It would be nice to have the ability to pass a Filter into any variant of this function. You could change the methods above to accept `filter::Filter` instead of `tff::TFFilter` and first call `tff = convert(TFFilter, filter)` (this will be a no-op if `filter` is already a TFFilter), or change the method signature here to accept `hz::Union(Number,Array)` and add a second method like this to accept the angular frequency.