What you're calling "Software AHS" is not just for harmonic suppression. It
also allows you to transmit outside your bandpass (virtually). So even though
your rig may only have 2400Hz of width you can still transmit at 2900Hz -- you
just can't receive there. It also helps avoid signal loss at the low end of
the bandpass so you can transmit at 200Hz even though your bandpass may roll
off higher.
Mike W9MDB
On Tuesday, May 2, 2023 at 02:30:10 AM CDT, Alan via wsjt-devel
<wsjt-devel@lists.sourceforge.net> wrote:
Interesting discussion but I find it a bit confusing so I've gone back to
basics:
The transmission is in the form of amplitude modulation using single (upper)
sideband suppressed carrier. That means the RF may not be a single frequency
but a band of frequencies with the bandwidth defined by the normal USB filter
width - the sideband. The carrier and lower sideband aren't there in usable
form.
The USB filter width is about 300Hz to 2800Hz, depending on rig settings, so a
single tone of exactly 1500Hz will produce RF at 1500Hz into the sideband width
and for all practical purposes nowhere else in that sideband.
A lower tone of 500Hz will produce RF 500Hz into the sideband but also probably
some harmonics every 500Hz up to 2500Hz.
If there is no audio signal at a particular frequency within that filter width
there will be no RF in that part of the sideband either.
A WSJT-X signal is generated as a narrow band of frequencies about 50Hz wide.
On transmit RF will be present within the sideband only for that 50Hz width.
WSJT-X by default begins that 50Hz wide signal at 1500Hz. If we move the TX
marker to say 1200Hz the 50Hz wide RF signal will begin at at 1200Hz into the
sideband width, not 1500Hz.
WSJT-X in receive sees and displays all of the signals within the USB filter
width
We pick one signal that will have a particular position within the received
sideband, let's say 400Hz up from the lower bound and reply to it.
Without split our transmission would occupy RF between 400Hz and 450Hz into the
sideband. This is non-optimum for WSJT-X.
If we have split set then on our reply WSJT-X will change the rig VFO to put
our transmission back into the optimum location in the sideband for reception.
If we have fake it set then WSJT-X shifts the transmit audio so that the
resultant RF is again in the optimum location within the sideband.
The location of RX and TX signals within the sideband are different, so the RF
frequencies involved are different.
That's the definition of split frequency operation, but it all occurs within a
single USB sideband width, so may not be immediately obvious.
The important thing to remember is that in USB mode the frequency shown on the
rig is not a single transmit frequency. Its the start of a band of
frequencies, the upper sideband produced by amplitude modulation.
Does that all make sense?
In the context of WSJT-X it might be better to use something like "Hardware
AHS" and "Software AHS" with AHS meaning audio harmonic suppression rather than
split?
Alan G0TLK, sent from my mobile device
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