The reason for pre-emphasis has as much to do with the modualtion technique and the
inherent limitations with the method.

If you're familiar with the sideband model for frequency modulation (the one
represented on a spectrum analyzer), you may remember that 100% modulation (+/-
75kHz) will produce many more sidebands at 60Hz than it will at 6kHz. Because the
amount of power in the sidebands for lower-frequency modulation is so much greater,
the low frequencies are inherently quieter (because the limiter stages in FM
demodulators have more to work with to strip out AM noise). Pre-emphasis was added
to provide a modicum of noise improvement (by increasing the amount of power in the
sidebands) for modulation frequencies which have a relative deficiency in modulation
sidebands.

It's arguable whether the North American standard of 75uS is too much, and as I've
never heard European-standard FM radio, I don't know if their 25uS preemp is too
little or still too much, but the adoption of preemphasis compensation has nothing to
do with the technology of the time - it's simply trying to circumvent the laws of
physics, and those haven't changed since the beginnings of frequency modulation
techniques.

Frankly, Chris, if you're having trouble dealing with high-frequency clipping with
your processing set up, you've got it set up wrong, anyway. There's no way that the
high end should be slamming against the pre-emp limit. No wonder you think Texars
are trash, since it's so easy to use too much high end. Finesse is the key.
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"Chris Boone" wrote in message
...
Actually, the preemphasis on FM nowadays with all the high end
processing can be done away with...to try to process (compress, limit,
clip) the 0-15 kHz spectrum and especially the high end, means you will
overshoot and overmodulate the xmtr unless you clip/limit the hell out
of it..but when you clip, you cause even more problems....
MOST engineers today agree that the PRE AND DE emphasis curves could be
done away with entirely...because the field has changed a lot since
Armstrong's days and the FCC's early development of the FM rules.
With today's processing and EQing, you really donot need PRE
emphasis...and if you donot need it, why have DE-emph in the rcvr???

The RIAA curve for records was done because of the material used & the
freq response in early days...technology has left that behind and that's
a moot point today...CD's donot have such a requirement...and I always
record my cassettes with Dolby C ON and then play them back with Dolby C
OFF so the high end sounds cleaner and brighter

If the rcvr makes had wanted to, they COULD have had flat response to
10-15 kHz in an analog rcvr and put a filter switch on it (like my Sony
XRA-33 AM STEREO car rcvr has)...but they didn't want to listen to car
buyers bitching about the noise and whine while listening to AM adj
channel...so they made the rcvrs narrow....same reason why 50% of the
time your radio FM stereo light is ON, you are actually listening in
blended mono!!!

Chris
WB5ITT
Houston

WBRW wrote:


Because there has to be a level playing field. Why do you think the
RIAA curve for phonograph records was established? Because there was
too much variance and too much confusion, and it was inhibiting the
goal of providing the consumer with increased fidelity and
convenience.

This also applies to AM radio. Whether it be today's 10 kHz or the 5
kHz that IBOC proponents want, consumers will never get to experience
analog AM radio at its full fidelity unless there is a level playing
field and a universal standard which manufacturers can design their
receivers to meet.


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