Vocoder – A Holy Grail Project

Recently, a visitor to the Music From Outer Space Facebook page remarked on the latest project to come out of MFOS, the MFOS – 12 Channel Vocoder. He called it one of the “Holy Grail Projects,” and I have to say he’s right.

Once I discovered that vocoders were a very clever mix of filters, voltage followers, and VCAs—things I have become very familiar with—I was determined to build one and share the project with the rest of the DIY community. I do feel that the publishing of a viable vocoder project on the MFOS website is a milestone for our company.

It is a formidable project just from the sheer number of components involved. The vocoder uses several sets of two identical filters, one voltage follower, and one VCA. I tried to find the most economical circuit I could for each of the bands, but even so, R304 is on the board.

The filters ended up being Multiple Feedback – Lowpass, Bandpass,  and Highpass types. Each filter only uses one op amp, so there are the two op amps (TL072 Dual Op-amp) and the passive components for two multiple feedback filters: one to drive the voltage follower, the output voltage of which controls the channel’s VCA; one to filter the program material to be fed to the VCA’s signal input. So, you have 12 sets of all that as well as the unit’s control circuitry, square wave oscillator, and white and pink noise sources.

If you like to solder—this project is for you.

I found a lot of vocoder projects documented on the web and the architectures differed mainly in filter types and the center frequencies selected for the filters. Another marked difference was in the VCA circuits.

I chose to use the LM13600 even before I found the same thing in one of the designs out there, but Craig Anderton and the Okita vocoder both used a novel concept. Craig used a dedicated chip (the NE571 compander chip) to cover the voltage follower and VCA portions of the circuit. That’s a nice savings in components but I shy away from specialty chips.

These days, as more and more sound equipment uses DSP modules, analog chips like this will become as scarce as Isaac Newton autographs. I want a new generation of builders to have a fighting chance to build an analog vocoder and have some fun with it, so I chose the LM13600/LM13700 route for the VCA and a whole quad op amp chip (LM324) with a few discretes and diodes for the voltage follower instead. There are still several chip foundries manufacturing the LM13600/LM13700 and I find them very easy to use and very reliable.

The filters in the vocoder must be made using resistors and capacitors with a value tolerance of 5% or better. People often write to me to ask if they can substitute 1% resistors for 5% resistors in a circuit. I tell them you can always go to a tighter tolerance, more temperature-stable component. Going the opposite direction can cause issues, as too much tolerance can make a circuit misbehave.

Of course, as soon as you build anything you find some things you could have done, but I feel fine with this circuit and project. Could it have more bells and whistles? Certainly, but to me it contains the foundational elements needed for a vocoder. I did not include a sibilance detection circuit which compares the energy in the upper bands to the energy in the lower bands to determine if white noise should be emitted to simulate sibilant sounds. I simply allow a settable amount of white noise to be passed through the unit’s highest pass filter whenever frequencies in that range (> 3330 Hz.) are present in the control signal. Will I ever develop an add-on PC board with the  sibilance detection circuit on it for those that want it? Well, yes but not for a while. MFOS is developing exciting new projects so it will have to get in line.

I hope you’ll get a chance to check out the website to see the complete project details. If you decide to build one, I’ll be offering a PC board as well as a components kit.

Next time, I’ll go into a bit about the filter design process and some cool tools you can use to design your own filters.

[By Ray Wilson]