[BC] Re: SUPER MODULATION & AM STANDARDS

[gRAdy Moates]

>The 125% limit was put into effect because of splatter.  Stations 
> that supermodulated (especially ones near the top of the dial) 
> used to splatter like crazy, wiping out most stations within 20 
> or 30 kHz away. I remember 1510 in Boston being 1/2 an inch 
> wide on my Mustang car radio's dial. This was done on purpose 
> to have dial dominance.   

While I feel that the increased energy of supermodulation has the 
direct effect of pushing out the contours of a station beyond the 
locations predicted by the allocation (and that this was one of the 
reasons that the FCC instituted the 125% pos-peak limit), we 
have discussed, on another list, about a year ago, the splatter 
issue, which is also real.  Specifically, on the other list, there was 
a fairly lengthy discussion of the transmitters used by WBAM, 
Montgomery, AL and it's 'sister stations' owned by the Brennan 
family.  These 50 kW stations were using transmitters that had 
been custom-built, and had pos-peak modulation capability that 
was near 200%.  By hard-clipping the negatives, these stations 
became 'splatter monsters', but they covered an amazing amount 
of ground during the day.  I remember WBAM coming in clearly on 
a Sears Silvertone 6-transistor radio in Cottondale, Florida during 
the daytime.  All the kids in my High School listened to them.  
They were 'the only game in town', and they weren't even in town.  

Two good reasons to institute the limit. . . enforcing contours 
and reducing splatter.  

Of course, later, when NRSC filtering was instituted, it became 
simple to supermodulate without splatter.  Matter of fact, 
when I was at WOL, WZYQ, WYRE and WPRW, several years 
before NRSC was instituted, I was running clippers with embedded 
two-pole low-pass filtering at around 10 kHz.  My reason for this was 
to reduce the amount of the transmitter's modulation capability that 
was being wasted on generating sideband energy that did not 
contribute to loudness.  I stole the idea directly from Bob Orban's 
design in the Optimod 8000 for FM, but, of course, my circuitry was 
not NEARLY as elegant as Bob's.  

When NRSC became law, I just bought a bunch of Bob's 9100 
NRSC upgrade cards, pulled my filters out and stuck Bob's filters 
in.  That was a lot of fun.  

A couple of years ago, new owners of WZYQ finally replaced that 
stuff with new digital processing boxes, and Ken Sleeman kindly 
asked me if I'd like to have my old stuff back.  It feels good when 
you can build something and have it still be running after 20 years.  

THANK YOU, Ken.  

Grady 

Grady Moates 
 
Owner, LOUD & Clean Broadcast Science 
  Voice        800 946-7007 
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