"Earl Kiosterud" wrote in message
news
t%Yi.403$CI1.60@trnddc03...
"Robert Orban" wrote in message
...
In article ,
says...
"SFTV_troy" blabbed:
... this new receiving technique would not improve the sound
(it would still be limited from 100-6000 hertz), but would only reduce
interference.
At least in the States, AM & FM broadcasting is limited to 50 Hz to
15KHz.
There is no low frequency limit for either AM or FM; 50 Hz was the
minimum
performance standard that would meet the now long-deleted FCC Proof of
Performance measurements.
The effective HF limit on FM is about 18.5 kHz; this leaves a +/- 500 Hz
guard
band for the stereo pilot tone. Again, 15 kHz was the minimum spec that
would
pass a Proof of Performance, not a limit on bandwidth.
Currently, the legal FCC-mandated HF limit on AM in the US is a hair less
than
10 kHz, which almost completely protects second-adjacent stations from
interference. This was changed around 1990 as a result of work done by
the
National Radio Systems Committee (NRSC). More recent work by the NRSC has
indicated that 7 kHz is probably the optimum compromise between causing
interference and loss of audio quality on typical AM radios (which are
down 3
dB at about 2.6 kHz). However, limiting bandwidth to 7 kHz is voluntary.
Robert,
Was AM radio ever allowed audio to 15 KHz? I read many years ago that it
was, perhaps before the NRSC recommendation was adopted by the FCC. I
presumed that the stations either were allowed to overlap 5 KHz
(doubtful), or that stations in a given area were separated by at least 30
KHz.
--
Regards from Virginia Beach,
Earl Kiosterud
www.smokeylake.com
I was a broadcast engineer in the late 1970s to the late 1980s. At that time
(before NRSC) AM was required to transmit a minimum 5KHz bandwidth, but the
maximum modulated bandwidth was not really defined. There were limits on
"spurious" emissions, caused by audio distortion products and carrier
harmonics. I don't recall the exact mask, but 15KHz was legal at that time.
Our studio transmitter link was a Mosely PCL-505, which was flat to 15KHz,
and we employed no artificial band limiting, so the station was flat to at
least 12KHz. Our tower was the limiting factor for bandwidth. It sounded
just like monophonic FM on the modulation monitor.
During the day there was no overlap, because stations were allocated on
second alternate channels in most markets. Local stations that did overlap
usually worked out a solution amongst themselves if the interference was
objectionable. At night it got quite a bit noisier as distant stations would
skip into the area, but it wasn't generally sidebands that caused the
problem, it was the carriers themselves, each whining away at 10KHz. That is
still a problem, even today.
The real problem was that in the late 1980s, AM stations began adding
proprietary "pre-emphasis" -- high frequency boost to make their station
sound brighter on typical pathetically band-limited AM receivers. This can
and did cause severe interference in some congested markets. Partially to
address this, and to standardize the pre-emphasis, NRSC limited AM sidebands
to 10KHz in the early 1990s. Since most AM radios do not even come close to
being flat to 5KHz, 10 KHz is still two or three times more bandwidth than
most listeners can use.