GB1243365A - Method and apparatus for the high speed transmission and reception of two-level images - Google Patents

Abstract

1,243,365. Facsimile systems. V. MOSCA, and M. BONFA. 15 Oct., 1968. [26 Oct., 1967], No. 48754/68. Heading H4F. A method for the high-speed transmission and reception of an image comprising highspeed scanning the image to produce a video signal, stopping the scan on detection of the video signal reaching a predetermined threshold and generating a pulse signal of predetemined duration for transmission to the receiver, maintaining said stopped scan state for said duration and thereafter causing the resumption of said high-speed scan. In the transmitter of Fig. 1, the video signal A is passed via an adder circuit 1 to a level discriminator 2 which converts the video signal into a binary-quantized form referenced to a predetermined threshold. Each upward step of the output of 2 (a threshold up-crossing) is converted to a spike by differentiator 3. Said spike sets a bi-stable 4 which 'set' output closes a switch 6 whereby pulses from a 465 Kc/s. oscillator are gated to a counter 7. By counting up the requisite number of pulses, the counter 7 produces a pulse commencing with the production of said spike and ending 500+0.064Á sec. thereafter. The accuracy figure of 0.064Ás is obtained from a scanned image line being made up of 390 points, each line being scanned (in the absence of a stop due to a detected threshold attainment) in 5 ms., such that each point is scanned in 12.8 ÁS. It is assumed that a maximum of 200 threshold attainments may be detected per line and for a total error time at the end of a line scanning of one point on 12.8 ÁS., the accuracy of the stopping times, i.e. the durations of said pulses, 12.8 must be-= 0.064Ás. This pulse appears at 200 output B and is transmitted to the receiver. Other outputs at C provide, by means of timer 8 and adder 9 and via inverter 10, an input to adder 1, whereby, under the control of the output of discriminator 2, the total signal applied to the discriminator 2 is kept constant during the stopped scan period whereby interference pulses &c. have no effect. Line sync. pulses are added to the transmitted signals, distinguished from the threshold attainment indicating pulses by their greater amplitudes. In the receiver, Fig. 2, the received pulses are fed via interference filter 1<SP>1</SP> and amplifier 2<SP>1</SP> to level discriminators 3<SP>1</SP> and 9<SP>1</SP>, which respectively separate out the line sync. pulses and the threshold attaimment pulses. The former pass to a sync. pulse generator 8<SP>1</SP> and the latter to an assembly comprising switch 6<SP>1</SP>, 465 Kc/s. oscillator 51 and counter 7<SP>1</SP> which, is the same manner as in the transmitter, produce at output C<SP>1</SP> the pulses stopping the reproducing scan thereby indicating a threshold attainment. A two-level image reconstruction is thereby enabled.