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June 10th 04, 06:53 AM

In article , wrote:

John Byrns wrote:

1. It has been variously stated that the audio bandwidth of AM
broadcasting is either 3.5 kHz, 5 kHz, or 10 kHz. In the US AM broadcast
channels are 20 kHz wide, so audio is effectively limited to a maximum of
10 kHz by law/regulation. It is my impression that most AM stations
transmit audio out to this legal maximum. Of course as HD-radio takes
hold this will change with the analog signal cutting off somewhere around
5 kHz. I know there are at least 2 active broadcast engineers that read
this group, perhaps they could fill us in on what the stations they are
involved with are actually doing as far as audio bandwidth goes?

We've kicked this around the block before - but I guess it won't hurt to
kick it one more time. (I'm only going to address US standards here).

The AM Bandwidth is 10Khz.
However - what has to be taken into consideration are "real world"
filters - and the "stop band" specifications of the NRSC-1. I.E. the
signal must be down 15db (from 100% modulation) AT 10khz(!!!)

Further - it must be:
-30db at 10.5Khz;
-40db at 11Khz; and
-50db at 15Khz.
-50db is .32% modulation (that's point 32 percent - not 32 percent).

Now - depending on how good your processor / final filters are - figure
your "real world" bandwidth from there. Consider "good" filters at 12db
per octave - and "really, really good" filters at 24db per octave (an
octave is 1/2 (going down) or double (going up) a given frequency.

So if you put a 12db per octave filter in front of your transmitter -
the highest unattenuated frequency through that filter will be around
4.5Khz.

You can do much better with a 24db per octave - somewhere around 7.5Khz.
Of course - NRSC-1 is no longer the "newest kid on the block" - the
new one is ITU-R (Recommendation 328-5). To meet that spec. - the
processor filters are set to 6.0Khz.

Amigos and Optimods are set up to meet NRSC-1 power spectrum
requirements "out of the box". The Optimod 9200 (the current top of the
line digital AM processor) is adjustable from 4.5kHz to 9.0kHz in 0.5kHz
steps, plus NRSC - is guaranteed to meet ITU-R (Recommendation 328-5)
and NRSC-1 power spectrum specifications without the need for further
low-pass filtering prior to the transmitter. And as noted -- is
typically set for 6.0kHz for ITU-R.

Amigos or Optimods are probably in 90%+ AM stations in the US (We're
(WMER) is running an Amigo).

OK, now we are getting down to brass tacks as my Grandmother used to say,
whatever that may mean. The $64 question is does the Amigo as used at
WMER only do NRSC, or does it have a selection of cutoff frequencies like
the Optimod 9200, and if it does what is the cutoff frequency set to at
WMER?

Regards,

John Byrns

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

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