[opendtv] Re: FCC rules for digital radio

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Sun, 25 Mar 2007 19:30:44 -0400

John Limpert wrote:

> Many AM stations have to shutdown at night.

Good point. So I wonder exactly what the FCC means by "Authorizes AM
nighttime operations." Do they mean that now all AM stations are allowed
to continue at night? Or do they mean that *digital* over the AM band is
allowed to continue at night?

> Wasn't there a restriction against nighttime AM digital operation,
> as it hadn't been properly tested for those propagation modes
> (skywave) and the effects on other stations were unknown.

It's the simple fact that in hybrid mode, each AM station with digital
simulcast takes up 30 KHz of spectrum. So at night especially, each
digital station in the AM band will clobber two other AM stations that
might be in that slice of spectrum, for quite a long distance.

If the station transmits only in digital, then the IBOC scheme has an
option where the station crunches down all of its signal in the same 10
KHz bandwidth that is now given to regular AM. So nighttime operations
could presumably be the same as they were in the analog-only days. This
is different from the FM band, in which stations retain their entire 400
KHz of channel width even if they are all-digital.

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