Patent GB-526984-A (GB)
Title: Method and apparatus for deriving delayed secondary signals from a primary signal
IPC: H03H7/30
Filed: 1939-03-30
Abstract: 526,984. Optical sound recording and reproducing. GOLDMARK, P. C. March 30, 1939, No. 10000. Convention date, June 30, 1938. [Class 40 (ii)] [Also in Groups XXXIX and XL] A number of secondary signals are derived from a primary signal by making a fugitive record which gradually decays and picking up from the record a number of times during its period of persistence. The record may for instance comprise a phosphorescent surface influenced by a light beam modulated by the primary signal. The invention is particularly useful for the production of artificial echo effects in sound currents, the secondary signals being combined with the primary signal. The amount of echo and its delay time may be controlled as desired, and it may be variable with frequency. In the embodiments described, the primary signal modulates the light from a mercury arc lamp or from a cathode-ray tube, but a light valve or Kerr cell may also be employed.. First arrangement, Figs. 1, 2. The light from a mercury arc lamp 20 is focussed through a slit 20a on the phosphorescent peripheral surface 30 of a rotating disc 31, and the image record is picked up through a slot 33 by a photoelectric cell 35. The phosphorescent material may be zinc sulphide, and the speed of the disc is such that a record persists for several revolutions. 'The sound currents from a microphone 1 are applied through a transformer 13 to modulate the light from the lamp 20, which is in series with a triode 21 and a protective resistance 26. The arc is started by momentary closure of a switch 27a. The average arc current is maintained at a suitable low value by applying the sound currents through a transformer 17 to the grid of the triode 21 in series with a rectifier 43 under the control of a delay circuit 40, 41, so that the biassing current varies in accordance with the envelope of the sound currents. The secondary currents from the pick-up 35 are amplified and are fed into the signal channel to be mixed with the primary sound currents. An infra-red lamp 58, which is rendered operative by a switch 59a, may be employed for erasing the record after the first pick-up when desired, e.g. in the case of an impulsive primary signal, or may be adjusted to erase the record partially at each revolution of the disc. Second arrangement, Fig. 4. The sound currents are applied through the transformer 13 to the mercury arc lamp 20, as in Figs. 1, 2, but in this case the record is picked up in succession by four photo-electric cells 35 spaced around the disc 31. The output currents are fed over individual switches 61 and an attenuator 62 to a mixing transformer 65, which receives the primary sound currents through an attenuator 63. The attenuators are ganged to vary the proportion of primary. and secondary currents while keeping the total output constant. The lamp 20 is maintained at the proper operating point by automatic cooling by means of an air blower 45 which is brought into operation by a marginal relay 19 when the current becomes excessive. A condenser 29a in parallel with a protective resistance 29 assists in preventing extinction of the lamp when the voltage falls. Third arrangement, Fig. 3. A cathode-ray tube 50 which serves as the light source has its deflecting plates 51 supplied with high frequency voltage modulated at the sound frequency by a modulator 53, so that a variable width record is formed on the phosphorescent surface of the rotating disc 31. Alternatively, the sound currents may be applied to the control grid to give a variable density record, or both amplitude and grid modulation may be employed. The record is picked up through three spaced slits 33 by a single photoelectric cell 35 by means of mirrors 55, 56 and a prism 57, or separate cells may be employed. In a modification (Fig. 5, not shown), the disc 31 is enclosed within the envelope of the cathode-ray tube, so that the phosphorescent surface is influenced directly by the electron beam. The disc is driven by external induction magnets. A portion of the secondary currents from the pick-up may be fed back to the deflecting plates of the cathode-ray tube. Fourth arrangement, Fig. 6, not shown. The recording takes place substantially as in Figs, 1, 2, and three spaced pick-ups are provided, the outputs being mixed in adjustable proportions. An output switch is provided for passing either the primary or secondary currents or a mixture of both. A switch is also provided for cutting out the amplifier in the primary circuit so that the secondary currents are fed back to the input of the recording lamp.