com/id/Build-a ... g-vocoder/

The pairs of multiple-feedback bandpass filters will undoubtedly work great, and the multiple inputs and stereo output (channels alternate left and right) with inverted phase look good. The input and output buffers looked very reasonable to me. It's necessary to invert the output of every other channel, otherwise destructive interference occurs, and the flat-ish frequency response is ruined. I re-worked the the bandpass filters into a configuration I liked better (using only E12 resistors) using this extremely helpful design calculator: http://sim.okawa-denshi.jp/en/OPttool.php

However, there are several things I didn't like. The envelope follower (EF) and voltage-controlled amplifier (VCA) specifically stood out as needing some work. The EF in the Instructables design turns out to work a bit like an on-off switch with a rather high threshold, rather than the linear gradient I would like. So, I did some research and modified the design, borrowing the clever (and functional) EF from the Rothwell Love Squeeze pedal, with a couple modifications for a more rapid recovery after a signal spike. I also replaced the Instructables VCA with the super nifty and economical JFET VCA I found here: http://electronicdesign.com/article/ana ... rolled-amp

I'm also not completely satisfied with the indicator LED (tri-color) system. It does indeed turn on each LED color progressively, but a fairly high voltage must be reached for the first color to turn on, then the others turn on within 0.7 volts of the first. It would be more ideal if the first one turned on at 0.5V, the second at 1.5V, and the third at 2.5V, for example. I don't know to achieve this, so I'm open to any suggestions.

Simulation
Falstad has an excellent analog filter simulator which allowed me to tweak the bandpass filters to a state I liked better than the original. The Java applet is here: http://www.falstad.com/afilter/
And here's the code to import:

Code: Select all

$ 1 5.0E-6 5 69 5.0 50
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I also used Falstad's time-dependent circuit simulator to model the EF, VCA, and display LED sections. Here's a direct link to the Java applet with all components already in place:
http://www.falstad.com/circuit/#%24+1+4 ... .0+2+-1%0A

Outstanding design issues
1) Tri-color LED indicator triggers at too high a voltage, and not spread out enough
2) I don't know of a source for PCB sockets. I'm sure Mouser and Digikey have them - the trick is actually finding them.
3) Envelope follower tracking speed can always be improved
4) PCB layout not done yet (I will do this, but any suggestions or criticism would be appreciated)
5) Enclosure scheme has not yet received much thought

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deltafred
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Post by deltafred »

Sounds like the sort of project I would have tackled 40 years ago but I am afraid I am way past that now but good luck with it and I will be watching with great interest.

(I still have a half built mono analogue synth that I started in the early 70s. A real shame I never finished it because when I prototyped a simpler version it sounded pretty good. I should just have boxed it up and gone with that.)
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Post by lithiumdeuteride »

I've done some more work and determined that having too much amplification in the filters is a bad idea, because of the danger of clipping the carrier signal when a large control signal spike occurs. Here's how it works now:

1) Assume control and carrier signals, each at the center frequency of a channel, each with 100 mV amplitude
2) Control and carrier signals are boosted by a factor of 1.37 (previously 4.75) going through their respective bandpass filter stages, now each 137 mV in amplitude
3) Control signal is boosted by a factor of 11 going into the envelope follower (EF), control signal is now 1.51V, which easily activates the Schottky diodes
4) EF's voltage pump produces a DC voltage of +2.2V
5) Band level potentiometer, when centered, cuts this DC voltage in half, to +1.1V
6) +1.1V sets voltage-controlled amplifier (VCA) gain to about 1.26, resulting in a carrier signal amplitude of 172 mV passed to the output section

Set up like this, a loud spike of 300 mV in the control signal results in an excursion up to 610 mV in the carrier signal, which is very loud indeed, but in no danger of clipping an op amp with a +/-12-volt split supply. This is right on the edge of switching off the JFET completely, which would maximize the VCA gain at about 4.6. This condition would give an output of 630 mV - still safe.

I've also eliminated some complexity with the LED indicators. Now there's just a single monochromatic LED for each channel. I've used an op amp to boost the voltage going into the transistor-LED by a factor of 2, so it will turn on sooner. This sounds like a waste of an op amp, considering the band level potentiometer cut the voltage in half right before it, but if the transistor-LED are attached directly to the EF, the LED will flicker rapidly. The low-pass filter formed by the band level potentiometer, 4.7k resistor, and 1 uF capacitor helps in eliminating LED flicker, as well as preventing the control signal from bleeding into the VCA amplifying the carrier signal.

Here's the latest design for a vocoder channel, which I believe can be packed onto a PCB of about 5 square inches (a cost of only $4.50 per board!):
http://www.falstad.com/circuit/#%24+1+4 ... -5+2+-1%0A

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Post by uncleboko »

What on earth is all that gobbledegook?

