Some might get confused sometimes when compressor, expander and gate are discussed and especially when concepts like “upward”, “downward”, “parallel” or such-like are thrown in. Fortunately, things can easily be explained just by looking at the according transfer curves and as an added sugar some more sophisticated insights can be obtained es well.
The “ordinary” compression transfer curve which is used in most audio dynamic compressors is the downward compression curve somewhat similar to the image above where a 1:1 curve is shown for reference as well (and so in all other diagrams here). It basically works in a way that a signal level above a certain threshold – in this case roughly around minus 10dB - leads to an attenuation which is linear as well (ok, almost – and just in case of a hard-knee compressor) but with a lower amplification ratio. The so-called upward compression works the other way around: A signal above the threshold level remains unaltered level wise but those below are getting amplified to a certain degree now. This is shown in the diagram below where a threshold is set somewhere around +20dB:
Since the upward compression transfer curve looks quite similar to a *parallel* downward compressed one (which is an uncompressed dry signal mixed back into downward compressed one) some might guess that this is completely the same but actually it’s not. The difference lies in the fact that envelope curve based dynamic treatments are only partially applied to the parallel compressed signal and the other part remains completely unmanaged. This is different in the upward compressor: the whole signal will be shaped by the envelope curve and therefore the transient response is actually different.
Opposed to a compressor an expander features basically just different transfer curves as shown in the diagram above where a typical 1:2 downward expansion curve is shown: In this case, below a threshold at around -10dB downward expansion occurs which even lowers the already quieter signals. Upward expansion (not shown there) is just the other way around and preserves the signals below a threshold and amplifies the already louder ones. The downward expansion is often referred to when it comes to the so-called Gate/Expander device. This can easily be understood just by looking at the according transfer curves again which in this case shows that the classic gate curve is just a special case of the downward expander where the ratio becomes infinite:
Additionally, other concepts like soft-knee curves (already shown above) or processing range limiting (like a range control in a compressor or a floor setting in a gate) can be implemented just by proper transfer curve designs as well. This leads to the question, if one could implement just one single device for all that three purposes – compressing, expanding and gating – by simply utilizing the very same basic engine and just swapping the respective transfer curves. Well, in theory this idea might be attractive and sound but practise has shown that this does not lead to optimal results. All the other relevant aspects such as time or frequency dependent behaviour or even non-linearity is not that interchangeable between such devices and their specific application domains in general.