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Modulation Explained

Rochester Democrat and Chronicle Newspaper

August 11, 1922


When we talk into the ordinary line telephone transmitter which is operated from a direct current source, the current which ordinarily flows through the transmitter circuit is varied to correspond to the voice-produced sound waves, or in other words the direct current is modulated by the voice waves so the the electrical apparatus at the other end of the line reproduces the sound waves as spoken into the transmitter. Were it not for the losses in the apparatus the sound waves would be exactly reproduced but in line telephony there is always a certain amount of distortion.


In radio telephony instead of employing direct current as a carrier for the voice-produced waves, we employ very high frequency alternating currents which radiates waves into the ether carrying the modulations corresponding to the voice waves to the distant receiving stations. Notwithstanding the face that the transmission of speech by radio-telephone requires more delicate and complicated apparatus than is employed on the ordinary line telephone, by proper adjustment of the instruments and circuits it is possible to secure clearer reproduction than can be obtained over wires.


The accompanying diagram shows the characteristic form of a voice wave and the effect produced on a constant amplitude continuous wave of radio frequency when the voice wave is super-imposed upon it by means of a proper modulating system.


Modulations of the output of a continuous wave transmitter of the usual vacuum tube type my be accomplished in different ways. One of the simplest and probably the lease efficient ways is by the insertion of a carbon microphone transmitter directly in the ground lead. This method has been employed in small power installations with some success especially in the old are type of telephone transmitters.


Posted February 22, 2018