Amplitude modulation is a fundamental synthesis technique that can be heard in all types of music from the avant-garde to dub step (wobble bass). This technique is the idea behind an LFO and can be related to tremolo because only the amplitude is being affected.
Amplitude Modulation is similar to Ring Modulation but the modulation is uni-polar instead of bi-polar. this means that the modulation frequencies samples get scaled from -1.0 -> 1.0, to 0.0 -> 1.0. This is very important because in amplitude modulation we are not trying to change the frequency.
As you can se the wave “bounces” or the volume increases and decreases at a steady rate. This occurs when the modulation frequency is at sub audio rate or below 20 Hz.
Just like Ring Modulation, Amplitude Modulation creates other frequencies that are so close together they are not heard when the modulator is below 20 Hz. These frequencies can be calculated by Carrier Frequency + Modulation Frequency, Carrier Frequency – Modulation Frequency and just the Carrier Frequency. Unlike Ring Modulation the Carrier Frequency will always be heard and will not change even if the Modulation Frequency is above 20 Hz.
When the Modulation Frequency is above 20 Hz the C+M and C-M start to get further away from each other and eventually sound like 2 separate frequencies with the Carrier remaining the same. In the above example the Carrier Freuqnecy is 440 and the Modulation Frequency is 2000. This would give us 3 frequencies at 440, 2440 and 1560. When the Modulation Frequency is lower the frequency are closer together, but still can be heard separately.
With complex Carrier Frequencies the spectrum becomes ver full because each harmonic is producing 2 other frequencies and keeping the harmonic heard.
This is a Square Wave run through an Amplitude Modulator. As you can see it looks like a Square Wave.
This is a Square Wave through an Amplitude Modulator with a Modulation Frequency of 2000 Hz. The spectrum fills out and the sound changes from a classic Square Wave sound to a sound that has a lot more buzz/crunch to it. As the Modulation Frequency gets higher higher partials start to be heard more and the structure of the Square Wave changes.
While Amplitude Modulation is always thought of with the modulator below 20 Hz, I encourage everyone to break that thought and experiment with all frequencies and even different types of waves for the modulator. Amplitude Modulation is a building block to creating interesting and complex sounds. By “breaking the rules” new sounds can emerge like the one below that uses a Sawtooth Wave for the modulator with a frequency of 5000 Hz.