Okay, time for a little unofficial survey -- for those of you who
engineer AM stations, how much pre-emphasis do you use?

In the late '80s, the NRSC established a standard "modified 75 uS" AM
pre-emphasis curve, but the FCC never officially made its use a
requirement, except for AM Stereo stations. And today, it seems that
most AM stations are using a higher amount of pre-emphasis -- even
many AM Stereo stations.

In fact, the Optimod 9100B offers four different pre-emphasis curves:
"Blue", "Red", "Yellow", and "Green". "Blue" is the NRSC curve, while
the others provide increased amounts of high-frequency boost. The
"Green" curve can be maxed out to provide over _twice_ as much
pre-emphasis as the NRSC curve!

The mono Optimod 9200 offers a similarly wide choice of pre-emphasis
curves, and in fact, it comes shipped from the factory with a default
setting which exceeds the NRSC curve by up to 3 dB at mid-range
frequencies (~4 kHz).

The result of this variation, combined with the overwhelming
predominance of Optimod processors on the AM band, is that we're
basically back to square one, before the NRSC standards were created.
Some AM stations sound dull and muddy, while others sound tinny and
shrill -- and you don't even need a "wideband" receiver to hear the
difference.

Over the decades, adjacent-channel interference is the number-one
complaint which has led to almost universally narrow-bandwidth,
low-fidelity AM receivers. But Optimod-ized AM broadcasters are only
aggravating this problem by using extreme amounts of pre-emphasis.

And now, IBOC proponents want to "solve" this problem by chopping off
everything above 5 kHz. They even claim analog AM radio will sound
_better_ as a result, because receivers can be opened up to 5 kHz
bandwidth, as compared to the 2 - 3 kHz they typically offer today.
But says who? The hundreds of pages of iBiquity, NRSC, and FCC
documentation I have read make absolutely _no_ mention of what kind of
pre-emphasis, if any, IBOC stations should use, or how much bandwidth
the analog portion of IBOC receivers should provide.

Meanwhile, the Omnia 4.5 AM audio processor curiously appears to offer
no user choice of pre-emphasis. It does offer a choice of 4, 6, or 10
kHz audio bandwidth, but the user's manual makes no mention of
pre-emphasis at all. I guess Omnia chose a curve that "sounds good"
to them, and that's that?