Experiments with Frequency-Shifting CW (FSCW)

(C) 2015, G. Forrest Cook, WB0RIO


This article describes an experiment involving the modification of standard Morse code in order to improve reception in high-noise environments. In normal Morse code (CW), the dots and dashes are both sent at the same frequency so that they come out of the receiver at one pitch. With Frequency-Shifting CW (FSCW) the dots are sent at one frequency and the dashes are sent at a slightly different frequency. FSCW should not be confused with FSK-CW, where the dots and dashes are at one frequency and the spaces between are at another frequency. FSK-CW has a 100% transmit duty cycle.

During reception of regular Morse code in the presence of significant noise, it is possible to confuse dots and dashes due to the noise blanking out the signal at critical points. A burst of noise in the middle of a dash can sound like two dits. By varying the pitch of dots and dashes, the person receiving the code has a better chance at differentiating the two code elements. This comes at the price of slightly higher signal bandwidth. If the frequency shift is kept small, the shifting signal should pass through most narrow receiving CW filters with no trouble.

Some standards and definitions have been set for this experiment. The transmitted dot frequency is higher than the dash frequency. The received tones can be reversed on the receiving end depending on whether one listens to the upper sideband (USB) or lower sideband (LSB).

The frequency spacing is defined as the dot-dash offset in hertz, FSCW-25 specifies a dot signal that is 25 hertz higher than the dash signal. After trying a number of QSOs, the 25 Hz frequency shift seems "about right". Note that the perceived musical pitch difference between the dots and dashes will decrease as the two tones are shifted higher in frequency. This effect can be easily observed by tuning up and down while listening to an RTTY signal.

Here are two mp3 audio files with example FSCW signals:

It should be noted that two-tone Morse code was used by Ludwig Koch for teaching Morse code, and that it sped up the learning process considerably. See page 172 of The Art And Skill of Radio-Telegraphy by N0HFF for details. (Thanks to KB3IPY) This mode was apparently used for teaching Morse code, but it was not transmitted over the air.

Another related varient of Morse code is DFCW which is sometimes used for QRSS (extremely slow speed) transmission. DFCW uses different tones for the dots and dashes but the length of the dots and dashes are the same. This is similar to a two-tone spark gap system that was patented in 1904 by Joseph Murgas.

The Hardware

In this experiment, the two-tone shaped sine wave Morse code audio signal was produced by connecting a WB4VVF Accu-Keyer circuit board to my Smooth Tone Clickless CW Sidetone Generator circuit. The Accu-Keyer's keying signal drives the sidetone circuit's keying input. The Accu-Keyer produces dash/not-dash signals on U3A pins 5 and 6 which are perfect for shifting the sidetone frequency during dashes. The not-dash signal is routed to the sidetone circuit's LM324 pin 13 through a 1M fixed resistor in series with a 1M variable resistor, this shifts the sidetown frequency down during dashes.

The sine wave audio signal is used to modulate an SSB transmitter for generating the transmitted CW signal, this is similar to transmitting a PSK31 signal coming from a PC soundcard. An active signal is also required for keying the transmitter, this is provided by my QSK Timing Generator circuit. The active signal keys a 555 timer which sends a 1 Khz keying tone to my TS-430s soundcard interface. In effect, the keyer now simulates the two output tones produced by a computer when running Fldigi. As with PSK31, it is important to keep the audio level below the ALC threshold to prevent wide-band splatter.

This is defnitely a circuit-bending hack, a better solution might be to modify a program such as Fldigi to produce the appropriate tones in software. Another approach that has been tried is to take the FSCW signal from the Accu-keyer and use that to key a varactor on the master oscillator of a DDS-based VFO during dashes, causing a small frequency shift.

I have modified my CW beacon controller project so that it now produces the FSCW pitch-shifting information along with the standard Morse code signal. The beacon controller can either be implemented as an EPROM-based device or as a self-contained Arduino microprocessor project.

Putting it On The Air

The FSCW signal sounds close enough to regular Morse code that most hams should instantly be able to copy it. One might expect to get some funny RST reports back, especially in the tone digit. A report of 5999 might not be too surprising if the remote operator were to rate the quality of the two tones.

Your author has modified the VFO for his 40-30 CW Transmitter and added a switch so that FSCW mode can be easily turned on and off. Numerous QSOs have been made with this setup. In the first contact, the other operator said that he "got it" after a few rounds and did not have any trouble copying the FSCW signal. In another contact, an extended rag-chew was conducted on a the 40 meter band during a summer night with extremely noisy atmospheric conditions, the operator on the other end reported good copy.

Some operators have had a lot of difficulty copying the two-tone signal and one even came back with a report of QRM, probably from hearing two tones instead of one. Others were quite interested in the mode and encouraged me to use it more.

I have been operating an experimental FSCW beacon transmitter on 28.2715 Mhz. An Arduino processor is used to control the beacon.

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