For radio reception on medium and longwave and also in the telephony mode of early military radios, amplitude modulation is used. These can be demodulated with simple receivers, starting from detector sets to complex superhet receivers, without the need for a beat frequency oscillator. The modern designation for this type of modulation is A3E.
The audio frequency from the microphone modulates the width of the envelope of the transmitted signal; the degree of modulation is calculated from (a-b)/(a+b) * 100%. A simple crystal detector or a tube or semiconductor diode is sufficient for demodulation.
The energy as well as the bandwidth requirements of amplitude modulation is quite large: a large part of the transmission energy is in the carrier signal, in which, however, no information is transmitted. Due to the fact that identical sound information is transmitted in both sidebands, the signal needs much bandwidth in the frequency spectrum, it is wider than twice the frequency of the highest transmitted audio frequency. Thats why a signal with high audio quality requires more bandwidth or channel spacing, a narrowband signal becomes dull and difficult to understand.
Ampitude-modulated signals are susceptible to propagation interference (fading), noise and crackling due to static discharges and ignition sparks from generators, motors, etc.