What is Amplitude Modulation?
It’s all in the name really. We take two signals the first is our carrier, the second is the modulation. If you remember the days of medium, and long wave radio that was Amplitude Modulation applied to RF instead of AF signals. The diagram below shows a block diagram of AM in operation in a radio transmitter.
This makes quite a versatile effect in a synthesizer or FX pedal, some might say similar to Ring Modulation. But personally I class it as entirely different effect. The major difference is where a Ring Modulator (A true balanced one that is) should suppress the carrier signal, amplitude modulation does not suppress the carrier. I’m not going to get too technical (or go into the maths too deeply), but if you want to there’s a good article on Wikipedia on the subject.
Here we have 0% modulation depth:
Then 50 % modulation depth:
Next 100% Modulation depth:
You can see here how we always have an output from the modulator, and that as we apply modulation to the carrier signal we get two other signals, these are sidebands, and are created by the sum of the carrier and the modulation frequency, and the difference of the carrier and modulation frequencies.
Example: (Sorry a little bit of basic math here).
If the carrier (fc) is 440 Hz and the modulator (fm) is 200 Hz then we get a lower sideband of fc-fm=240 Hz and an upper sideband of fc+fm=640Hz. So our output will now contain 240 Hz, 440 Hz and 640 Hz. What about the 200 Hz modulation signal? This never appears as a direct signal at the output. That’s correct it only appears in the form of the sidebands.
440Hz modulated by 200 Hz:
440Hz modulated by 330Hz.
440Hz modulated by 880Hz.
Where has the lower sideband gone? Well 440Hz-880Hz= -440Hz, negative frequencies are an impossibility so it disappears. This is why we need an HP filter in the output to prevent DC and LF rumbles distorting the output.
Howe can we do this in SynthEdit?
It’s fairly easy using VCA modules if we keep one or two details in mind.
1) The VCA Response curve must be set to Linear mode for the proper modulation effect, anything else will not give the correct modulation.
2) With some combinations of carrier and modulation frequencies we can get some very loud rumbling sounds due to the difference between carrier and modulation frequencies. A high pass filter set to about 50 Hz, after the modulator will cut these out.
3) Unless the modulator structure has a bias voltage on the Modulator VCA you won’t get the true AM effect, there will only be an output when modulation is applied. This is not correct, we need to apply a 5V bias to the modulator’s Volume plug for true AM operation.
Note: Don’t be tempted to go beyond 100% modulation unless you want some horrible distortion, and aliasing by products.
Note: The Output VCA is there so that the oscillators will behave properly. If you put this in a synthesizer it’s not really needed.
Note: any Module names with an addition inside curly brackets {added} is just a comment to clarify what the module does…please don’t try and find them in SE or 3rd party modules!
The modulator structure.
The “guts” of the modulator.
Tremolo Effect.
Because the carrier signal is always present at the output we can use this effect as Tremolo as well as an effect to add extra harmonics to a signal. All we need to do is reduce the modulation frequency down to a range of say 0.1 to 5 Hz.
Are there sidebands when used as Tremolo?
Do we get any additional frequencies in the output when used with say 0.5Hz modulation as a Tremolo? No we don’t, just the original 440Hz carrier pulsing up and down in time with the modulation. Any sidebands are so close to the original carrier frequency as to be completely indistinguishable – see below.
