[Elecraft] K3 SSB Contour Filter (reduce total bandwidth)

[David Cutter]

Didn't they use something like this in the space programs?

David
G3UNA


> K3 SSB Contour Filter
> 
> Recently I was reminiscing about "Narrow Band Voice Modulation" which 
> was developed in the late 1970s by Tom Lott VE2AGF and R.W.Harris, 
> WB6CZX. An important part of this system relies on the fact that between 
> the low frequency vowel sounds and the high frequency consonants is an 
> almost void of several hundred Hertz. Lloyd Butler VK5BR developed a 
> hardware filter but I was looking for a computer filter program to 
> investigate and experiment with this.
> 
> After hunting around on the internet, I suddenly realized that my K3 
> with KRX3 was all I needed. I locked the VFOs and set the filter on the 
> main receiver to 350-750 Hz and the sub-receiver to 1400-2400 Hz. 
> Listening to both receivers is equivalent to a a single receiver with a 
> bandwidth of 2050 Hz and a notch from 750 Hz to 1400 Hz. This gives a 
> total bandwidth of 1400 Hz with acceptable audio quality. On some SSB 
> signals it is possible to reduce the total bandwidth down to 900 Hz. 
> Reduced bandwidth means better S/N ratio.
> 
> I had one signal (USB) with QRM from 1 kHz below. Almost no copy with 
> normal tight filter but 100% copy with 700 Hz to 1500 Hz notched out. 
> So, it looks promising but needs some more testing.
> 
> Setting filters on CW is relatively simple. On SSB there are so many 
> variables as you have to consider the transmitted signal, QRM and also 
> the preferences of the receive operator. It is almost essential to have 
> preset settings that can be recalled to suit the transmitted voice 
> characteristics and various levels of noise and QRM.
> 
> I am not an audio engineer or a programmer but it should be possible to 
> develop a firmware (SSB contour) filter with a wide adjustable notch. I 
> think it would be a worthwhile addition to the K3.
> 
> Give the receiver/sub-receiver method a try and see if it helps you 
> combat noise and QRM while contesting and chasing DX. If you like it, we 
> will have to talk nicely to Lyle!
> 
> 73
> Tony Fegan VE3QF
> K3 #137
> 
>