Joe,
I believe you are mixing 'zero beat' with zero frequency audio (or DC) -
unless I misunderstood your note.
It makes no difference if the DSP is working at audio (baseband) or at
an IF frequency, the filtering is set to a specific width and the math
figures it out - the resulting signal that you hear is at baseband, but
that is only the output, not necessarily the processing, and IF DSP will
convert to baseband at its output so we can hear the result.
Properly done, a 500 Hz roofing filter would be centered above the audio
zero frequency entirely, so the roofing filter response would extend
from (say) 150 Hz up to 650 Hz - nothing on the opposite sideband, and
even if it does spill over a bit on the filter slope, the DSP can clean
it up.
Then the DSP takes over, and your 200 Hz Hz bandwidth at a 250 Hz pitch
would extend from 150 Hz to 350 Hz - still nothing on the opposite
sideband, and if the roofing filter had allowed a bit of opposite
sideband through (up to 150 Hz), the DSP would not allow it through.
Of course, this is much supposition on my part, I do not have the
details yet myself. If they get a break from pushing the K3 out the
door, maybe Wayne and Lyle will provide additional information about
'how it really works'.
I don't really think it is anything to worry about, but if there are low
frequency limits placed on the sidetone pitch, that may be the reason
why it is limited at the low end.
73,
Don W3FPR
Joe Subich, W4TV wrote:
How does (if it does) this change when the second filter is a DSP
filter/demodulator working at IF instead of at audio? For example,
in the K3 with the 500 Hz (565 Hz SF=3.1) roofing filter set for
a 250 Hz pitch the IF will have significant bandwidth "below zero
beat." If the DSP is set for 200 Hz bandwidth, will it include
response at the "audio image" or does the K3 DSP "filter before
demodulate?"
73,
... Joe, W4TV
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