Implemented as a first-order highpass filter

whitespaces instead of tabs, see ad76693

Might not apply to the fm standards, but since you are not supposed to use it publicly, it doesn't matter

previouse filter was described by:
y[n] = bx[n]-bx[n-1]+ay[n-1]
filter is now described by:
y(n) = x(n) - a x(n-1)
with 'a' = old 'b'
even i have no idea if that coefficient 'a' is selected correctly, it sounds much better than before

by adding -preemph [eu/us/{cutoff freq}] to your Pi FM parameters, you can now select the right preemphasis for your region

with -cutoff [compliant/quality/{cutoff freq}] you can now select the cutoff frequency