[AMRadio] AM Broadcast Quality


Donald Chester k4kyv at charter.net
Wed Dec 18 13:41:05 EST 2013


>>From: "W. Harris" <nbcblue at hotmail.com> 
The specs on my Ranger audio chain and other Johnson transmitters is flat
response from 250-3000 cycles. For the Collins 32V-3 it is 200-3000 cycles,
which is all very adaquite (sic)  for good AM good voice  communications. I
get very good audio reports on the Ranger. Any more response just ads to the
bandwidth. For AM broadcast stations, 40-7,500 cycles is typical. We are not
broadcast stations.>>

Bill,

The selectivity of your Drake 2-B  is much narrower than the audio specs of
your Ranger or 32V, even when it is set to receive in the AM mode. I have
listened to AM on a 2-B in the past and recall having to tune to one side of
the  carrier to make the AM signal fully intelligible, because the
bandwidth was so pinched, even in the nominal "AM" position, that it was
impossible to  receive both sidebands at once, rendering the full frequency
range of a double-sideband AM signal is impossible to hear in any case.
Could this have something to do with your lack of interest in anything
beyond the frequency range of a typical SSB signal?

>>97.307 Emission standards
(a) No amateur station transmission shall occupy more bandwidth than
necessary for the information rate and emission type being transmitted, in
accordance with good amateur practice.

10 or 20 kHz bandwidth is not necessary for good AM communications on the
ham bands. We are communicating, not broadcasting.  >>

It depends on your definition of "communication". As I said before, simply
getting the basic words of a message across can be accomplished perfectly
well, if not better, with CW. "Communication" is multi-faceted; there is
more to it than merely getting a string of words or data across to a second
party. Recognition of who is  doing the talking by the sound of their voice
with all its subtleties is part of what is being communicated, as well.
Those of us who work phone instead of CW opt to use the additional bandwidth
because we prefer the pleasure and convenience of communicating by voice.
Those of us who work AM instead of SSB opt to use the additional bandwidth
because we prefer the pleasure of  communicating using the characteristics
of the AM mode. Those of us who use wider frequency  response instead of
space-shuttle audio, opt to use the additional bandwidth because we prefer
the pleasure of communicating  with a natural sounding human voice to that
of a tin-can telephone. Who is to dictate by what arbitrary standard, that
your preferred mode of operation is OK but mine uses "un-necessary"
bandwidth? 

Besides, this is a HOBBY, not a commercial communications enterprise. By
definition, the vast majority of amateur radio communication is trivial, and
tolerated only because resorting to public telecommunications services would
not, due to its unimportance, be justified. The exception is emergency
traffic, now termed "EMCOMM". Strictly  speaking, ALL amateur radio traffic
with the exception of emcomm, by definition, occupies "more bandwidth than
necessary".

Good amateur practice merely tells us to use the appropriate bandwidth to
match band  conditions and congestion in order to keep harmful interference
to a minimum. Dropping a strong, 20 kHz wide AM signal on  to 3875 during
prime evening hours, rendering 3860-3890 unusable to other AM and SSB
traffic, would hardly be considered good operating practice. But if this
same act is done in the wee hours or during the middle of the day when
no-one else is operating in this entire swathe of frequencies, no harm is
done, and there is no reason to say this isn't good operating practice,
assuming the 20 kHz bandwidth is due to the frequency  range of the audio,
not overmodulation, flat-topping on positive peaks or spurious distortion
products. 

The bandwidth "saved" by cutting off the low frequencies at 250 or 300 Hz is
negligible and serves no useful purpose, even when the total bandwidth of
the signal is limited to less than 3 kHz. I doubt if anyone would notice the
difference between the interference caused by a 2.7 kHz and that of a 3.0
kHz SSB signal. With AM, due to carrier and sideband spacing, cutting the
lower audio frequencies does  nothing at all to reduce the total bandwidth.
We don't want to boost the low frequencies to the point that they overpower
the rest of the frequency range and make the signal difficult to read. But
that doesn't mean  we have to cut them off, either.

Furthermore, what does "broadcasting", which by definition means
transmitting the same information to many listeners, versus point-to-point
communication, which by definition means transmitting the information to
only one listener, have to do with audio quality or bandwidth?


Don k4kyv


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