From: "Tim Shoppa" on Tues 11 Oct 2005 06:00

AM wide 12-16khz
Am narrow 6-10khz

Maybe it's my aging male ears but I've never heard an AM transmission
where a 10kHz filter was needed or even desirable. Whenever I switch
on the wider filter I just hear more hiss and static, and no more
fidelity.

I can hear a small difference between 6kHz, 8kHz, and 10kHz with local
AM BCB stations. My preference has always been to abhor hiss so I tend
to go towards the narrower side. I don't think my ears are as
sensitive to hiss as when I was younger but it still bothers me
especially with extended listening.

I am told that the 16kc wide filter on my R-390A was used largely for
when multiple channels were being multiplexed (e.g. multi-RTTY or
multi-voice channels).

As a point of historical reference, the "390" series was intended
for fixed-point to fixed-point communications reception as well as
in the land-forces' AN/GRC-26 truck-mobile HF hut. The wide
bandwidth was intended to be used with "commercial-government"
SSB that had a 12 KHz wide bandwidth to accomodate four 3 KHz
"voice grade" channels frequency multiplexed. Each voice channel
could handle 4 to 8 TTY circuits, each using tone-pairs (one for
Mark, another for Space) via auxilliary "carrier" equipment. A
common arrangement was to have one voice channel as the "order
wire" or command channel, a second voice channel as an "overseas
radiotelephone" circuit, the remaining two voice channels used for
TTY circuits. Depending on the carrier and auxilliary equipment,
TTY tone pairs could be doubled to take care of selective fading
effects common on HF long paths.

For some illustration of actual use of R-390s at a large receiver
site, go to http://kauko.hallikainen.org/history/equipment and look
down the page at "stations." Click on the 1962 communications
station link for a repro of a booklet produced by the Japan Signal
Service Battalion about Army station ADA in Tokyo. ADA shared the
receiver site with the USAF and the antenna field was over dozens
of acres of small farms northwest of Tokyo. PDF is about 10 MB.
I was assigned to the transmitter and control sites at ADA.

The "commercial" 12 KHz wide SSB was standard practice on HF from
the 1930s. Not many left in service now although there are a few
unmistakable continuous-roaring-multi-tone commercial SSB signals
to be heard outside of ham bands. The "roaring" sound comes from
the TTY tone pairs (at least 8 circuits per station).