|[AMRadio] Re: Proposed AM 9 KHz BW limitation |
recycler at swbell.net
Mon Aug 16 23:10:58 EDT 2004
Ok, I'll ramble on about it. I have a few vague points.
The cited article seems to be mostly about digital bandwidths and
segregating the subbands by bandwidth rather than emission type.
It does mention an AM bandwidth limitation of -26dB@ 9 KHz. (I
wonder how many 6SN7's it would take to build a sharp low pass
audio filter with +/-1dB from 50Hz- at 6KHz and -26dB at 9KHz and also
have minimal phase shift? heh.)
It does not appear however as though they really want to take
away anything from the AM group.
As for AM bandwidth, there was, what I believe was a petition for
rulemaking some time ago, brought about by some fellows who were
apparently repeatedly interfered with by a few SSB operators
experimenting with bandwidth which was as wide as the usual
boatanchor/converted-broadcast AM phone bandwidth.
9 KHZ seems like more than enough for AM, after all, broadcasters
are spaced 10KHz apart, and have very good quality signals. The
real issue is freedom from all forms of distortion in the
modulation system. I cited some texts by Terman on the audio
bandwidth issue here, and the most generous of them said that
audio of 3500Hz was sufficient as a minimum for long distance
telephone quality of the day (1947):
So, a 9 KHz limit would make some sense, and should be plenty.
This is not about broadcasting or demanding telephone-quality,
and what I am saying that I would not object to a 9KHz -26dB
bandtwidth limit as matter of a station's good practice, but I
hate to see more and more rules, which can do more harm than
good. The homebrew KW AM rig I am restoring is capable of 30Hz to
11KHz +/-2dB at 100% modulation. All this means is that the audio
quality will be excellent when limited, by good operating
practice, to a reasonable 3000-3500 Hz audio bandwidth. So, there
should be no 'static' from the AM enthusiasts..
Since I do not have a General license, I fiddle with the AM iron
using dummy loads, and sample the RF. For voice, even higher
pitched voices, I think the 9KHz -26dB (4.5KHz -26dB audio)
bandwidth proposed is fine. It's not quite good for cymbal
crashes, bagpipes, and banjo music, but then those don't get
transmitted over the air on the ham bands ;-^
And what do they mean by "multimedia"?
I do object to a 200Hz limit on digital bandwidth on some bands,
as the excellent MT-63 mode uses 1KHz bandwidth. Maybe MT-63
could be authorized outside of the digital sub-bands. I don't
think it's authorized for ham use at this time, (anybody know?)
but I have experienced it on a military net and would like to see
it available for robustness and data rate. I understand that 1KHz
is alot of space, so..
I do object to a 'strict' 3KHz limit on SSB bandwidth, as some
older rigs are a bit wider, such as the GRC-106 military HF
radio, which is about 3300Hz wide at -3dB and 4KHz wide at -40dB.
Maybe it won't cause a problem.. ?? (by the way, AM mode on that
rig uses a carrier and one sideband, so it's quite narrow for an
AM compatible rig) After all, the audio frequency response
references above certainly apply to SSB audio bandwidth as well.
Wide SSB rigs -- (as supplied by the manufacturer, not modded by
'hi-fi' users) -- are few and far between, and they should
grandfather those few transmitters as they (apparently) want to
do with the AM stuff. I have never seen an SSB rig wider than
4KHz, but I could well be ignorant of them, as I prefer old tube
type AM stuff.
I do appreciate that "Amateurs would not be required to be able
to measure the bandwidth of their signals", as this relieves the
requirement for costly equipment. I understand that it does not
relieve the responsibility for compliance. I'm sure it will be
easy to comply using simple instruments.
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