What is a Ring Modulator?
A ring modulator multiplies two audio signals together to create two brand-new frequencies which are the sum and difference of the input frequencies.
What does all that mean, though?
A ring modulator is basically a VCA with a special modulation input. On a normal VCA, you want complete silence when there is no CV, so it won’t let any carrier signal pass when the modulation signal is 0 (or below). Negative modulation signals are just ignored in a VCA. But in a ring modulator, on the other hand we want it to pass the carrier when the modulation signal goes negative as well (see below). For this reason they are also known as “Balanced Modulators”
This makes Ring Modulation different from the modulation we get from a VCA because it also has the added effect of cancelling the carrier frequency from the output signal, leaving only the sum and difference frequencies.
The frequencies from both the carrier (Input 1) and Modulator (Input 2) are not present at the output of the ring modulator, but two new signals (sidebands) will be generated at the difference and sum frequencies for the two inputs. The more complex the frequency spectrum of the modulator signal, the more sidebands that will be produced.
What does it sound like? Well just think of the classic Dalek “Exterminate” from Dr Who…that’s a normal voice passed though a ring modulator with a 30Hz sine wave, you can also create bell like sounds and some really otherworldly discordant sci-fi film type effects.
Comparison of Ring Modulation and Amplitude modulation at various frequencies.
1 kHz Modulator 3 kHz carrier.
Note how the 3 kHz signal has disappeared from the ring modulator’s output, and two new frequencies (AKA Sidebands) have appeared these are;
3 kHz – 1 kHz = 2 kHz, and 3 kHz + 1 kHz =4 kHz.
However the Amplitude modulated signal still contains the 3 kHz carrier, and more sidebands on to of the 2 kHz and 4 kHz.
2 kHz Modulator 3 kHz carrier.
3 kHz Modulator 3 kHz carrier.
4 kHz Modulator 3 kHz carrier
Note: This set of spectra looks odd, 3 kHz + 4 kHz = 7 kHz, is fine and it’s present which is OK. But where has the seemingly unrelated 1 kHz sideband come from? well 3 kHz – 4 kHz = -1 kHz, and a negative frequency just isn’t possible – so it gets “reflected” back from 0 up to 1 kHz. This always applies if the result of the
carrier- modulator equation results in a negative number.
(Well as far as we know negative frequencies don’t exist… and if they do I don’t want to think about the implications or the physics, it makes my head hurt!)
5 kHz Modulator 3 kHz carrier.
As you can see from the frequency spectrums above, the 3 kHz carrier is never present in the ring modulators output signal, but is always present in the amplitude modulators output.
Note: When the carrier and modulator are at the identical 3 kHz frequency there is only one 6kHz signal present in the spectrum, this is because 3 kHz + 3 kHz = 6kHz, however (you can probably guess this) 3 kHz – 3 kHz = 0 Hz. This is obviously DC so we can ignore this in fact we really don’t want it, so to prevent any DC in the output, not to mention any LF growls, rumbles or cross-mod it’s a good idea to put a high pass filter in the output at say 100 Hz cut-off frequency.
Using the Synthedit Ring Modulator.
There are already a choice of some Ring Modulator SEM’s. This is a complete module using the stock SynthEdit SEM.
We can make a quite good Ring Modulator unit with just a few other modules added on.
1) The standard container module with an Input IO Mod and an Output IO Mod.
2) Add an internal Modulation oscillator using an Oscillator with a Tuner Module. Connect the Pitch control Plug to a Spare plug on the Input IO Mod.
This is so we can control the modulation oscillator using either the keyboard, or some other control voltage source.
3) We switch the Modulation source from internal to external using a Switch Many ->1 module
4) In the properties panel rename the Two Input plugs, we have used “Internal” and “External” so we get a meaningful choice in the List Box.
5) Feed this to the Input 2 of the Ring Modulator, and connect Input 1 of the Ring Modulator to the Spare plug on the Input IO Module.
6) Connect a Level Adj module to the Output plug of the Ring Modulator using Input 1, and a Slider Control to the Input 2 of the Level Adj Module.
7) Select the Slider Control, and in the Properties panel give it the name “Output level”, and a Maximum value of 20. This is to compensate for the output of the Ring Modulator being a lower signal level (±5 volts, instead of the usual ± 10 volts).
Comparing the SE Ring mod and the TD Diode Ring Mod.
As you can see below there’s a noticeable difference in the frequency spectrum from the two Ring Modulators, the standard SE gives a cleaner harmonic spectrum of the sort you would get from a modern Balanced Modulator chip, and the TD_RingmodDiode module has a “dirtier” frequency spectrum like you would get from a diode modulator with it’s inherent distortion. For those with an electronics background the diode modulator will have a region of voltage where the diodes are not conducting (0 to +0.3 Volts for Germanium diodes, and 0 to +0.7 volts for Silicon diodes) so there will always be a small portion of the audio that is not passed through the modulator causing a small amount of a “crossover” type harmonic distortion.
