It is a mastering compressor AU and VST plugin.
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Voxengo Polysquasher is a mastering compressor AU and VST plugin for professional audio and sound production applications.
Polysquasher was designed with a main goal: to be transparent. Transparent compression action is a cornerstone for any mastering compressor. Transparency of audio signal processing is necessary to preserve all existing elements in the mix, while compression itself is necessary to make the mix sound uniform to other mixes (tracks) and to make it sound more solid and
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