What is ring modulation?
Ring modulation is an effect where two waveforms are combined to create a new frequency. The effect that this creates is described as a metallic ringing sound used for gongs or bell type sounds, hence where the name ring modulation comes from.
In technical terms, ring modulation can be described as a form of amplitude modulation (AM). AM is where the amplitude of one signal (the carrier) is modulated by another signal (the modulator), the result is that you have two sidebands within the frequency range.
Ring modulation is different because it takes the two input signals to create a different output of frequencies, the output is the sum and difference of the two input frequencies. So, in simple terms, you are creating an output with a different frequency range than the original input signals.
So this new waveform, with the new frequency range, now contains harmonics that have no relation to the frequencies of the original two input signals. These inharmonics cause the metallic and bell-like qualities of the sound.
Ring modulation is a very popular effect with many synth users and professional sound engineers, even the Doctor Who creators found some interesting ways to take advantage of ring modulation, using it to great effect in the soundtrack and for the Daleks voices as well.