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Post by Ben N »

Analog vocoder? :shock: Is that what they're calling an amp, a sawn-off horn driver and a length of plastic tube tied to a mic stand these days? :lol:

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Post by lithiumdeuteride »

I made some additional modifications to the envelope follower (EF) section. When a large control signal suddenly disappears, the EF's voltage output (and therefore the carrier amplification) decays to zero at a certain rate. Previously, it took about 120 milliseconds for the output amplitude to decay to one-tenth of its original value (-10 dB reduction in amplitude), then another ~120 milliseconds for it to decay to one-hundredth, etc. But if you want to speak rapidly into the machine, or resolve short percussive syllables, it's desirable to have a more rapid decay than this. Even a robot voice should be capable of halting abruptly without a noticeable fade-out effect.

I changed the values of the EF's capacitor and drain resistor to achieve a -10 dB decay time of about 65 milliseconds, at the cost of getting a slightly lower overall voltage output from the EF. A new capacitor value (2.2 uF) is required, but capacitors seem not to have the large bulk discounts that resistors do, so I think this is acceptable. The project still requires only nine different resistor values.

I also got rid of the NPN transistor controlling the level indicator LED, after discovering that it adds absolutely no functionality. Now, an op amp simply boosts the voltage by a factor of 5.7, and a 47k resistor limits the current through the LED. One more component eliminated!

Here's the latest simulation of the channel board, in Falstad's circuit simulator Java applet: http://www.falstad.com/circuit/#%24+1+4 ... -5+2+-1%0A

Here's the PCB layout for the channel board: http://www.techwarereview.com/non-websi ... 0board.png

And here's the edge connector I plan to use to socket the channel boards: http://www.digikey.com/product-detail/e ... ND/2310830

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Post by merlinb »

Ben N wrote:Analog vocoder? :shock: Is that what they're calling an amp, a sawn-off horn driver and a length of plastic tube tied to a mic stand these days? :lol:
No, you're thinking of a talk box. A vocoder is a different beast entirely.

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Post by Ben N »

merlinb wrote:
Ben N wrote:Analog vocoder? :shock: Is that what they're calling an amp, a sawn-off horn driver and a length of plastic tube tied to a mic stand these days? :lol:
No, you're thinking of a talk box. A vocoder is a different beast entirely.
I think of the relationship between a talkbox and a vocoder as analogous to the relationship between a megaphone and a PA system--same basic objective, different generation technology.

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Post by lithiumdeuteride »

I'm getting a bit worried about the RDS(ON) value of the JFETs (drain-to-source resistance when the JFET is on). If a batch of JFETs have consistently identical RDS(ON), it won't be a problem. But if one has a resistance of 100 ohms while the other's is 150 ohms, the voltage-controlled amplifier won't be able to switch off properly, and it will amplify when it should stay quiet.

Does anyone know about the variance of RDS(ON) within a batch (same part number)? I'm no expert on JFETs, and the spec sheets don't give much statistical information about it.

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Post by lithiumdeuteride »

Upon further inspection, it seems the article about the voltage-controlled amplifier (VCA) addresses this issue. It recommends giving one of the resistors at the front of the VCA (nominally 10k) a variability of about +/- 5%. This covers for tolerances in the JFETs and enable the VCA to be extremely quiet when it receives zero control voltage. So, it will need a trimpot at the very least, which will be a nuisance to dial in, but only needs to be done once for each channel.

However, because you don't know which JFET will have the higher value of RDS(ON), you need both +5% and -5% variability. To avoid having to use a weird resistor value like 9.5k, in series with a 1k trimpot (giving a range of 9.5k to 10.5k), I think it's preferable to use two common resistor values in series or parallel.

Let's see what I can find using the resistor values already needed for the project... A 10k and a 180k give a parallel resistance of 9.47k! That will work well with a 1k trimpot. If you happen to get horribly mismatched JFETs and run out of trimpot travel trying to zero the gain, you could change/remove the 180k resistor to shift the range of adjustment. However, it's probably easier to dig around for JFETs with a better match.

Here is the v1.1 board layout:
http://www.techwarereview.com/non-websi ... 20v1.1.png

And here is a negative image with components labeled, for printing.
http://www.techwarereview.com/non-websi ... values.png

Here's the latest simulation, including trimpot:
http://www.falstad.com/circuit/#%24+1+4 ... 55+2+-1%0A

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Post by lithiumdeuteride »

I've done more work on the vocoder and now have PCB layouts for the main board and channel board. The main board has both standard (1/4") and differential (XLR) input, and both mono and stereo outputs.

