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The most efficient means of using Morse code is by on/off keying of the carrier. Receivers using envelope detectors produce a tone for the time that the carrier is present by injecting a locally-generated r-f signal (a “BFO”*) into the passband of the i-f amplifier. The pitch of the audio tone produced at the receiver output is the difference in frequency between the down-converted carrier and the local signal present in the i-f strip.
Another means is to amplitude modulate the transmitted carrier with an audio tone, where the tone is comprised of the Morse code characters. This is less efficient, as only the a-m sidebands carry the transmitted intelligence, rather than the full power of the carrier.
*Beat frequency oscillator