Patent GB-688139-A (GB)
Title: Transmission system for the transmission of characters in accordance with the hell or similar facsimile method
IPC: H04N1/36
Filed: 1950-05-16
Abstract: 688,139. Facsimile printing telegraphy, synchronous movements. SIEMENS & HALSKE AKT.-GES. May 16, 1950 [Jan. 21, 1950], No. 12244/50. Class 40 (iii). In a character transmission system operating according to the Hell method or a similar facsimile method, synchronous operation of the receiver with the transmitter is effected by combining a continuous train of synchronizing impulses with the image impulses. The synchronizing impulses bear a predetermined frequency and phase relation to the image impulses and are arranged to have the same amplitude. At the receiver, an alternating voltage generator is controlled by the received combined impulse train to produce signals at a frequency and hase coinciding with the synchronizing impulses, and these signals are then applied to control the receiver drive synchronously. Impulses corresponding to the synchronizing impulses are also derived, either by squaring the alternating voltage or by means of a contact actuated from the receiver drive, and these impulses are combined with the received combined impulse train to cancel out the synchronizing impulses and restore the original image impulses for application to the recording mechanism. In Fig. 1, curve a illustrates the image impulse train corresponding to the letter E and curve b a train of synchronizing impulses. The impulse trains are combined in a manner such that simultaneously appearing impulses cancel whilst single impulses remain. The combined train which is transmitted appears therefore as shown in curve c. At the receiver, the synchronizing impulse train is reproduced as shown in curve d and is recombined with the impulse train c in a similar manner so as to re-establish the original impulse train as shown in curve e. As illustrated, each synchronizing impulse corresponds in duration to a unit image impulse L. In another embodiment, Fig. 2 (not shown), each synchronizing impulse corresponds to the character field of duration 5L and the interval between impulses corresponds to the marginal zone. In a modified method of combining the image and synchronizing impulses, transmission takes place when both impulses are present and when neither impulse is present, whilst transmission is suppressed for single impulses. Transmitter, Fig. 3. Cam roller 1 actuates contacts 2 (of which only one is illustrated) to generate image impulses in the known manner. In accordance with the invention, a cam 3 mounted on the main shaft operates contact 4 to generate periodic synchronizing impulses, and the contacts 2, 4 are connected respectively to short-circuit the arms 5 and 6 of a bridge circuit 5, 6, 7. A source of alternating signalling potential is connected across one diagonal and on output is taken from across the other through transformer 8. With this arrangement, which operates in accordance with the method illustrated in Fig. 1, impulses generated simultaneously at contacts 2 and 4 maintain the bridge in balance and transmission takes place only when impulses are generated separately. The bridge circuit may be replaced by a differential relay or a multivibrator type of valve circuit. Receiver, Fig. 4. The combined impulse train from receiver 9 is rectified at 10 and applied to two channels 11, 22. The first channel 11 consists of an alternating voltage generator, such as a valve oscillator, connected through filter circuits tuned to the frequency of the synchronizing impulses or a multiple thereof. The generator produces a voltage coinciding in phase and frequency with the synchronizing impulses and drives a synchronous motor 12 on the shaft of which is mounted the printing spindle 13 of the recording mechanism. The output of the generator is also applied through a rectifier 15 and limiter 16 to produce a train of impulses 17 corresponding to the synchronizing impulses. The second channel comprises a voltage limiter 22 from which is obtained the combined impulse train 23. The impulse trains 17 and 23 are then applied to a rectifier bridge 18-21 which combines the two trains to derive the original image impulse train which is applied through valve 24 to control the printing magnet of the recording mechanism. As an alternative to the rectifier bridge, the printing magnet may employ two differential windings to which the impulse trains 17 and 23 are applied directly. In another embodiment, Fig. 5 (not shown), an alternating voltage generator controls the driving motor as shown in Fig. 4, but the impulse train 17 is generated by a contact actuated by a cam disc on the motor shaft. The two impulse trains are combined by differential windings on the printing magnet. The angular position of the cam disc on the shaft is made adjustable in order to correct for phase errors, The receiver may be arranged to record during the intervals between impulses so that a negative record is obtained. Such a record may also be obtained by reversing the significance of the impulses generated at the transmitter. Specifications 380,217 and 421,960 are referred to.