Moog Spectravox – detailed overview, and How Vocoders Work

Moog’s Spectravox was Moogfest 2019’s Engineering Workshop synth. It’s not available commercially, at least not yet. The only way to get one is to have built it at the workshop (or buy it on eBay…). To see what it’s like to attend this workshop, I made a whole video about the experience last year building the Moog Subharmonicon (see video here). Building Spectravox was a very similar experience, except this time I didn’t make any mistakes and the 30-instrument unison patch ritually played at the end of the workshop was even more impressive, as is shown towards the end of the video below:

So, what is Spectravox? It’s built as an analog vocoder bass on Homer Dudley’s 1939 patent, but to call it a vocoder would be to miss out on quite a lot of “synthy” things it can do: It’s a single oscillator semi modular synth with a bank filters, spectral shifting (meaning, central control of all the filters’ cut-off frequency) and a dedicated shift modulation LFO.

On the vocoder side it also has an interesting twist – a hold function lets you “freeze” a single moment in the program audio for further processing on the synth side.

Main components

Spectravox’s synth, or carrier section has one oscillator, which can be either a sawtooth or variable pulse width. 

You can control the oscillator’s frequency manually with an on board knob, or by sending it control voltage via an input in the patch bay.

Spectravox also has an internal noise generator and you can control the mix between the oscillator and noise generator using the Carrier Mix knob. You can also replace the oscillator in the audio path by plugging an external source into the carrier input in the patch bay.

Filter bank

The thing that makes Spectravox special as a synth is its filter bank, with 8 band pass filters in the middle and a low pass and high pass filter on either end. 

The filters have a 12 dB per octave slope and you can shift their cutoff frequency together. They also have joint resonance control. Cranking up the resonance doesn’t cause the filters to self oscillate on their own but you can get them to sing on the slightest of harmonics. 

Bringing up even two or three filters gives Spectravox’s sound a distinctly formant/vowel like character.

Finally the filter has its own dedicated LFO – with both mod rate and mod depth controls, and it goes well into audio rates. A slower modulation rates, multiple resonant filters give the carrier audio a comb filter/flanging character.

VCA and Envelope Generator

Spectravox’s VCA can either be always on, or be controlled by the Envelope Generator – which is a simple one-stage decay envelope.

You can trigger the envelope generator either by using the trigger button, the trigger input in the patch bay, or when the program audio level passes a certain threshold, assuming the Prog Trig switch is on.

The Envelope Generator can also be applied to other destinations using the patch bay.

Hiss and Buzz

Hiss and Buzz is a synth feature intended for use along side the vocoder, but applies to the synth section as well. Buzz just turns the Hiss feature off… and Hiss causes noise to be sent to filters 9 and 10 regardless of the position of the Carrier Mix knob. Hiss was added to help with the legibility of consonants, which is a great Segway into the vocoder features of Spectravox.

How Speech Works

If you think of our vocal cords, they’re a simple monophonic oscillator with pitch control – occasionally paraphonic with falsetto – that is then passed through our head, mouth and nose which act as sort of like a filter. 

Our lungs can be considered the VCA in case you were wondering, which really is the only time you should put the VCA before an oscillator.

If we stretch this analogy a little bit further, vocoders are cool because they let you chop the head off the neck and either part will work just fine standalone or with the body parts of other synths altogether using the patch bay.

How Vocoders Work

Vocoders work on two sets of audio – the modulator, typically the speech we want to “encode”, and the carrier, typically the pitch and timbre we want our speech to have.

The Vocoder was invented in 1939 by Homer Dudley and Spectravox uses pretty much the same design. Vocoders work by using two sets of filters. The first set uses band pass filters at specific frequencies to split up incoming audio into different frequency bands – typically based on the frequencies of formants, or vowels types, though vocoders can do pretty cool things with any type of audio, like beats for example. This audio is called the modulator, or program in Spectravox, which can come in through the combo XLR input or through the patch bay.

Then the output of each of the bands in the first filter set gets sent to individual envelope followers, one per band. This greatly simplifies the audio coming in to a single amplitude envelope for each range of frequencies that a single band pass filter represents. The more bandpass filters you use, the more accurate the reproduction.

The envelope follows are then sent to the individual VCAs of band pass filters applied to the carrier audio, or , using the previous analogy, they put the head on a completely different neck.  

The carrier in Spectvox can be either the internal oscillator with out without noise mixed in, or any external audio source, such as a polyphonic synth.


The idea behind Spectravox’s hold function is quite simple – it’s like analog freeze. Hold takes the level of each of the 10 envelope followers, one per band, and samples and holds it. When applied to the carrier filters, this has the effect of freezing the incoming program formant.

Patch Bay

Spectravox’s patch bay is quite small compared to other Moog synths in this form factor but does give you access to a few of the important features you might want change, like swapping the program or carrier, the LFO, pulse width control, the spectral shift control and more.


Synths are about creating sound and with Spectravox, Moog has taken a well known synthesis idea and injected it with a few interesting twists. The result is a synth which I think achieves the goal of creating a different kind of sound. It sounds quite unlike anything Moog makes, or anything else sold today for that matter, but in a good way… Check out the video and particularly the two outro jams to see if you agree.

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