Good question. I'd like to know too. I have seen them refered to as "ROMpler". I think the meaning mean there is a sampled sound in there but it is really only Read Only. Filters and effects can be applied over the sampled waveform. But I too would like to know what these type os synths are really called.
They're basically the only kind I use. I think there's a whole art to creating sounds. I've not dabbled into this. My art is more focused on playing the music, writing ,music, sight reading. I do admire those that can create their own sounds and use them.
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Bob