. . . "Consequently, as shown in FIG. 7b, the duty ratio drops below 50% to cause V.sub.Moto decline, and control for delaying the rotation of the capstan motor 6 is thereby effected, so that a constant rotational speed is automatically maintained.The repeated frequency of saw-tooth waves generated by the sam-tooth oscillator 102 is determined by requirements of such as the time constant of the circuit c" .