Topband: DSP Units on Topband, Info Wanted

[Tom Rauch]

Hi Gale,

> make.  I'm wondering what improvement people have found by using some of
> the various outboard audio DSP units.  I'm specifically interested in any
> improvements noted while operating on topband.  Some of these units are
> relatively expensive so the question is - are they worth the money?  If
> so, which ones? Or would my money be better spent on one of the noise
> canceling units? BTW, I'm using a TS-850 with IRI filters. All info
> appreciated!

Since I worked on some DSP designs, I've had most types here to 
"gut and inspect". There is very little difference in the way the good 
ones work, and they all have very limited advantage with 160 QRN .

They will work OK, within limits, on ignition noise or other pulse 
type noise...if the receiver is on wide bandwidth.
 
The reason for this is receiver selectivity, no matter HOW it is 
obtained, slows the rise an fall times down and makes the noise 
"ring". The DSP simply can't tell the difference between a narrowed 
bandwidth filtered noise that is ringing, and desired signals. There 
is no way to eliminate this, except to run very wide IF filters. 

I do use a DSP unit on very rare occasions, but with the receiver 
filters wide open and the receiver AGC turned off. I mainly use the 
DSP to pick through the crud pouring out and try to extract signals, 
and to provide some AGC for volume.

They have other uses, but this is how it works for me on 160 DXing.

On the other hand an RF noise canceler can allow you, under 
some conditions, to null noise from one direction or peak the signal 
from another. I use an original homebrew version of a noise 
canceller MFJ makes to combine antennas just for this function, 
and it generally gives me 3 to 6 dB improvement most days. The 
improvement can be as little as nothing, and as much as many 
many dB on others on rare occasions.



73, Tom W8JI
w8ji@contesting.com


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