A slower modulation rates, multiple resonant filters give the carrier audio a comb filter/flanging character.
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.īringing up even two or three filters gives Spectravox’s sound a distinctly formant/vowel like character.įinally the filter has its own dedicated LFO – with both mod rate and mod depth controls, and it goes well into audio rates. The filters have a 12 dB per octave slope and you can shift their cutoff frequency together. 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. You can also replace the oscillator in the audio path by plugging an external source into the carrier 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 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’s synth, or carrier section has one oscillator, which can be either a sawtooth or variable pulse width. 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. 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.
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: 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). The only way to get one is to have built it at the workshop (or buy it on eBay…). It’s not available commercially, at least not yet. Moog’s Spectravox was Moogfest 2019’s Engineering Workshop synth.
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