Distortion Produced by Telephone Transmitters

It was shown by equation 4 that distortion resulted from normal transmitter action. For transmitters of given resistance R_t in circuits of resistance R_c, the least sensitive transmitters will produce the least distortion since they have the smallest total resistance change r. It follows that a given transmitter in different circuits will produce the least distortion in high-resistance circuits. In such circuits a given transmitter will, of course, produce less useful output.

Much distortion is caused by the diaphragm which is mechanically resonant at certain frequencies. At these frequencies the diaphragm motion and the electric output will be greater than for sounds of the same intensity but different frequency. In modern transmitters the diaphragms are ribbed or otherwise stiffened so that flexure is reduced and piston motion is approximated. The resonant cavities caused by the air spaces around the diaphragm and by the mouthpiece also cause distortion unless these cavities are designed properly.

A small amount of distortion is caused by the unequal travel of the diaphragm. When moving inward it meets an increasing opposition as it tends to compress the carbon granules, but this opposition does not occur when the diaphragm moves outward. This is an important factor leading to the use of the double-button carbon-granule microphone to be considered later.