> I don't know where the rumor comes from that excessive bandwidth
> somehow results in "cutting QRM or QRN" better. It certainly is not
> based on any type of engineering or research!
I'm not sure of the methodology, but I recall a graph from a Navy study in
which keyed envelope shape rise/decay was plotted against minimum
discernable signal strength. Another graph depicted CW speed against MDS
conditions as well...and a third graph combined both factors. I'll look for
this...
>From my own experience, I know that I can copy harder keyed waveforms in MDS
conditions at relatively high CW speed, than softer waveforms. Clearly, my
own error-free reception is a function of 1) CW transmission speed and 2)
the keyed waveform shaping. I do not believe you can have this discussion
without addressing these two factors together. For example, if the
transmission speed is ~ 5 WPM (traditional, non Farnsworth), I would expect
that little difference would be detected in the copyability between hard and
soft keyed waveforms.
Another interesting source is found in my 1998 ARRL Handbook on page 15.7:
"It so happens that we always need to hear one or more harmonics of the
fundamental keying waveform for the code to sound sufficiently crisp. If
the transmitted signal will be subjected to propagation fading - a safe
asumption for any long-distance radio communication - we harden our keying
by making the transmitter's output rise and fall more quickly. This puts
more energy into more keying sidebands and makes the signal more copiable in
the presence of fading - in particular, selective fading, which linearly
distorts a modulated signal's complex waveform and randomly changes the
sidebands' strenght and phase relative to the carrier of each other. The
appropriate keying hardness also depends on the keying speed (W9AC: there it
is...). The faster the keying in WPM, the faster the on and off times - the
harder the keying must be for the signal to remain ear-readable through
noise and fading...A transmitter's CW waveshaping is therefore usually
hardwired to values appropriate for reasonabley high-speed sending (35 to 55
WPM or so) in the presence of fading. As a result, we generally cannot vary
keying hardness at will as we might vary a voice transmitter's modulation
with a front panel control (W9AC: two notable exceptions include Kachina and
Kenwood's menu-selectable DSP-based CW waveform generator). Rise and fall
times of 1ms to 5ms (5ms rise and fall times equate to a keying speed of
36WPM in the presence of fading and 60WPM if fading is absent) are common."
Arguably, perhaps the best solution allows: 1) variable control of the keyed
rise/fall time from a front panel control, or 2) a circuit in which the
speed of the waveshape changes as a function of keying speed. In order to
accomodate hand keys and external keyers, the keying input would require
analyzing...which may result in additional delay from the time of the keying
until RF is generated. In contest conditions, the front panel control can
be set to "soft" and for other band conditions, depending on propagation and
crowded condition, the control could be set between the hard and soft
extremes.
73,
-Paul, W9AC
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