That constancy of record speed should be sought in sound recording and reproduction has been recognized throughout the history of the art, but standards of performance have usually fallen far short. Especially have the speed variations due to sprocket-tooth action failed to receive from many engineers in the sound-picture industry the consideration which their harmful effects warrant. This is due to imperfect understanding of inherent limitations and to the fact that the impairment of sound quality is of a kind which may easily be ascribed to other causes. An analysis is made of the loss of true tones and production of spurious tones, as a result of speed modulation at sprocket-hole frequency. A recorder in which not only are disturbances of this kind eliminated, but all speed variations reduced to a minimum, has been available to the film recording industry for several years. Film phonographs, re-recording machines, and projector sound heads of which this is true are now also available, thus making possible an over-all performance which sets a new standard in the sound reproducing art.