Amplitude Modulation

Shaping Sound Through Volume

Amplitude Modulation (AM) is a fundamental concept in audio synthesis an communication. It's a way of varying the loudness (amplitude) of one sound wave based on the shape of another. This seemingly simple process can create a surprisingly rich variety of timbres, from subtle tremolo to aggressive, metallic sounds.

A Brief History of Amplitude Modulation

While we often associate AM with synthesis, its roots are firmly planted in radio communication. The very first successful transmission of speech by radio, achieved by Reginald Fessenden in the 1900's. Using amplitude modulation. Before this, early radio relied on on-off keying (Morse code), but Fessenden's work paved the way for transmitting continuous audio signals.

The principle was straightforward: a high-frequency carrier wave's amplitude was varied in proportion to the audio signal (the message). At teh receiver, the carrier was removed, leaving the original audio. This technology became the backbone of early radio broadcasting, and "AM radio" remains a common term today.

How AM is Used in Synthesis

In synthesis, AM takes on a more creative role. Instead of transmitting information, it's used to sculpt the harmonic content and perceived movement of a sound. The core remains the same: one oscillator (the carrier) has its amplitude controlled by another oscillator (the modulator).

When the modulator is at an audible frequency, the results are particularly interesting. The interaction between the carrier and modulator frequencies creates sidebands. These sidebands are new frequencies that weren't present in either the carrier or modulator alone. They appear at sums and differences of the carrier and modulator frequencies.

• Carrier Frequency (Fc): The main sound source.

• Modulator Frequency (Fm): Controls the amplitude of the carrier.

• Sidebands: Fc + Fm and Fc - Fm.

The presence and strength of these sidebands are what give AM its distinctive timbral qualities.

Kinds of Timbres Created by AM

The timbres generated by AM depend heavily on the frequencies of the carrier and modulator, as well as the modulation depth (how much the modulator affects the carrier's amplitude).

• Tremolo (Sub-Audio Modulation): When the modulator is at a very low frequency (below 20 Hz), the effect is a rhythmic fluctuation in volume, known as tremolo. This adds movement and expressiveness to a sound without significantly altering its harmonic content.

• Ring Modulation (High-Depth, Audible Modulation): As the modulator frequency enters the audible range (above 20 Hz) and the modulation depth is high, the sound takes on a more complex and often metallic character. This is often referred to as ring modulation, especially when the carrier and modulator are multiplied together, resulting in only the sidebands and no original carrier frequency. Ring modulation is known for its:

  • Bell-like sounds: Particularly when the frequencies are harmonically related.

  • Metallic textures: With inharmonic relationships, creating clangorous or industrial sounds.

  • Robotic voices: Often achieved by modulating a voice with an oscillator.

• Harmonic Richness: If the carrier and modulator are harmonically related (e.g. one is an octave or a fifth above the other), the resulting sidebands can create a richer, more complex harmonic spectrum that still sounds musical.

• Inharmonicity: When the carrier and modulator are in-harmonically related, the sidebands will also be in-harmonic, leading to dissonant, abstract, or "out-of-tune" sounds. This is often desired for experimental sound design.

  
  

  

  Simple Patch to Achieve Amplitude Modulation

Experimentation:

1. Change Modulator Frequency: Hear how different modulator frequencies create different sidebands and thus different timbres. Try harmonic relationships (e.g. modulator at an octave or a fifth above the carrier) and in-harmonic relationships.

2. Change Waveforms: What happens if you use a square wave or saw tooth for the carrier or modulator? The richer harmonic content of these waves will interact in more complex ways.

3. Modulation Depth: Adjust the output level of the modulator (VCO 2) before it goes into the VCA's CV input. This will control the depth of the amplitude modulation. Higher levels mean more aggressive modulation.

   
   

   With this simple patch, you'll quickly discover the power and versatility of Amplitude Modulation in sound design!

   
   

   -Seán