🚀 go-pugleaf

In article <1k25kgINNb73@geraldo.cc.utexas.edu>,
ccdo017@bongo.cc.utexas.edu (David Kirk) writes:
|> I am beginning a DSP project and need some assistance as to which
|> direction I should head.
|> 
|> The project entails taking the frequency response of any signal
|> after
|> it has passed through a simple filter (ie RC filter) and producing a
|> new signal which has its response reversed:
|> 
|> |				|
|> |------------  			|          ----------
|> |            \	       ----->   |         /		
|> |	      \			|        /
|> |	       \		|       /
|> |___________________ f          |_____________________  f
|> 
|> I know that taking the FFT will give the spectrum, but it would
|> be different for different signals. The only constant is the
|> (analog)
|> filters response. My design should be able to take any signal of any
|> spectral make-up.
|> 

Hi !

I am only guessing what you want to do.  If you change the sign of every
other sample of your signal, you will mirror the frequency response
around Fs/4 (process also termed spectral inversion: swapping the highs
with the lows).  If you want to mirror the frequency response of, for
example an FIR filter, change the sign of every other sample in the
filter impulse response (be sure not to flip the central sample. 
Otherwise you will invert the signal).  This is a simple way to convert
a low pass FIR to a high pass FIR with the frequency response mirrored
around Fs/4.

Changing the sign of every sample is the operation of multiplying the
signal by a full amplitude sinusoid at f=fs/2.  This process amplitude
modulates the signal out of band such that it aliases perfectly into the
base band.