[Icom] External Audio Filter

[George, W5YR]

For several years I have used the JPS NIR-12 DSP Audio Filter with my
IC-765. While no longer available new, the NIR-12 is exceptional in its
capabilities and a good used one is a treasure. since the 765 employs
conventional crystal filters and no DSP noise reduction, the NIR-12 is a
worthwhile accessory.

The NIR-12 is exceptional in using two 40-MHz DSP processors and in
offering a wide ranger of filter settings, a multiple notch filter, and two
forms of adjustable noise reduction. The unique aspect is that all these
functions can be used concurrently.

If you choose to use an external filter, I highly recommend that you use a
DSP filter in order to obtain generally better filters than an analog
filter can provide and in order to obtain, in the case of most current
units, DSP noise reduction.

I believe that the most nearly comparable currently available new unit
approaching the NIR-12 is the filter made by Timewave. I do not recall the
exact name but it is something like the DSP599X. Such a filter will augment
the 706 filters and DSP that you already have and may be of some help. 

The problem with add-on filters is that by the time the radio output signal
reaches the outboard unit the AGC within the radio has already done its
work to allow stronger signals within the internal filter passband to set
the operating conditions for the receiver. Thus, the "weaker" signals that
you are having problems with have been made even weaker by the presence of
stronger signals within the internal filter passband. 

The external filter can do nothing about this situation except narrow the
passband around your now weakened signal in an effort to improve S/N ratio
and attempt to remove adjacent QRM. This may or may not produce the results
you desire.

The effect of external DSP upon a weak signal already subjected to internal
DSP noise reduction is problematical as to the outcome, but the effect of
cascaded filters is predictable and generally productive.

72/73/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771
Icom IC-756PRO #02121  Kachina #91900556  IC-765 #02437

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Gary Smith wrote:
> 
> I have an ICOM 706MKIIG which I have had on the air
> for a couple of months now.  I have worked a
> reasonable amount of DX with it, including a couple
> rare ones. I have noticed that sometimes I have
> difficulty digging stations out of the noise...there
> are times when the DSP and narrow SSB filter just
> don't help.  Are any of you using external audio
> filtering with this radio?  If so, what are you using
> and are you getting good results?