> As I said before, having the high order sideband does no good at all
> if we are using 500Hz or narrower filters. We give up S/N ratio in
> direct proportion to the bandwidth increase, if we use a 1kHz filter
> S/N is 6dB less than with a 250 Hz filter. If we use a 250 Hz CW
> filter, the rise and fall limit is about 5-6mS with a sine shape rise
> and fall, any sidebands wider than that simply do not make it
> though the filters.
Agreed 100%. With narrow filters, I notice no difference regardless of
rise/fall time. The filtering rounds the edges and extends the rise/fall
time. However, under MDS conditions, I almost always use as a wide of a
filter as I can, notwithstanding QRM. The wide filter provides the most
readable copy on faster waveforms, due to the extra bandwidth if the
transmitting stations is using faster rise/fall times. I am not advocating
excessive bandwidth (less than 2 ms rise/fall) in order to accomplish
this...but again, the faster rise/fall time under MDS conditions almost
guarantees more readable copy when listening with wider filters (i.e., wider
than 500 Hz).
I also realize the actual shape, not just the rise/fall, is of great
importance: smooth transitions produce less bandwidth than sharp edges along
the envelope. So, my position is: keep the waveform as close to sine or
guassian as possible, and change the rise/fall to match band conditions,
particularly if the receive operator prefers wider filters under MDS
conditions, and the band is not crowded to the point where the advantage
gained by the wider filter is not offset by interfering stations. Tom, I
assume your response is...if all the stations were using smooth transitions
and greater than 5 ms rise/fall times, then under crowded band conditions,
closely-separated stations will not interfere with each other, regardless of
the receive filter bandwidth. And I agree with this. Contest conditions
where the vast majority of stations have ample signal strength is an
example. Weak signal DX work on a less crowded band is different altogther.
The bottom line is that there is a place and a need for faster rise/fall
times when no interference to other stations results.