Analog algebraic sinusoidal band-pass filter spectrum encoder-decoder day (a.k.a. Vocoder)

For this semester, Max and I plan on building a vocoder from scratch.

A vocoder, put simply, is a device that filters an audio input (typically a microphone) in accordance to a second input (usually a keyboard). This is done by feeding the first source (the “Program”) through a series of ‘band-pass filters’– or ‘bands’ for short. Once the first input is separated and decoded, the second input (the “Carrier”) is analyzed and processed with a circuit to provide the amplitude levels for the program (An “Envelope Follower”). Finally, this spectrum of signals are mixed and re-encoded to a single output.

The vocoder adds a synthetic, artificial tone to the voice, as if the words are being spoken through a robot or an early voice module.

Here is one that Kraftwerk had custom built for them in the ’70s (and used in the album “Ralf und Florian“)!

Kraftwerk Vocoder

The vocoder we will be building have a total of 14 bands (each separately adjustable in volume), optional mono or stereo out, and LED readouts for the levels of each band! The stereo effect is done by sending the odd numbered bands to the left channel, and the even ones to the right.

The page where the info on this specific vocoder may be found here, mind you, it’s only for reference- I merely retrieved the schematics from this page, along with a bit of miscellaneous data.

I have ordered the parts for it off of Tayda electronics roughly an hour ago, and the package should arrive in a week! (Over 500 parts- the soldering will be mind-numbing…)

Here are the schematics for this thing:

This is the amplifier for the “Program” input (With option for Mic Jack, or regular patch cable):

Schematic 1

And for the “Carrier” input:

Schematic 2This is the combination Band-Pass Filter and Envelope-Follower Circuit. This circuit will be repeated 14 times, but each time will have different values for the capacitors marked “C”, value chart is up next.

Schematic 3

And now for the values of the filter capacitors (a.k.a. “Condensers”):

Vocoder Schematic 5

Filter Capacitor Value Chart

After all the algebraic fun of spectral decoding, we finally get to the mixer, where the re-encoding happens!

Schematic 4

So, however ambitious it may seem, the “output” of this project will be very much worth it!

This concludes today’s post, and once again,

“Weird sounds abound!”

–Secoyatree

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