Patent GB-622221-A (GB)
Title: Facsimile system
IPC: H04L21/00
Filed: 1946-04-12
Abstract: 622,221. Facsimile transmission systems; photo-electric telegraphic transmitters. STANDARD TELEPHONES & CABLES, Ltd. April 12, 1946, No. 11330. Convention date, Jan. 30, 1943. [Class 10 (iii)] In a communication system of the built-up character or simulated facsimile type in which a series of electrical impulses depending on the shape of a particular character are transmitted during a period of time which is short relative to the time between characters, the transmitted impulses have a sharp wave front and a high ratio of peak to average power, receiving and visual reproducing means being provided including means for eliminating all energy below a predetermined level which is substantially lower than the peak energy level of the received impulses. A drum 20, Fig. 2, driven by a synchronous motor is provided with perforations 21 representing a series of signal elements forming a particular character to be transmitted and which are arranged to interrupt light from a source and mirror 23, 24 impinging on a photo-cell 22 when shutter elements 25 are selectively operated by the keys 26, the resulting signal impulses generated by the cell 22 being transmitted by radio transmitter means. The drum 20 is maintained continuously in rotation at a high speed and the shutter arrangement is such that upon depression of any selected key 26 a single transmission of one character results, which is timed to commence at the beginning of the selected character. The shutter mechanism, shown in detail in Fig. 2a, comprises a support plate 20a provided with an aperture 21a normally covered by the opaque portion of a shutter plate 23a to prevent passage of light to the photo-cell. When key 26 is depressed, the shutter plate is pulled to the right, tensioning the spring 28a, until the portion 27a of a pull-bar is positioned above a cam 22a, arranged for rotation with the drum 20, which releases the shutter plate when the proper drum position is reached. Upon release, the spring pulls the shutter plate towards the left, the momentum of the system being sufficient to cause an opening 24a in the shutter plate to register with aperture 21a, permitting the passage of light to the photo-cell, and at the same time slightly compressing the spring which, on expansion, again puts the openings out of alignment. This arrangement provides only one shutter operation for a single key operation. To prevent the simultaneous depression of more than one key, any known form of blocking mechanism, such as the notched bar 27, may be provided. The rotational speed of the drum is made several times higher than the normal operating speed of the keys, thus permitting a high peak signal power to be employed for each character element transmitted although the average power transmitted remains relatively low. Receiving means for signals transmitted by the system described above comprise a conventional radio receiver 36 (Fig. 3) feeding the control grid 37 of a cathode-ray tube 35, a clipper circuit 38 comprising a high-pass filter being included to remove interference signals, intentional jamming &c., below a predetermined energy level. The motors 30, at both the transmitting and receiving stations, are preferably operated at substantially the same speed but, unless the characters are to be reproduced only once on the screen of the receiving cathode-ray tube, synchronism between the motors is rendered unnecessary by the following arrangement. A drum 40, Fig. 4, mounted on the same shaft as the drum 20 and provided with a single contact element 42, is arranged to trigger a circuit consisting of a gas discharge tube 47 and resistor and condenser network 48, 49 to produce a sawtooth voltage waveform such as a, Fig. 5, which is mixed, in mixer valve 52, with a square-wave voltage such as b, derived from oscillator 50, to produce a resultant waveform c which is applied to the vertical deflection plates of the cathode-ray receiving tube. A similar arrangement comprising a drum 41 provided with seven contact elements produces a waveform d which is applied to the horizontal deflection plates. With the particular scanning frequencies described, four separate complete representations of each received character will be displayed on the cathode-ray tube screen if the driving motors (of transmitter and receiver) are in synchronism, Fig. 6 (upper Figure). If the motors are running out of synchronism, the received character will be split up into four portions, one of which, however, is produced in its entirety, Fig. 6 (lower Figure). Instead of four images, additional character images may be produced by employing scanning sawtooth waves having a greater number of " steps " than are employed in the described example. Specification 606,138 is referred to.