Patent GB-1145163-A (GB)
Title: Transmission time reduction system and method
IPC: G08C21/00
Filed: 1966-10-14
Abstract: 1,145,163. Television; sawtooth generators. INTERNATIONAL STANDARD ELECTRIC CORP. 14 Oct., 1966 [18 Oct., 1965], No. 45940/66. Headings H3T and H4F. Television.-The transmission time and/or bandwidth which is required for a video signal representing black areas on a white background (e.g. a typewritten document) is reduced by using two scanning speeds, a high speed for the portions of white signals in excess of a predetermined minimum period and a slow speed for the initial portions of the white signals and all the black signals. The existence of a white signal in excess of the predetermined period is detected by: (1) differentiating and clipping the video signal to determine black-to-white transitions, (2) triggering a monostable multivibrator having a trigger duration equal to the selected predetermined period by the differentiated and clipped signal, and (3) applying the multivibrator output and the video signal to an AND gate so that the gate is enabled only during the excess of white signal over the predetermined period. Scanning normally proceeds at the slow speed and the gate output is used to switch the scanning to the high speed. The video signal is transmitted to a receiver where a similar detecting arrangement is used to control the scanning speed so that the video signal during reproduction is re-established with the correct time scale. Operation at the transmitter and receiver proceeds as illustrated in Figs. 3 and 4 respectively. The transmitter, Fig. 1 (not shown), uses a vidicon pick-up tube and the receiver, Fig. 2 (not shown), uses a cathode-ray reproducing tube. The line scan is controlled at the two speeds, whilst the frame scan is by a staircase wave generator which is stopped at the end of each line by a signal from the line scan generator. In a modified arrangement, Figs. 7 and 8 (not shown), the high speed scan is also used for black signals in excess of the predetermined minimum period. The detecting arrangement is modified so that the multivibrator is triggered for both white and black signals and the AND gate is omitted so that the multivibrator alone controls the scanning speed. In a further arrangement, Figs. 9 and 10 (not shown), a grey scale video signal is first quantized into predetermined number of levels and then twospeed scanning is used such that the initial part of the signal at each level proceeds at the slow speed and any excess over a predetermined period proceeds at the high speed. Transistor sawtooth generator circuits.-In Fig. 5, capacitor 130 normally charges through resistor 144 to develop a sawtooth signal via emitter follower 146 but may be caused to charge at a more rapid rate by the application of a variable duration pulse to terminal 51 to render transistors 140 and 143 in parallel with resistor 144. The sawtooth signal is terminated by reset pulses applied through resistors 132 and 133 to render transistor 134 conducting. One reset pulse is obtained from another (the frame) scanning circuit and the other is obtained from an amplitude discriminator responsive to the sawtooth signal. In an alternative circuit, Fig. 6, capacitor 130 charges to the base potential of emitter follower 156 which may be that of either terminal 150 or 152 depending whether or not transistor 154 is rendered conducting in response to a pulse at terminal 51. The sawtooth signal developed across the capacitor is terminated by reset pulses applied to render transistor 158 conducting.