[CQ-Contest] 756Pro: definition of 'ratty'

jljarvis n2ea at arrl.net
Thu Nov 29 23:01:43 EST 2001

Continuing with K1KP's observations, two things were 
noticed in reviewing the 756pro, shortly after I got mine.
W3EEE, who uses DSP for professional audio consoles, observed
a 'zipper' problem, relating to ADC aliasing on the rising slopes
of cw signals, which caused him to return his radio.

That didn't bother me so much...and the key clicks which are
passed by the sharp digital filters don't bother me terribly,

HOWEVER....there is an AGC artifact which drive me nuts.  With
AGC on medium, default setting.  A strong signal within the 15KHz
wide upper IF pipe, but not in the passband of the lower IF filter,
will modulate the audio baseline.  If this signal is cw, it 
will sound like low frequency thumps.  If it's digital or ssb, it
sounds like a 'shadow' or 'ghost' of the original signal, passed
through a lowpass filter.  a musician would say it was 3 or 4 octaves
lower in frequency.  It's at a low level.  

Two aspects of this are troublesome:  First, if you tune onto the
offending signal...once you've identified the 'shadow'....you can
hear it UNDER the signal itself.   Secondly, fill the 15KHz IF Pipe
with signals, as in a contest, and you've got a sea of rumbles, they
will add root-sum-square, and the noise will build.

I believe that THIS is the mechanism which caused the TOI tests
to be degraded, in Radcom and DLARCs review of the radio, as well
as private tests done by a consulting engineer on my radio.  

Icom has denied that this mechanism exists.  From my perspective,
this casts their technical acuity in doubt.  I would be very, very
cautious about the PRO-II.  

Apart from that, Mrs. Lincoln.....I like the radio.

Jim, N2EA

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