A simple subtractive synthesizer consists of a VCO, a VCF, an envelope (ADSR), a mixer and an amplifier.
VCO (Oscillator)
With analog synthesizers, the frequency (pitch) of the oscillators is usually regulated by a control voltage this is known as a „Voltage Controlled Oscillator“ (VCO). This synthesizer consists of two oscillators whose frequencies can be superimposed. The three classic waveforms are sawtooth, square and triangle. Each of them has its own sound character and is suitable for very specific sounds.
Mixer
In most synthesizers, a mixer follows the oscillators in the signal path, which mixes the signals of the oscillators.
VCF (Filter)
In addition to the oscillators, it is above all the filter that is decisively responsible for the sound of a synthesizer. In a subtractive synthesizer it can remove certain frequencies from the sound. That‘s why this type of synthesizer is also called „subtractive“.
The two most important controls are Cutoff (Frq) and Resonance (Res). A low pass filter allows frequencies below an adjustable point (cutoff) to pass unhindered, while the frequencies above it are filtered out. With the second control Resonance you can adjust the emphasis of the cutoff frequency.
Envelope ADSR
Envelopes make it possible to change sound parameters such as volume, pitch or filter cutoff over time. The most common form of envelope is called ADSR, which stands for Attack-Decay-Sustain-Release. The attack time determines how long the envelope takes after triggering to reach its maximum level. This is followed by the decay phase, in which the envelope drops to an adjustable level selected with the sustain knob. As soon as you release the button, the release phase begins, in which the envelope drops to zero. So the Attack, Decay and Release controls control times, while Sustain controls a level.
VCA (Voltage Controlled Amplifier)
The VCA controls the volume. Without the amplifier you would hear the oscillators all the time.