Today I took some time to assemble a small project that had been in my to-do list for a while. A Ring Modulator. This is a wonderful little sound effects device that has been around since the early days of electronic music. In addition it has been at the heart of one of the most famous science fiction villain’s voices ever– The Daleks from the Dr Who series. If you have ever wanted to know how they did the famous voice yelling, “Exterrrminnnate!” then you are about to find out. It was done with a passive ring modulator. That’s right kids, all analog!
A ring modulator gets its name from the arrangement of the diodes used. Diodes pass current in only one direction. Normally used in radio receiving circuits and power supply applications to turn an AC wave into a DC current, in a ring modulator the diodes are arranged like a bridge rectifier except that all of them are pointed the same way so you can imagine a flow of current in a circle. If you apply a sound source in from one side and then add a sine wave audio carrier signal in, the result is a frequency mix, or Heterodyning, that gives an additive AND a subtractive waveform that is a mix of the original sound with the carrier superimposed upon it. Ideally the output shall have none of the raw signal or raw carrier at the output. All easily visualized graphically on an oscilloscope.
Other uses of ring modulation include frequency multiplication by feeding the same signal at both input and the carrier, although the output will show some distortion due to the forward voltage drop across the diodes; and spectral inversion of a signal which is sometimes used in other audio filtering processes.
In the case of the Dalek’s voice….the audio input from the actor was modulated using a 30Hz carrier tone. The output was then fed to the soundtrack recording of the show….and the rest is history as they say. Dr Who would be proud of course, the basic ring modulator really seems Gallifreyan in the since that such a tiny box can seem so big on the inside. (sorry, I could not resist)
I had planned to pick up my four diodes and a couple of audio matching transformers from my local shop but then found a complete kit from Synthrotek for less than I could buy the individual parts. It consists of the board, four Germanium diodes, the two transformers, and three phone jacks. Simple and only takes a few minutes to put together. I plan to add in a bypass switch and then mount the entire assembly into a small box. For now, however, it is strewn across my little field desk in the back of the travel trailer (Yep, we are still on the road.) and for a first test I have used an old Heathkit audio oscillator for the carrier source and then connected a little amplified speaker to the output. The input source for this test is my special little Theremin.
A Theremin can be made to make some odd sounds by itself. Fed through a ring modulator and the sounds pick up a new dimension of oddity. It is great fun! And as soon as I can, I will have to put a mic on it so I can record my own Dalek voices! (My daughter is gonna love getting THAT phone call!)