UNDERSTANDING RING MODULATORS

Combining two analogue signals of different frequencies – a process known as mixing or AM modulation – produces a new waveform. In fact, any periodic waveform can be synthesised by mixing pure sine waves, and this method has been used for synthesising the sound of real music instruments, all of which have characteristic sounds that are quite different from the boring sound of a sine wave. The same technique is used to produce the AM radio signals that are still broadcast on the medium wave and long wave bands, although instead of two audio frequencies, the audio signal is used to modulate a much higher frequency radio signal.

When two signals are mixed in this way, the output contains four frequency components, the two input frequencies, their sum and their difference. So, for example, if a 1khz signal is mixed with a 4khz signal, the output contains frequency components of 1khz, 3khz, 4khz and 5khz. However, if a so-called ring modulator is used instead of an AM modulator, the input signals don’t appear at the output. So, in the case of our previous example, the output would contain frequency components of just 3khz and 5khz. This produces a very different “edgy” sound and is an important technique in the classic early analogue simulators that were used by the likes of Rick Wakeman and The Moody Blues. And if you’re wondering why it’s called a ring modulator, that’s because the common analogue way of implementing it uses a ring of four diodes.

Ring modulators aren’t used only to generate musical sounds, but also to modify the sound of physical instruments such as electric guitars via effects pedals.