An expander is used as part of a gate to wipe out noise (and anything else) below a predetermined threshold. Generally this is an all or nothing response.
The comparison of an expander with a compressor is based on the fact that a compressor acts on signal above (as opposed to below) a predetermined threshold.
And that, as far as I can see is where the similarity ends.
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my toons Brought to you by Dr BadGAS
my toons Brought to you by Dr BadGAS