Why do CW signals get converted to pure sine wave tones at 750 Hz?

Question

I've been reading about the history of telegraphy, and I knew that back then radio signals were produced in massive strength by using powerful dynamos and magnetos, with a telegraph key to interrupt this current, as described in Tom Swift and his Wireless Message, by Victor Appleton. I first read that book in eighth grade, and since then my fascination with radio grew.

I know that lots of technological advancements have been made since, which doesn't require as much energy to produce radio signals, especially when there are repeaters to carry on the signal.

Now, I'm wondering why a continuous wave will cause the transceiver to emit a 750 Hz tone in accordance to the signal's interruption. Wouldn't it sound like white noise, or mains hum, whose alternating frequency is 60 Hz, or a harmonic above that, 120 Hz? Of course, with the latter example, one would have to actually be connected to a power source. I also wondered what the mayday call sounded like on the Titanic, and it sounded like this.

Answer 1

The tone you hear when receiving a CW signal is generated inside your receiver.

Originally, a motor-driven variable capacitor or interrupter was used to put an AC component into the detector circuit, which sounded as a tone in the earpiece when the detector conducted the carrier. Here's a 1917 example on the OneTubeRadio blog. Once active components were available, and electronic oscillator fulfilled this function.

Later, regenerative receivers could be adjusted to the edge of oscillation. Oscillation is bad if you're receiving an AM audio signal, but for CW, it allowed the set to enter oscillation when the carrier was detected, and stop when the carrier shut off, thus generating a tone in the earpiece; the pitch was controlled by adjusting the regeneration coupling.

Modern CW transceivers listen on a slightly offset frequency (typically between 400 and 1000 Hz difference, but usually operator adjustable) from their transmit setting; when the transmitter is tuned to the same frequency as the station you're receiving, that offset causes the internal oscillator to "beat" against the (nearly identical) received frequency; that beat produces the tone you hear, and the oscillation that produces it also produces the "side tone" that you hear when keying the transmitter.

Answer 2

"CW" means continuous wave, that is a signal of constant amplitude and frequency which is switched on and off. The transmission itself has nothing to do with 60 Hz, 120 Hz, or 750 Hz. The transmission is simply a sine wave at whatever frequency the transmitter is tuned to.

The sidetone that the transmitting operator hears is generated by the radio, and could be at any audible frequency. Most modern radios allow adjusting the sidetone frequency though a menu option.

The receiver produces an audible tone by mixing the received signal with a locally generated carrier. According to the basic function of mixers, the resulting audible tone is at the difference between the two frequencies. So if the receiver's local oscillator is set to 10,000,000 Hz and the signal is at 10,000,500 Hz, the result is an audible 500 Hz tone. The receiver usually sets its oscillator to some fixed offset from the frequency shown on the dial, again often configurable in modern radios to suit the operator's preference. A slight mistuning of the receiver results in the tone being heard at a different frequency, as long as the frequency is still close enough to fall within the filter passband.