Now - depending on how good your processor / final filters are - figure
your "real world" bandwidth from there. Consider "good" filters at 12db
per octave - and "really, really good" filters at 24db per octave (an
octave is 1/2 (going down) or double (going up) a given frequency.


Randy, those figures are not characteristic of modern processors that
use DSP filtering, which is capable of extremely rapid rolloff.

Take a look at http://n2.net/k6sti/speech.jpg . This is a screen shot
of my HP 141T/8553B/8552B spectrum analyzer tuned to a local AM radio
station broadcasting speech. The analysis-filter bandwidth was 300 Hz,
the vertical scale 10 dB/div, and the horizontal scale 5 kHz/div. I
set the storage-screen persistence to maximum and accumulated spectra
for 10-15 seconds. It is easy to see the extremely sharp rolloff at 10
kHz. http://n2.net/k6sti/music.jpg shows a different AM station
broadcasting classical music. The music spectrum is evident, but so is
the brick-wall filtering at 10 kHz. These spectra are typical of what
I observe for AM stations here in Southern California.

If you have a receiver capable of SSB reception, you can easily check
the spectral limits of any AM station. Put the receiver in LSB mode
and tune down frequency from the carrier (or use USB and tune up).
Regardless of program content, it will be obvious where the response
ends. You'll hear the modulation sidebands suddenly vanish. Whenever
I've tried this, my dial has always read more than 9 kHz away from the
carrier.

Brian