These are not digital - they are real analogue ring modulators, the same circuit design as used to create the original voice of the Daleks in 1963.
They have been made using carefully matched germanium diodes, housed in a rugged metal case. The units are 'passive', meaning they don't require batteries, but lower signal levels may require a preamp.
Connect a synth, drum machine, microphone or guitar to the input socket, a simple waveform (e.g. VCO or note held on a keyboard) into the 'carrier' input, and the ring modulator will produce interesting metallic/robotic sound effects. Sound files below.
Voice combined with swept sine wave:
Roland TR-808 combined with swept triangle wave:
Roland TB-303 combined with saw wave:
Roland TR-606 combined with slowly swept saw wave:
Sweeping oscillators in both inputs:
According to Wikipedia, the original voice of the Daleks used a 30Hz sine wave as the carrier input.
"In a 2006 BBC Radio interview, [Nicholas] Briggs said that when the BBC asked him to do the voice for the new television series, they instructed him to bring his own analogue ring modulator that he had used in the audio plays. The BBC's sound department had changed to a digital platform and could not adequately create the distinctive Dalek sound with their modern equipment."
I have included some of them in my homemade synthesiser:
So far the ring modulators have all gone to interesting new homes, including Martin Horntveth of Jaga Jazzist, and Chip Chapman of Le Forte Four, who emailed to say:
"I enjoyed the first one so much, I need a second for a live performance coming up in a few weeks!"
Chip used the first ring modulator during a gig in Los Angeles in December, as part of the live setup pictured below:
Email: dave@ [this website]