"Phil B" wrote in message
...

AM broadcast stations are required to cut off their high audio frequency
abruptly at 5kHz to prevent interference to adjacent channels spaced +
or - 10kHz.


I've read articles which claim there was no specific limit on AM audio
bandwidth, but the FCC required the stations to limit bandwidth to limit
interference. Given the normal minimum station spacing of 30 kHz in a given
area, this would imply a maximum audio bandwidth of 15 kHz. If sideband
splatter is any indication, I know the old rocker WCFL at 1000 kHz would
splat out the lower sideband of KDKA at 1020 kHz in the Chicago area. Now,
KDKA is an easy nightthime catch.

As stations were added to markets, the FCC limited the audio bandwidth to 10
kHz.

I've read textbooks which claim a maximum allowed 5 kHz audio bandwidth but
I don't trust the textbooks, so I searched the FCC website.

I came up with:

3. Sound Broadcasting

Sound broadcasting, double-sideband..

BINFn/INF=2M, M may vary between 4000 and 10000
depending on the quality desired

This defination was among a group above the FCC's formulas:

BINFn/INF = Necessary bandwidth in hertz

So, if I'm reading this correctly, the necessary bandwidth for standard AM
will be twice the audio bandwidth, which must be between a minimum audio
bandwidth of 4000 Hz and a maximum audio bandwidth of 10,000Hz.

This is from:

http://frwebgate.access.gpo.gov/cgi-bin/get-cfr.cgi?TITLE=47&PART=2&SECTION=202&YEAR=2001&TYPE =TEXT

Frank Dresser