Main board v1.2, input section:
http://www.techwarereview.com/non-websi ... ection.png

Main board v1.2, output section:
http://www.techwarereview.com/non-websi ... ection.png

Channel board v1.2:
http://www.techwarereview.com/non-websi ... 20v1.2.png

The hole pads drawn in black indicate where a component (LED or potentiometer) mounted off the board should be soldered to the board. The hole pads for input and output jacks aren't given any special color.

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Post by lithiumdeuteride »

It looks like the PCBs are going to be a bit cheaper than I thought. Ignoring the setup fees and looking only at the recurring costs for producing boards, it's about $100 (USD) for a main board and 14 channel boards.

I'm going to purchase enough boards to make 3 vocoders, so I'll have two spare sets. If anyone wants to buy a set of boards, I will sell them for the recurring cost ($100), plus shipping. It will probably take another $150 of components to populate the boards, though this could decrease if you make particularly efficient purchases. I realize nobody will buy anything before hearing the finished product, so I will assemble one and make a recording.

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Post by Zokk »

if it's in the same range as the last JH project, your work is really amazing!
But his was a 20 band... and proto pcbs cost 300€.

http://www.jhaible.de/vocoder/living_vocoder.html

I'm really impatient to hear something coming from your design.

Congrats :thumbsup

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Post by lithiumdeuteride »

That J. Haible design is very impressive. I could not hope to match such a work, which I'm sure will outperform my design. But that person seems to be aiming for maximum features and performance (8-pole filters, attack/release controls, freeze mode, noise generators, etc.), at any cost. I'm aiming for simplicity, low price, small footprint, and the ability to construct it out of components that people already routinely use for guitar effects pedals.

Let's compare some of the part counts. First the J. Haible design:
  • 144 integrated circuits, spanning 12 types
    39 transistors, spanning 6 types
    104 diodes, spanning 5 types
    899 resistors, spanning 77 values
    600 capacitors, spanning 33 values
And now my rather simplistic design:
  • 30 integrated circuits, spanning 1type
    28 transistors, spanning 1 type
    42 diodes, spanning 2 types
    363 resistors, spanning 9 values
    116 capacitors, spanning 17 values
The J. Haible design has about 3 times as many components as mine! The only unusual component I use is the edge connector, which allows for packaging in three dimensions, as well as the ability to swap out a faulty board. Once assembled, the boards for my design will fit into a 10.5"x2.5"x4.0" (inches) box, excluding the external power supply. I have no idea what the appropriate enclosure would be. If I used surface-mounted components, I could have made it smaller still, but that stuff is awful to work with.

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Post by lithiumdeuteride »

I decided the channel board had an inefficient use of space and cleaned it up a bit. I was able to shorten it by two tenths of an inch, which should save a couple bucks on the overall order, and package in a smaller space. Here's the v1.3 layout of the channel board:
http://www.techwarereview.com/non-websi ... 20v1.3.png

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Post by lithiumdeuteride »

I made some final improvements to the main board. It now has 9/64" holes at the corners, for PCB standoffs. A couple components had to move slightly to make room.

Here's the input end of the version 1.3 main board:
http://www.techwarereview.com/non-websi ... ection.png

And the output end:
http://www.techwarereview.com/non-websi ... ection.png

You may notice a lack of a socket for the ATX power supply I mentioned earlier. I don't want to waste space when it's likely that people will use a variety of things as power sources. It needs a split supply, but this could easily be two 9V or 12V AC adapters connected in series. So instead of pushing people towards an ATX power supply, I merely supply holes on the V+, GND, and V- rails, to which you can connect the voltage source of your choice. This minimizes board cost and size.

I also learned that J. Haible unfortunately died last year. Maybe one day someone will finish his extremely ambitious vocoder project.

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Post by lithiumdeuteride »

Boards have arrived! They look good, but I forgot to account for router machining tolerances, so the channel boards are a bit too wide to fit into the edge connectors. I'll use a grinding wheel to trim them down.

Here are some pics (256 colors for bandwidth saving):
ImageImage
Image
Image

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Post by DaveKerr »

lithiumdeuteride wrote: I'll use a grinding wheel to trim them down.
Damn, what a cool project. Not that you don't probably already know this, but that dust is pretty awful stuff - do it outside and with a decent mask.
... multiple LFO waveforms (saw up, saw down, triangle, square); a more flexible envelope with attack/release controls as well as inverted envelope. I am afraid it will have more knobs than the TGP annual convention - frequencycentral

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Post by lithiumdeuteride »

Got another shipment of components today. The 14-pin sockets and 4.7k and 10k resistors have been soldered to the channel boards. Note the discoloration where the grinding wheel took off a bit of material.
Image

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Post by Greenmachine »

Awesome pics!
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