Journal of the Society of Motion Picture and Television Engineers (1950-1954)

the magnetic coating. Insufficient force has its worst effect on the reproduction of high-frequency signals in the form of low and erratic output voltages. Excessive force will distort the unsupported edge of the film carrying the magnetic track to such an extent that the complete width of the head will not contact the track, and low output voltages will result. If excessive force is allowed to remain on the head and the position of the head is not changed, the head will finally wear to fit the curve of the film and produce the proper output voltages. But such a situation will decrease the life of the head many times. The proper force is one which is the minimum to overcome the pivot friction, and which will yet produce the proper output voltages over the required frequency range with a minimum of amplitude modulation and maximum head life; The best contour for the head is the maximum radius allowable that will be less than the radius of the sound drum and will also permit lateral adjustments to bring the gap of the head in contact with the film. This provides the maximum film support by the pole pieces as the head wears and also insures maximum head life. There is a direct relationship between the radius of the contour of the head and the smoothness of the low-frequency response of the head. The larger the radius, the lower the frequency of the bump in the frequency response due to the geometry of the head and the recorded wavelength. Since this bump is relatively high in magnitude and covers about half an octave in the response, it is difficult to compensate for in reproducing. Therefore, the most direct and effective way to produce a smooth low-frequency response is to design the head so that the bump will be below the lowest frequency required. The surface condition of the head is important to both the wear and the electrical performance. The finer the finish on the pole pieces, the less wear will result during the first portion of the hea< life. Of course, the cost of finishinj a head for the least possible wear \voulc be prohibitive, and a compromise ha | to be reached. The compromise finis! must be one which will produce a well defined nonmagnetic gap, so that th« head will perform as soon as it is assem bled into the complete projector, anc not waste production time by "\vearin} in" the head. The two items which contribute ti 1 wear on magnetic heads, the effects o which cannot be minimized by hea< i design, are the surface condition of th« magnetic coating and the film speed All the present-day magnetic tracks oij films are very abrasive as compared t( magnetic tapes. If magnetic tracks wit! j ' a surface equal to that of present-da}! tapes could be used, other factor j remaining the same, it is reasonable t( j assume that heads would last at leas i four times longer. There is one grea I factor in favor of the tape as a magnetic medium as compared with a trad! on film and that is the flexibility J of its base stock. In tape machines thd tape conforms to the shape of the heac; whereas in a projector the head has tcj be forced against the stiff film to get thcj contact required. This abrasive condition of magnetic tracks on film is a prob. lem the film vendors and service agencies must remedy. Slower film speeds would undoubtedly increase head life but the frequency range would be decreased and poor picture quality would result. The actual testing procedure used in evaluating these heads is straightfor j ward. A standard projector from the. production line was used, with the< amplifier separated from the rest of the projector so as to eliminate any chance j of the hum pickup affecting the readings, j Loops of brand new film, 1 0 ft in length, '• were used to simulate extreme operating ! conditions. All the film used was ex-] posed and processed, so that it was suffer ( than clear test film. A new loop was 502 April 1953 Journal of the SMPTE VoL 60