A legacy radio like a K3 or Orion uses a signal chain that is mostly in
hardware. The signal goes
[ant-bandpass filter-preamp-mixer-if amp-roofing filter-mixer-more filters-more
amplifiers-demodulator-DSP stage-audio amp]
Some of these stages may be rearranged or left out but its basically some
variation of this
The F5K or 3K goes [ant-banpass filter-preamp-mixer-ADC-software-audio]
In a legacy radio the reason you need "roofing filters" is because of all those
amps and mixers done in discrete components. Each of those mixers and amps
lends itself to overload, so in order to avoid overload you stick in a big
attenuator called a "filter". That attenuator however does not attenuate
things in its passband, so in order to keep things from distorting, roofing
filters keep getting narrower and narrower If you have 2 strong signals
inside a 500hz roofing filter for example they distort the hell out of each
other.
The Flex radio instead of all these amps and such uses equations in the
computer [software] to do all the magic that turns RF into audio. Equations
unlike discrete components do no distort so there is no reason to have "roofing
filters". This in its essence is the reason the Flex radios do not need
roofing filters.
Another reason is the flex radio as opposed to using crystal filters to slice
off sidebands, uses a skind of "phasing" technique to get rid of things like
images and unwanted sidebands
If you can imagine a double sideband signal looking like this 54321-0-12345
where 0 is the carrier in order to get rid of the "12345" and get SSB in a
legacy radio you need a $200 filter to chop off the "12345". In the flex
scheme in order to get rid of the "12345" you just set those values to 0 in the
computer program. You do it mathematically with no "filter" at all just math;.
Since you do it with math you miss all the distortion that things like filters
add to the signal, so you wind up with a very clean very well behaved very
undistorted SSB signal. Because you don't need a roofing filter you also can
have a panadapter with a wide bandwidth for click tuning and frequency
management
Hope that is of some help
73 W9OY
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