Disc playback system with speed control of a belt drive

Claim 1

In a playback system wherein a prerecorded signal is recovered from a disc record by a pickup device when relative motion is established between said disc record and said pickup device by rotation of a turntable supporting said disc record, said playback system including braking means for controllably applying a braking force to said turntable, said braking means being responsive to the output of a speed detecting means for varying the effect of said braking force in accordance with deviations of the velocity of said relative motion from a desired operating velocity; a drive system comprising: a pulley; a motor, subject to substantially constant frequency energization independent of the output of said speed detecting means, for rotating said pulley; a belt of elastic material stretched to encircle respective cylindrical surfaces of said turntable and said pulley, the diameter of said cylindrical surface of said turntable exceeding the diameter of said cylindrical surface of said pulley, for transmitting rotational motion to said turntable in a manner establishing a given step-down ratio between the respective rotational rates of said pulley and turntable when said motor is subjected to said energization in the absence of operation of said braking means, said given ratio being chosen to establish said relative motion between said disc record and said pickup device at a given velocity exceeding said desired operating velocity; and the length of said belt, the distance between the axes of rotation of said pulley and said turntable, and the elasticity of said belt material being so related as to permit the ratio of rotational rates of said pulley and turntabe to vary over a range of ratios differing from said given step-down ratio, when said motor is subject to said energization in the presence of operation of said braking means, without slippage between said belt and said cylindrical surfaces.