[Icom] which is better? IF or AF dsp filters

[Dana Hoggatt]

On Jan 21,  9:52am, "Hart Engel" wrote:
} Subject: [Icom] which is better? IF or AF dsp filters
} 
} As you know some rigs come with the dsp filters in the
} IF stage and some come in the AF stage.  Which is better?
} please e-mail me off line. va3kis@hotmail.com


All other things being equal, IF dsp is usually better
than AF dsp.  Why?  For one thing, most AF dsp systems do
their work outside the AGC loop, thus making them prone to
"pumping".

"Pumping" occurrs when the dsp is rejecting a strong
signal, like when the auto notch filter tries to suppress
someone tuning up near or on top of your contact.  In an
AF based system, the AGC can see that stronger signal
and responds to it, causing abrupt (and severe) changes
in your audio volume.  An IF based system bases its AGC
response on the final signal, so isn't fooled or "pumped"
up and down by the unwanted signal.

However, things aren't always equal.  IF dsp is built into
a rig at design time and usually can not be upgraded or
changed.  AF dsp systems can be external to the rig and can
be easily replaced with newer and more powerful algorithms.

The IC-746 is a good example of these effects and
tradeoffs.  Having IF dsp, the 746's AGC loop responds
very well to dsp processing.  Little, if any, AGC pumping
occurrs.  However, some claim that the latest Timewave
device outperforms the 746 dsp in noise reduction.  Sadly,
I don;t know of any quantitative NR comparisons between
differing dsp systems.

Remember, your ears may vary.  The one true test is the
one you make by actually listening to the results.  I
always strongly reccomend that you try some of these
things out for yourself so that you'll know what's
important to you.

Dana, KB9SSS