Copy CW signals more easily with two tones

A couple of days ago, someone posted the article, “A novel way to hear weak CW signals” (CQ, January 2018) to the Long Island CW Club mailing list.

In the article, the author, Pete,, N8PR (SK) writes that you should set up your receiver to produce two tones 65 to 80 Hz apart to make copying weak CW signals more easily. The theory behind this is that the dissonance between the two tones makes copying a CW signal more copyable than just a single tone. I like this  idea. I played around a little bit last night with this technique, and it did indeed seem to work better than using just a single tone.

The problem, as I see it, is that even if this works, it only works if your radio has two receivers. I can do this with my Flex 6400 because it has two receivers, but most radios, such as the IC-7300 have only a single receiver, making this impossible. I’m also not sure this is the best use of the second receiver should your transceiver have one. For example, if you’re working a pileup, then you may not want to use the second receiver to generate the second tone, but rather use it to listen to the pileup to more easily find the frequency of the station that the DX station just worked.

So, what’s the solution? It seems to me that one should be able to devise a DSP algorithm to take a tone and then generate the second tone from the first, and then mix the two to get the final output. Ideally, this would be done in the receiver, but failing that, it seems to me that one could do this externally.

I’m even thinking that one could use GNURadio to do this. At the very least, you could use GNURadio to develop a prototype. I’ll have to do some research and see what kind of modules that GNURadio has that can accomplish this.

I just searched for “dsp audio frequency shift algorithm” and came up with the post, “Shift frequency of an audio signal” on the website DSPRelated.Com. The replies list several ways as to how this might be accomplished. With this algorithm, you could shift the frequency of an incoming signal, then mix it with the original signal to get the two tones.

I also suppose that this could be done in analog, just as audio peak filtering was done at first. The input signal could be mixed with a beat signal to produce the second tone, which would then be re-mixed with the original tone. I’m not really an analog guy, but it might be fun to try and design a circuit that would do that.

The nice thing about an external solution—either digital or analog—is that it wouldn’t require continual retuning. Using two receivers, you’d have to continually be tuning both to give you the proper two tones. In any event, I”m going to continue to play around with this technique some more and see what I can do with it.