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

Dennis OConnor k8do at diamondcs.net
Fri Nov 30 08:50:53 EST 2001

When I am out in the shack, which is also my work shop, I always flip on 
one of the rigs and leave that tuned on someone's qso while I am doing 
other things... As I was reading your message I suddenly realized that even 
though I have a selection of DSP rigs on the operating bench, it is always 
the Onmi VI+ that gets turned on...  I wonder if there is some subtle 
distortion level with the DSP rigs that makes me subconciously reach for 
the non DSP rig to listen to for the hours that I am working on the 
tractors, etc... (Yes, the Omni has a DSP filter that can be switched on, 
but I don't use it because the audio annoys me)


> 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,
> either.
> 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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