Patch of the day – Augmented vocoder


I got an Expert Sleepers Disting Mk 4 module on Friday. It is Eurorack’s designated swiss army chainsaw. Amongst its 76 modes, it can be a vocoder. I took it for a spin with a John Cage lecture as the modulator signal on Friday evening and the above is what came out.

Analogue and analogue-style vocoders are a system of band-pass filters, envelope followers and amplification. Two signals enter the vocoder: a carrier signal which supplies the timbre of the output sound, and a modulator signal which supplies frequency and amplitude information.

A bank of band-pass filters cut each signal into strips of frequency (think bass and treble, but much more precise). The vocoder tracks how loud the modulator signal is at each strip, then adjusts the volume of the carrier’s strips to follow the modulator’s strips. The carrier’s strips are combined back together as a whole signal and out comes Cylons.

One of the challenges with vocoders is getting output that’s as intelligible as the input. The intelligibility of the Mk 4’s vocoder was not great, so I mixed in a filtered version of the original speech. The filter is tuned to sibilant sounds (like s, f and th) which were getting lost in the vocoder process. It’s an old trick – the EMS Vocoder used for the Cylons has a cleverer version which lets sibilance through as it’s required – but a dumb version works in a pinch and as a result you can understand Mr Cage just fine.

For further fun, I’m modulating the pitch of the carrier signal with a random signal. This makes Mr Cage sound much more floaty than usual as the carrier floats up and down in pitch, giving him a rather more vague delivery.