Equipment of instrumental accuracy for recording and reproduction of electrical signals, using cinematographic film

Equipment has been made for the recording of electrical signals and their reproduction or play-back, providing an accuracy comparable with that of typical electrical instruments. The errors are about ?1% over most of the range, but increase at the maximum amplitude and frequency; the frequency range is 0?1 kc/s. The equipment is primarily intended for the recording of experiments of short duration or a non-repetitive nature and their subsequent reproduction for study or analysis; a particular feature is the facility for reproduction on a changed time scale, which may be faster or slower than real time. It is thought that the apparatus offers a significant addition to experimental technique, particularly when used in conjunction with an electronic analogue computer. Cinematographic methods are employed. A simple film-traversing mechanism is used, with a variable-speed drive. Appropriate optical units are attached to it for recording or reproduction. One type of recording unit gives variable-density tracks on the film; it uses variable-intensity discharge lamps the outputs of which are monitored by photocells, to provide negative feedback to the amplifier. Precision photographic processing methods have been evolved for this application. An alternative optical unit gives variable-width tracks, using modified Duddell oscillographs and being based on standard cinematographic practices, including processing. This unit was made to determine the relative suitability in use of the two techniques, and practical experience shows that it gives significantly greater accuracy, constancy of calibration and reliability in operation. Subsidiary equipment provides for recording on film, mechanical signals, data transcribed from other recording apparatus or numerical data applied manually; this is of value for certain types of analysis using the analogue computer. The importance of making the equipment suitable for use by non-specialist operators has been recognized during the design - period. On reproduction from the records, the light transmitted through each track is measured with a photocell, the outputs from the equipment being presented as voltages; the film provides two tracks carrying independent information of high accuracy, and a third track for timing marks or a reference frequency. A thorough discussion is given of the considerations underlying the design of the equipment, and results of experiments are presented to show the performance attainable and the types of error which occur. In Appendix 11.2 a summary is given of the major features of the cinematographic, magnetic tape and disc methods of recording, when used for this application. It is concluded that a comprehensive installation might include equipment of more than one type.