Patent GB-753172-A (GB)
Title: Improvements in systems for transmitting television or like images
IPC: H04N11/12
Filed: 1954-05-10
Abstract: 753,172. Television. PHILIPS ELECTRICAL INDUSTRIES, Ltd. May 10, 1954 [May 13, 1953], No. 13564/54. Class 40 (3). The invention relates to a system for transmitting signals representing television images or similar images scanned in line-by-line fashion in which, within the band of one broad-band image signal, O-fa, Fig. 2, there are located two auxiliary carrier waves fh1, fh2 which may be modulated to convey additional image information, e.g. colour information. The invention is concerned with the elimination of interference in the reproduced images due to the use of a common band for three signals. According to the invention, the auxiliary carrier wave fh1 of lower frequency is controlled so as to have a phase difference of # radians at two instants spaced apart by one image period and also at two corresponding points of successive evennumbered or successive odd-numbered lines of one image, whilst the other auxiliary carrier fh2 is controlled so that its phases are equal at two instants spaced apart by one image period and differ by # radians at two corresponding points of successive lines of one image. When the image is scanned with an odd number of lines forming two interlaced fields, this is achieved by making the auxiliary carrier of lower frequency equal to an odd-numbered multiple of half line frequency and the other auxiliary carrier equal to a multiple of line frequency, the phase of said other carrier however being shifted by # radians at the beginning of each field (i.e. half image). The interference due to the lower frequency carrier cancels visually due to the phase reversal on successive scans of one line and the phase reversal on successive lines of each field, whilst the interference due to the other carrier, although it does not cancel visually on each line, is compensated by the phase reversal of the interference on adjacent image lines. Interference at the difference frequency of the two carriers occurs at a frequency equal to an odd multiple of half line frequency and therefore cancels visually in the same manner as that due to the lower frequency carrier. As a further feature, certain other interference effects due to the lower frequency carrier may also be reduced by arranging that the phase difference at corresponding points in successive # lines of one image is - radians. In an odd-line 2 interlaced system this is brought about by subjecting the carrier to a phase shift at the beginning of each field alternately of + #/2  and - #/2 radians. Fig. 6 illustrates a transmitter according to the invention incorporating this further feature. Camera I generates a brightness signal which is applied as a first input to adder A through filter F1 having a pass-band of O -fa as shown in Fig. 2. Cameras II and III produce first and second colour signals which are applied to modulate auxiliary carriers fh1 and fh2 respectively in modulators M2 and M3, the resulting signals being applied to adder A through filters F2 and F3 having pass-bands respectively equal to fb-fc and fg-fk. The combined signal is modulated on to a radio carrier from oscillator O1 in modulators M. Filter F4 restricts the output for vestigial sideband transmission. Auxiliary carrier fh1 is generated by oscillator 02 which is synchronized at an odd multiple of half line frequency by line synchronizing pulses L and produces out- # puts at a and b differing in phase by - radians. 2 Switch S2 applies the outputs alternately to modulator M2 and is changed over at the beginning of each field by field synchronizing pulses R. Auxiliary carrier fh2 is produced by oscillator 03 which is similarly synchronized by the line pulses but in this case at a multiple of line frequency and with outputs at a and b differing in phase by # radians. The outputs are applied alternately to modulator M3 via switch S3 which is also changed over at the beginning of each field by the field synchronizing pulses R. A receiver for use in the system, Fig. 7 (not shown), comprises a conventional superheterodyne with the detector output applied directly to a first cathode-ray tube intended to reproduce the information represented by the broadband signal from camera I and also to two band-pass filters arranged to select the modulations conveyed on the auxiliary carriers fh 1 and fh2. The filters feed detectors, the outputs from which are applied to second and third cathode-ray tubes to reproduce the signals from cameras II and III. Specification 737,136 is referred to.