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What sidetone frequency range (or VFO/BFO receiver offset) will allow one to copy CW when trying to learn to copy QRQ? (high-speed Morse code, 40 WPM and higher)

It seems like there will be very few audio waveform cycles in short QRQ dots when using a very low tone frequency at a very high WPM, so why might some CW operators say they prefer that?

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  • $\begingroup$ W8JI told me that the lower the tone, the less fatiguing (and I find the same thing to be true). And he used to be capable of 70 WPM (in his youth). How low can you adjust yours? On my IC-765, I have the CW pitch set almost as low as it can go; however, I can't go much above ~25 WPM. $\endgroup$
    – Mike Waters
    Commented Dec 15, 2018 at 1:56
  • $\begingroup$ I can't argue with "It seems like there will be very few audio waveform cycles in short QRQ dots when using a very low tone frequency at a very high WPM". Yes, but it should be enough to copy. Have you or anyone else tried this? $\endgroup$
    – Mike Waters
    Commented Dec 15, 2018 at 19:02
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    $\begingroup$ Does the on-off modulation of the audio sidetone by the CW keying produce additional tones that bear on this question? $\endgroup$
    – Brian K1LI
    Commented Mar 8, 2019 at 23:45

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For folks who may be unaware, sidetone is generated locally and except in the case of modulated CW, it has no effect on the on-the-air-signal.

This question is hard to answer definitively, in fact, it's impossible.

The research I did had answers all over the map from 400 Hz to 800 Hz with waveforms from sawtooth (short rise/fall times) to a more sinusoidal form depending on the taste and/or experience of the operator.

There are two basic camps, one that doesn't change the ~700-800 Hz frequency, ever.

The other camp changes frequency for a variety of reasons. Commonly these operators lower the pitch for weak signal work or Top Band work claiming it's easier to pull out a signal at lower pitches.

Other folks doing conversational QRQ change the tone, say 50Hz or so, from time to time just to keep fresh while copying.

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  • $\begingroup$ The OP has edited the question. Perhaps it's not so impossible to answer now. $\endgroup$
    – Mike Waters
    Commented Dec 15, 2018 at 1:44
  • $\begingroup$ "...operators lower the pitch for weak signal work or Top Band work claiming it's easier to pull out a signal at lower pitches." That has been the experience of many very proficient hams that I have known over the years. Myself included. $\endgroup$
    – Mike Waters
    Commented Dec 15, 2018 at 19:05
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It seems like there will be very few audio waveform cycles in short QRQ dots when using a very low tone frequency at a very high WPM

That's true; but the number of cycles are still enough to copy.

  • The lower the tone, the less fatiguing when operating CW for hours.
  • The lower the tone, the better we are able to separate and copy stations that are very close together.

A high-speed CW op who could copy 70 WPM (W8JI) told me that. I worked beside him during the 1980 ARRL 160 Contest, and can testify to the wisdom of that.

On my IC-765, I have the CW pitch set almost as low as it can go. Others have theirs set as high as it will go. To each his own. However, how low the sidetone can go depends on the operator.

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The Nyquist rate tells us in a frequency range of B cycles per second, a maximum of 2B code elements per second can be resolved.

The length of a dit is the fundamental unit of time in Morse code. "PARIS" is commonly used word for determining speed, and it's 50 dits long. So at a speed of 70 words per minute, that's 3500 dits per minute, or 58.33 dits per second. The Nyquist rate requires a frequency bandwidth of at least 29.17 Hz to encode something at that rate. Let's call it 30 Hz for a round number.

Given the lower limit of human hearing is 20 Hz, that would imply the lowest possible range is 20 to 50 Hz. That would imply a sidetone frequency in the middle: 35 Hz, with sidebands arising from the amplitude modulation of the sidetone on and off occupying the 15 Hz above and below that.

So for 70 WPM morse code, the theoretically lowest sidetone frequency which could be audible is 35 Hz. It could be as low as 15 Hz, but that would simply be below the audible range of humans.

70 WPM is pretty fast already, and the biggest limiting factor isn't the speed of the code but rather the limits of human hearing. So to a first order approximation, the answer to your question, "How low a sidetone frequency will allow QRQ CW to be copied?" is "as low as you can hear." Hearing loss, genetics, and the frequency response of the speakers or headphones in use will be more limiting factors than the speed of the code.

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    $\begingroup$ A Nyquist factor of exactly 2 is only valid for infinite length signals. Also human tone perception may require a certain minimum number of periods, much more than 1 (according to audiology and/or psychoacoustics textbooks). You can hear short duration "pops", but not as tones or beeps. $\endgroup$
    – hotpaw2
    Commented Mar 9, 2019 at 15:20
  • $\begingroup$ @hotpaw2 By the time the sidetone frequency is high enough it's audible, there will be much more than one cycle per dit. If you doubt such transitions are audible, try listening to some PSK62, which is about the same speed. My speakers can't reproduce a frequency low enough that I can't hear the phase reversals. $\endgroup$
    – Phil Frost - W8II
    Commented Mar 9, 2019 at 21:45

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