RadioBanter - View Single Post - Blue, Red, Yellow, or Green? (AM pre-emphasis survey)

January 2nd 04, 11:52 PM

In article , (WBRW) wrote:

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!

Hi Kevin,

I don't know anything about the Optimod 9100B, do you know what the
various curves look like? It would surprise me if they simply added more
and more "preemphasis". IIRC, and of course I may not, the original
Optimod 9000 had separate equalization controls for peaking the area
around 3,500 Hz where the typical AM radio is rolling off, and a separate
control for the higher frequencies up to 10 kHz or so. If I remember it
correctly, the idea was to be able to boost the frequencies that would
help the typical radio the most, without overdoing it at the higher
frequencies, and causing problems by boosting frequencies that weren't
going to be heard anyway, except on a minuscule number of radios. I
wonder if the four different colors don't mainly provide different amounts
of peaking in that mid frequency area where most radios need some help?

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).

If I follow you, this midrange boost is consistent with the equalization
in the Optimod 9000, and what I would guess the colors might do in the
Optimod 9100B, i.e. provide midrange boost beyond the NRSC curve.

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.

I assume that's the intention, the optional mid range boost is there to
help the typical radio.

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.

I wonder if it is "extreme amounts of pre-emphasis", or just the extra
midrange range hump in the 3.5 to 4 kHz area that you are hearing?

Why not just equip your radio with a complimentary four color equalizer switch?

Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/