Hi Guys:
Well, it is quite true that EU AM broadcast stations are spaced 9 KHz
apart, and US stations are 10KHz apart, it's not true that the audio
bandpass for those stations is about 5 KHz. AM stations may very well
modulate their transmitters so that audio frequencies up to 10 KHz (or
more) are transmitted. It simply depends on the design of the
transmitter and the audio chain that feeds it. Yes, this makes their
signals broader, but the FCC rules for AM stations provide (in effect)
that stations will be located far enough apart from one another that
the "wide" sidebands are not interfering with other stations. Or, the
stations will need to use a directional pattern for their signals, so
as to "protect" the other stations from interference.
In fact, at one time, many US AM stations would use high quality phone
lines that were certified to pass up to 15 KHz, for their connection
from a studio to a transmitter site, for those stations that had
studios "in town" and a transmitter elsewhere. If dedicated phone
lines were not used, then often a microwave link of equivalent quality
(or better) would be used. Back when AM stations were part of a major
network, like ABC, NBC, CBS, or Mutual, the phone lines used for the
network programs would only be rated to 5KHz. That's because of the
cost per mile for the high grade lines was so much more. Easy enough
to justify for a mile or ten, but not hundreds or thousands of miles.
Since many AM stations now primarily have talk programming, some
stations will deliberately limit audio response to 5K, so as to try to
pack more "talk power" into the signal, just as hams do with AM and
SSB rigs. SSB, after all is AM, too.
73 de Ray
K2ULR
On Nov 1, 2013, at 4:21 PM, Andrew Robertson wrote:
It’s true that, in the USA, the sidebands will be 5 KHz, at the
absolute maximum, which means that I am missing at most about .8 KHz
of audio at the very most. However, in the shortwave bands (above
MW BCB) there are many broadcasters using 12 to even 15 KHz of
spectrum these days. China, Cuba, Russia, and others have all
adopted a very hi-fi sounding very wide bandwidth in the past few
years.
I use the same speakers for all my listening with different radios
so it’s not likely that this is the issue.
Thanks.
-Andy KE7TMA
On Nov 1, 2013, at 7:31 AM, dave <[email protected]> wrote:
The audio bandwidth of an AM radio station is something less than
half the separation between stations on the dial. In the EU that
separation is 9 kHz and in the US that is 10 kHz. So the max
bandwidth one can hear on AM - at absolute most - is 5 kHz.
Remember that AM is double sideband. The audio energy is both above
and below the carrier (USB and LSB). The audio passband is approx
half the RF bandwidth used.
If the KX3 has an audio passband of ~ 4.2 kHz then you are
effectively missing nothing in terms of bandwidth.
If some other radio 'sounds' better that is likely due to listening
to a different speaker with different freq response.
73 de dave
ab9ca/4
On 11/1/13 5:25 AM, Andrew Robertson wrote:
While I do enjoy the AM SWL capabilities of the KX3, I would really
like to be able to appreciate a more hi-fi sound from the unit. I
do know that that roofing filter XFIL1 is probably the ultimate
limiter (I believe it’s 15 KHz? or 10?) but I think the DSP
filter in software is actually what is reducing my bandwidth
to 4.2 KHz. Since it is in software, I am hopeful that, perhaps,
a future software update could provide a wider bandwidth or even
shut off the DSP filter completely (pass-through mode).
I’m sure I’m not alone in using my KX3 as a SWL rig, and I hope
that consideration could be given to this suggestion. Thanks
Wayne and Eric and the rest of the gang at Elecraft for a
superb product!
-Andy KE7TMA
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