The first module is called the Oscillator. The Oscillator generates a sound. Its task is to create a waveform which will produce a different sound depending on the shape of the waveform. The oscillator does this continuously. The rate at which it generates each cycle of the waveform is what we hear as pitch. The most common waveforms are: sawtooth wave, square wave, sine wave, noise, etc...
Next, we have the filter. The filter is a module that allows only certain frequencies it receives to pass through, and at the same time is a barrier to others. In this way a filter is used to screen, or filter out, unwanted frequencies from the waveform so as to alter the timbre. The filters can be labeled VCF (Voltage Controlled Filter) or DCF (Digitally Controlled Filter) in our synthesizer.
The most common filters are:
1. Lowpass filter: Low frequencies are passed; high frequencies are attenuated.
2. Highpass filter: High frequencies are passed; low frequencies are attenuated.
3. Bandpass filter: Only frequencies within a frequency band are passed.
4. Band Reject filter: Only frequencies within a frequency band are attenuated.
5. Allpass filter: All frequencies in the spectrum are passed, but the phase of the output is modified.
With filtering, a sharp sounding wave can become smooth and better for listening.
Next, we have the amplifier. The amplifier is the module that outputs sound to a sound card or a digital file. It amplifies the signal before the output and it does it with the Envelope.
The envelope, also known as ADSR, is what controls the way the oscillator ‘plays’ the notes. ADSR stands for:
Attack time – how fast the note hits or swells,
Decay time – how fast the note goes from the full attack level to the sustained level,
Sustain level – the level at which the note is held while the key is still pressed,
Release time – how fast the note fades away after the key is released.
Lastly, we have the LFO. The LFO, or low frequency oscillator, is called as such because it’s frequency is below the human hearing range. The oscillator is used to modulate other aspects of the synthesizer to add a more player-like sound. We can adjust several settings to produce different effects. It’s great if we want to sweep similar to tremolo, vibrato or wah-wah. It acts below 20Hz and creates a pitch variation. It’s not a creator of sounds itself, it works when is connected with the oscillator or amplifiers.
Sources:
http://www.romicha.ru/coursera/production/lesson6