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

[Tony Fegan VE3QF]

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