Effects, while not technically a stage of any form of subtractive music synthesis, are commonly found on the end of most signal paths from synthesizers. Effects can come in an internal form (built into your synthesizer), and in an outboard form (where it is a separate effects box, such as a rack mount unit or pedal). While internal effects are useful parts of synthesizers as they save on space and power, they are usually not as high quality as a dedicated external effects unit, but will suffice for most users. Bear in mind when purchasing a vintage synthesizer, that many older models (from the 1970s and 80s) will most likely not have any form of built in effects.
Unison: In similar fashion to effects not technically being part of sound synthesis, unison is not really an effect, but more of a fixed option within your synthesizer. The effect of unison is simply to multiply the signal which is being produced by the synthesizer, and in most cases to detune each signal against the other (just like detuning oscillators). This creates a much bigger sound, and is especially useful for making up a lack of oscillators. Common numbers of signal multiplication in a unison effect can be 2, 4, 8 or even higher. Some synthesizers have become even more creative in their use of a unison effect, such as the Clavia Nord Lead 3 which pans 2 signals to the left of the stereo field, 2 to the right, and keeps 1 in the centre, while detuning all against each other, creating a massive stereo-wide effect.
Chorus: A similar result to the unison effect, but with a much more complex sound modification process, the chorus effect is a popular technique to embellish and enlarge a synthesizer’s sound. Some would argue that the technique of the chorus effect is identical to that of unison or detuning oscillators, and this is true in part, as it can be physically demonstrated in acoustic instruments such as the piano or guitar, where multiple strings are played at the same pitch, but are ever-so-slightly out of tune to create a subtle warming effect to the sound.
The standard interpretation of chorusing within the synthesizer signal chain however is of an artificial effect where the signal is copied and mixed with several copies of it itself which have their pitch (at a very small level) constantly swept by an LFO. The effect can also be used in stereo by panning the delay effect within the stereo field, providing that the delay-based pitch sweeps are offset from each other. Because an LFO is used to control the effect, the rate can be adjusted, as well as the feedback and depth of the effect. This ‘artificial’ chorus effect is also one of the methods used in the flanging effect.