For the reception of unmodulated Morse code signals (in the early days of wireless telegraphy this was called „unmodulated telegraphy“ or still today „continuous wave“), an additional oscillator signal must be mixed with the unmodulated signals so they can be heard in the headphones.
This is done by the BFO (Beat Frequency Oscillator) or heterodyne oscillator.
The signal of the BFO is mixed with the intermediate frequency signal before demodulation. Depending on the frequency difference, the Morse code can be heard with a different pitch. With a difference of 1 kHz, the Morse code signals have a pitch of 1000 Hz; the pitch can be varied with the BFO control.
At a frequency difference of almost 0, the frequency of the Morse code signal becomes so low that it becomes almost inaudible. This setting is not used for receiving Morse code transmissions, but the BFO is used as an auxiliary carrier for demodulating single sideband or SSB signals. For this purpose, tuning to zero beat is necessary to make speech intelligible and music enjoyable.