Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

PUSH-PULL CIRCUIT 463method it is very difficult, however, to obtain accurate results, since insome cases the. amplitude of the vibrations is extremely small. Bysubstituting a photoelectric cell for the human eye it is possible toimprove the accuracy to a large extent.Such optical cutter calibratorshave been in use for years. Since the measurements are taken in air,it is assumed that the difference in the measurements is small whencompared to such made while the head isThe problem of being able toactually cutting a record.calibrate the recorder under actualcutting conditions still remained, however, and it was not until thepush-pull FM circuit was developed that a solution to the problem wasFig. 11—Component parts of a complete FM calibrator for recording heads.realized. A device was needed which could be attached to the recorderwithout requiring much space or adding any mass to the movingsystem, as well as one which would not couple electrically to the drivingcoil of the recording head, and of course one which would not interferewith the cutting action of the stylus.The push-pull FM system meets these requirements very nicely.However, it is practically essential to use the push-pull circuit, becausethe distortion would be too high if only one side of the stylus were usedas one of the plates of a condenser, owing to the low ratio of thecapacitor plate spacing and the large variation of the spacing. Asillustrated in Figure 9, two small plates, one on each side of the stylusshank, insulated from each other and from the recorder, are spaced afew thousandths of an inch from the stylus. Since neither mass norrigidity is added to the moving system, no change in its action can