Sun May 13 10:46:53 EDT 2007
the FCC for specific bandwidth limits of 3 kc for SSB and 6 kc for AM.
I see a couple of problems with that idea, besides the fact that it would
limit phone signals to telephone-like "communications quality" audio. Even
the SSB'ers are getting away from that, with the current interest in "HI-FI"
SSB (an oxymoron?)
First, even if you wanted to limit the upper high frequencies to 3 kc, it
would be IMPOSSIBLE to maintain a flat response that high, without
transmitting a signal bandwidth of at least 4 kc for SSB and 8 kc for AM,
due to the rolloff characteristics of even the best filters. There is no
such thing as an audio or rf filter with vertical skirts. Even the famous
Collins mechanical filters are rated at -3/-30 dB points. Usually the -30 dB
point is nearly twice the -3 dB point. To limit the significant bandwidth of
a phone signal to 3 or 6 kc would require the audio to start rolling off at
2 kc or less.
Secondly, this would render most presently-used vintage AM rigs and many
older SSB rigs obsolete. Most AM rigs simply use a mic driving an audio
amplifier which in turn drives the modulator. There may be some bypass
capacitors across an audio transformer or two, and/or across the plate
resistors of some of the low level audio stages, but this generates, at best
(or worse, depending on what you think of the idea), a gradual rolloff of
highs. Extremely sharp audio filters are possible (I have one), but not
generally available, and to install them would require what would
undoubtedly be difficult and expensive modifications to existing rigs. If
this becomes mandatory, I suspect that many AM'ers will simply give up and
say hell with it. So maybe the bandwidth proposal would not immediately
kill AM as Docket 20777 would have, but in the long run it would have the
Hopefully, knowledge of what is presently going down is not limited to
members the AM Reflector and those who monitor the AM Window BB. There are
still many members of the AM community who don't have computers or internet
access. Also, I think many in tho SSB community would be allied with us on
this issue, if they were aware of what may be about to happen.
MSN Photos is the easiest way to share and print your photos:
More information about the AMRadio mailing list
This page last updated 16 Oct 2017.