Usage of the five most important synthesis modules
The five primary synthesis modules are, oscillators, filters, amplifiers, envelopes, and LCOs. Each module changes a particular element of sound and combined they comprise one complex whole. Today we will look at how we use these modules to create and modulate sound.
The first module is the oscillator. An oscillator creates sound electronically instead of mechanically. It does this by creating geometric waveforms. The main waveforms generated by oscillators are sine, square, sawtooth, triangle and noise. They are named based on the shape of the wave. Each waveform has different characteristics that produce certain types of sound. A sine wave produces a tone at a single frequency. A sawtooth wave includes a set of upper partials, or harmonics, creating a full, bright sound. A square wave produces only half of the harmonics, creating a hollow sound. A triangle wave is essentially a filtered square wave and a noise waveform is energy evenly spread over the entire frequency spectrum, creating simple white noise.
As we have said, each module modulates a specific part of the sound. In the oscillator, pitch is modulated. Because the pitch is modulated through changes in voltage another name for the oscillator module is a VCO, or voltage controlled oscillator. The other two modules concerned with the creation of the sound, the filter and the amplifier, are also controlled by changes in voltage and are called a VCF, or voltage controlled filter, and a VCA, or voltage controlled amplifier.
Next comes the filter module. The purpose of this module is similar to that of the EQ section of a mixer, removing or emphasizing certain frequencies and/or harmonics. However in a synthesizer the filtering changes over time. The main filter used in a synthesizer is the low pass filter. The waveforms generated by the oscillator are harsh, almost obnoxious. The low pass filter cuts out most of the overly bright high frequencies, which helps those waveforms sound more musical. The filter module can also use other types of filters, such as a band pass filter, to modulate other frequencies.
The filter is normally modulated by changing its cut off frequency over time. A filtered oscillator is a common phenomenon in the real world. The human voice is a filtered oscillator. The vocal cords are the oscillator and the mouth is the filter. Synthesizer filters tend to be resonant filters. All filters are delays and delays involve feedback, which can create resonance at certain frequencies. When the resonance level is raised it emphasizes the cut off frequency and makes the harmonics jump out at you as the filter sweeps through the frequencies. Increased resonance is best used when you want to hear the filter itself.
The amplifier module controls volume. A synthesizer’s amplifier, as previously said, is voltage controlled and designed to change volume very fast. The amplifier is modulated by the envelope, which is a set path that the sound takes each time the key is depressed and released. This path is defined by four controls, attack time, decay time, sustain level, and release time. Changing these parameters influences the shape of the note. The attack time determines how fast the note goes from zero to full value. Decay time is how long the volume takes to go from full value to the sustain level. Changing the sustain level determines at what volume the sound stays until the key is released. From that point until the sound reaches zero volume is the release time.
As you might imagine we can create many different envelope shapes, which greatly influence how notes sound. Different instruments have different shaped notes, and to accurately emulate them the amplitude envelope must match that of the instrument. For example an organ note goes on and off like a switch, and thus has a very short attack and release time with no decay and a high sustain. A plucked violin, a percussive sound, will have a short attack and decay with no sustain. In this case the decay time defines the end of the note, regardless of when the key is released. The amplifier envelope has a great deal to do with creating a note whose sound distinguishes itself from other notes of the same pitch and tone.
The final module is the LFO, or low frequency oscillator. The LFO is strictly a modulation module, because, in this instance, by low frequency we mean the sound generated is below the threshold of human hearing, or @ 20Hz. The output of the oscillator is therefore not heard and only controls another parameter of the sound. Most often the LFO controls the VCO. It works cyclicly and moves the pitch of the VCO up and down, over and over. This makes the LCO good for creating a vibrato, where the cyclic output of the LCO controls the frequency of the VCO output, making the pitch waver. It also controls the amount, shape and frequency of the modulation. Using different waveforms it can also create linear modulations, trills and other pitch variations. In a simple synthesizer the LFO output is often hardwired to the VCO input. But in a more complex synthesizer we can control more than one module to get a more natural vibrato that includes changes in amplitude from the amplifier and timbre from the filter.
One final thing to remember about synthesis is that we always need to be aware of the source of modulation, its destination and amount. This will help us keep balance and clarity in molding the sound we desire. Thanks for letting this old dude explain things from my point of view. I hope you have learned as much from this section of the course as I have.