> Well, they did have an upper freq limit spec as I recall. It was 7.5
The telco loops were probably 8 kHz on unloaded twisted pair. Nevertheless,
there was not a regulated upper-end frequency
limitation prior to the implementation of NRSC.
Someone mentioned 10 kHz channel spacing. Yes, as it is now, and was prior to
NRSC, channel spacing remains at 10 kHz in North
America. Although it was technically possible to operate an AM station with 15
kHz of audio bandwidth, channel and station distance
spacings were such that interference was not a problem during the day, although
night-time sky wave could and did present a problem,
especially stations that did not employ 190-degree towers.
For example, prior to NRSC, the legendary 50KW WAPE-AM here in Jacksonville ran
with 15 kHz audio into a home-brew Doherty-modulated
transmitter which was designed by the station's owner in the late '50s as part
of his master's thesis in electrical engineering.
Perhaps some of you had seen in person or photographs of the famous "Brennan"
transmitter. The transmitter was truly a sight to
behold. The front panel was approximately 25-feet long and made of
gold-anodized sheet metal. The peak and carrier tubes were
water cooled via distilled water brought in every week by Zephyrhills to top
off that amount of water that escaped through
evaporation. A swimming pool cooled the exhaust water through a series Pyrex
glass tubing at the bottom of the swimming pool which
then recirculated after cooling back into the tube water jackets. The swimming
pool was indeed used for swimming during the 1960s.
Half the swimming pool was located outside and the other half was inside and
formed part of the reception area. I don't think we
could get away with that arrangement today.
The master of that transmitter was Jim Henry, KF4IC, who is now Director of
Engineering for Infinity/CBS Radio in Orlando. Jim is a
main-stay on 75-meters and if you run into him there, ask him about that
transmitter. When I took control of the station in 1994,
the Brennan transmitter was dismantled and I installed a new Harris DX-50
solid-state transmitter in its place...where it still runs
When we ordered BC quality loaded lines
> from telco years ago it was 7.5 kHz for AM and 15 kHz for FM. I can see
> the correlation now with the later
> NRSC roll-off filters. Before NRSC the 7.5 kHz spec was probably enough
> protection for adjacent stations.
> >The NRSC mask now effectively
> >limits the audio passband to approximately 9.5 kHz....and the service
> >still sounds poor despite attempts by broadcast stations and
> >receiver manufacturers to improve the quality.
> AM quality today is really a lot better, with tailored processing and PDM
> modulation. (up to +125%)
> If the listener has a good quality RX it's not bad.....
> And..... for "talk radio," including sibilant sounds, 7.5 kHz (or 9.5)
> is more than enough.
> (I'm sure the majority are tired of this subject now.... I'm quitting
> before the flames arise!) :-)
> Perry w8au
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