International projectionist (Jan 1959-Dec 1960)

power source. Torque motors are used to drive the turntables directly. These motors are connected to rotate in opposite directions when power is applied — the supply motor opposing and the takeup motor supporting the normal direction of tape motion. In the record and reproduce modes these motors act simply to maintain proper tape tension and have no influence on tape motion, which is controlled entirely by the drive system. During this operation the supply motor imparts tension by opposing tape motion, while the takeup motor attempts to turn slightly faster than necessary to wind in the tape from the drive system. In the fast winding modes of tape 1 ravel, the reel motors do control the tape motion. Here one motor is operated under full power and the other with reduced power; the greater torque of the motor under full power overcomes the lesser opposing torque and tape is simply pulled from one reel to the other, again under correct tension. The Drive System The drive system utilizes a synchronous motor coupled either directly or through a pulley arrangement to the capstan. The circumference of the capstan and its rotational velocity determines the speed of the tape in the record and reproduce modes. It is very important that this speed be constant, because any variation will be reflected as a change in tone of a musical note. For instance, to consider an exaggerated example, if we were reproducing a sustained 1000-cycle tone at a tape speed of 7^2 inches per second and that speed suddenly dropped to 6 inches per second our tone would be reduced to TOO cycles; most musical directors would frown upon this effect. (Of course, this could occur either while recording or reproducing.) Tone variations of this type are called "'flutter" when they occur at a relatively high rate, and "wow" dien they occur at a relatively low rate. Therefore, we must have a precisely machined capstan that rotates at an unvarying speed. Head-to-Tape Contact Vital While tape speed is a function only of the capstan, tape motion in record and reproduce is instigated when a capstan idler (sometimes called a pressure roller) clamps the tape between itself and the capstan, thus providing a surface against which the capstan can drive the tape. The capstan idler is normally coupled to a solenoid, which in turn is actuated by the play or record switches. This arrangement allows a "fast start" condition in which the capstan motor is operating whenever power is applied to the equipment, and tape can be quickly brought to full speed, whenever the pla> or record switch is pressed. Good head-to-tape contact and proper placement of the tape on the heads is extremely important. An inherent characteristic of magnetic tape recording is that the effectiveness of the magnetized tape in producing a high-frequency signal in an ideal playback head varies directly with the distance of the magnetized particles from the head. Thus, any loss in good head-to-tape contact will adversely affect the high frequency response. If the tape does not track correctly across the heads both frequency response and level will be affected. Two precisely machined and positioned tape guides will thus bridge the head assembly. In professional quality equipment the positioning of the guides will ensure good head-to-tape contact and the machined grooves in the guides will accurately position the tape. System's Electronic Circuits There are three main electronic circuits which must be provided — a record amplifier, a bias and erase oscillator, and a reproduce preamplifier. These will normally be quite conventional audio circuits except for certain minor modifications made necessary by the special application. The function of the record amplifier is to present to the record head a signal of proper amplitude for the recording process. The record head is essentially an inductance whose a-c resistance (impedance) will vary directly with frequency. The magnetizing force idirectly related to the amount of currenl which flows in the head coil, so high frequencies would suffer if the rising impedance of the head coil at the shorter wavelengths were allowed to decrease the current flow appreciably. Therefore. the output circuit of the record amplifier will present a relatively high impedance in respect to the head coil, which will now have less effect on the complete circuit; a virtually constant current condition is thus maintained regardless of tlie frequency involved. In order to further ensure proper recording of high frequencies the record amplifier might also contain a preemphasis circuit which essentially provides more amplification as frequency rises. The magnetization curve of recording tape is non-linear, and considerable distortion would occur during reproduction if steps were not taken to provide correction. A high-frequency, constant amplitude "bias" voltage is mixed with the audio signal so that operation is obtained on the linear portion of the magnetization curve. (This is basically the same action we take when we employ a d-c bias with vacuum tubes to obtain (Continued on page 26) Achieving flat overall response necessitates equalization of the reproduce amplifier. This diagram illustrates various record and reproduce characteristics which result in the desired output. <§ ) © z at BC RECORD HEAD CHARACTERISTIC V REPRODUCE AMPLIFIER \^ CHARACTERISTIC < / < O > ^System Nois e\ I Q o u a: r System Noise i I 1 | SIGNAL 1 kc 10 kc SIGNAL 1 kc 10 kc v Reproduce Head * \. Ch a racteristic -^ \ / REPRODUCE HEAD < I— o > CHARACTERISTIC y f System Noise-7 O < O > \ / RESULTING Vv' OUTPUT 6 db per / \ octave / \ rise / \ / Reproduce — \ / Amplifier N^ y Characteristic ^ / ' 1 / System No ise | SIGNAL 1 kc 10 kc SIGNAL 1 kc 15 kc ® ® IN INTERNATIONAL PROJECTIONIST • APRIL 1959