Brian,
> I often hear people talking about preserving bandwidth and I wonder
why that is such a goal? Sometimes increasing bandwidth can improve
performance to the point where you get all the bits through in a shorter
period of time, making the channel available to others. Think about
spread-spectrum or packet radio vs. RTTY for example.
This the argument that Winlink tried to foist upon everyone over five
years ago when asking for unlimited space. The fallacy of that argument
is that the frequency (or channel) is dominated by one station that
keeps the channel occupied until "its" traffic is passed, whereas, if
more stations, say five, can fit in one of our limited frequency ranges
and pass traffic simultaneously at only 1/5 the rate, the chances of
more than one station successfully passing traffic, even at a slower
rate, is greater than the chances of a single wide station passing its
traffic. In the limit case, the wide station keeps trying to pass
traffic without any success at all, but prevents other stations from
using the frequency when one or more of them could have been successful
in passing traffic.
The same it true for spread spectrum, or even HSMM. The idea is that a
station is on for only a short time, so it creates QRM on that frequency
for only a short time, and that is supposed to be acceptable for the
harmed station, as they can just repeat their transmission, or perhaps
the signal is so weak it is not noticed. The problem occurs when many
spread spectrum stations try to use a band of frequencies at one time.
In the aggregate, they will QRM the majority of stations over the band
in question, since there are few places where others can operate without
any QRM at all.
In layman's terms, a single wide signal can blanket the entire passband
of a receiver (let's say MT63-2000), and it is not possible to filter
out that signal. You have to wait until it is finished passing traffic,
no matter how fast it transmits. In the extreme, if you use a 50 Hz wide
IF filter for PSK31, 40 stations can occupy the IF bandwidth at one time
without any of them capturing the AGC or interfering with each other.
Again, signaling theory can probably make a scientific argument for
conserving bandwidth so a band of frequencies is available to more than
one station at any given time.
All the "get on and get off fast" arguments may be valid on wired
circuits, where access is controlled, but they tend to fall apart in
practice on the HF bands where space is so limited and access is random.
As a recent example, ROS uses spread spectrum, occupies 2200 Hz of
spectrum, but currently is only able to handle two stations on the
frequency at one time without interference between stations, and the
weak signal performance is no better than MFSK16, for example, in 1/6
the bandwidth.
The simple reason that narrow bandwidth is such a goal is that it gives
more stations the "opportunity" to use a limited number of frequencies
at the same time, and speed of traffic passing is not so important for a
50 wpm QSO, which is the speed that G3PLX chose as a reasonable speed to
accommodate a most typists. PSK63 is twice as wide, so only half as many
stations can use the same frequency range, but the speed (100 wpm) is
greater than needed for ragchews. It was only intended to be a RTTY
replacement for contests, in less bandwidth, but just as fast as RTTY
overall. However, there is a penalty of 3 dB in minimum S/N for the
extra speed, so for ragchews, PSK31 is sufficient and twice as many
stations can use the same range of frequencies simultaneously.
That is how I see it from a layman's viewpoint...
73 - Skip KH6TY
Brian Lloyd wrote:
On Sat, Apr 17, 2010 at 1:03 PM, Brian Lloyd <[email protected]
<mailto:[email protected]>> wrote:
On Sat, Apr 17, 2010 at 10:24 AM, ke4lkq <[email protected]
<mailto:[email protected]>> wrote:
That would be an excellent idea and a must have for amatuer
radio operators. It would preserve bandwidth and also educate.
To educate is good. OTOH, good luck with that! One of my favorite
saying is:
"Five percent of the people think. Ten percent of the people think
they think. Eighty-five percent of the people would rather die
than think." -- Thomas Alva Edison.
I often hear people talking about preserving bandwidth and I
wonder why that is such a goal? Sometimes increasing bandwidth can
improve performance to the point where you get all the bits
through in a shorter period of time, making the channel available
to others. Think about spread-spectrum or packet radio vs. RTTY
for example.
--
73 de Brian, WB6RQN/J79BPL
--
73 de Brian, WB6RQN/J79BPL
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