Coherent CW

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From: Michael Bendio (unix dev) (mb@titan.wordperfect.com)
Date: Wed May 25 1994 - 15:00:45 EDT


The February '94 issue of QEX has an interesting article titled 'A DSP
Version of Coherent CW', by Bill de Carle, VE2IQ. I'll include the first
two paragraphs and summarize the rest.

        "Coherent CW (CCW) is an old technique for digging weak CW signals
out of the noise. The system synchronizes the receiver to the sender's
keying. Thereafter, due to the rules of Morse code (each "dah" equals
exactly three "dit-times" and spaces between marking elements are always an
integral multiple of one dit time) the receiver knows precisely when the
carrier is allowed to turn on or off. This is useful information that
would otherwise have to be transmitted--at the expense of additional power
and/or bandwidth. Of course, the sender must have an absolutely
rock-steady rhythm for the scheme to work. For this reason, CCW stations
employ special keyers or keyboards which can generate perfectly timed code.
Traditionally, CCW uses a keying rate of 12 WPM (100 milliseconds per
dit) and a received audio tone of 800 Hz.
        "The receiver divides its time into 100-ms windows, or frames.
Once synchronized, there is complete certainty that the incoming tone is
either present or absent for the entire 100-ms window--the rules of Morse
code prevent it from switching somewhere in the middle. This knowledge
makes it possible to use a matched filter (sometimes called an
integrate-and-dump filter) to reduce the receive bandwidth down to about 9
Hz (main lobe), eliminating most of the noise while letting the CW tone
pass through unscathed. All coherent 800-Hz energy received during the
window is integrated (accumulated), and only at the very end of each
window does the receiver make its decision--and the question is: 'during
the previous 100 milliseconds, was the key at the transmitter up or down?'"

Bill then goes on to describe how a hardware interface (the same one he
described in "A Receiver Spectral Display using DSP," in Jan 1992 QST)
used with a PC and a program he's written can implement this
integrate-and-dump filter to give a 20 dB advantage in S/N over regular CW
as 12 WPM. His hardware/program can track slight drift in the radio and a
slight deviation from 12 WPM. His 'Coherent CW' software package can be
ordered for $20 from:
        Bill de Carle
        29 Sommet Vert
        St. Adolphe d'Howard, QC
        J0T 2B0 Canada
The bare circuit board for his interface is $24, and an assembled and
tested version is $95. Prices are in US dollars. Add $5 for airmail
shipment.

He includes an address for further info on CCW:
        CCW Interest Group
        Peter LUmb, G3IRM
        2 Briarwood Ave
        Bury St Edmunds
        Suffolk IP33 3QF
        England

I have no financial interest in any of the above (and indeed, I don't
even know the author).

Michael Bendio WT7J mb@titan.wordperfect.com 801 222-5367
Opinions are my own and aren't necessarily shared by Wordperfect Corporation


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