Patent GB-764479-A (GB)
Title: Improvements in image signalling systems
IPC: H04N11/12
Filed: 1953-08-13
Abstract: 764,479. Television. ANTRANIKIAN, H. V. Aug. 13, 1953 [Aug. 28, 1952], No. 22326/53. Class 40 (3). The frequency band occupied by an image signal is reduced by translating frequency components within a lower part of the spectrum of the signal into components falling within the upper part of the spectrum and then combining such translated components with the remaining components of the signal in the upper part of the spectrum. Fig. 5 shows an arrangement in which a video signal S occupying a band F as shown in Fig. 1 is compressed within a band S<1> by translating components within the lower frequency part S" of the band into the upper part. The signal is received at 1<1> and is divided into the two parts S<1>, S<11> by means of a filter 21 and subtractor 9<1>. The upper part is applied directly to adder 19 whilst components in the lower part are heterodyned in unit 4<1> by means of oscillator 5<1> of frequency H and resulting components falling within the band S<1> extracted by means of subtractor 20 operating in conjunction with low-pass filter 21. The output of subtractor 20 is then combined with the original signal in adder 19. The resulting combined signal may be modulated on a carrier for radio transmission by modulator 22 and oscillator 23. A circuit 24 is included in the output to restore line and field synchronizing pulses distorted or lost in the process of compression. The frequency H of oscillator 5<1> is preferably made an odd multiple of half the line frequency, although small departures from this frequency are permissible. The phase of the oscillator may be reversed by phase shifter 61 after every field or frame. Part S" of the spectrum is chosen not to exceed half of the full spectrum S, and frequency H is chosen to fall within band S<1> with F-H equal to or greater than F<1>. In a receiver for utilizing the compressed signal, the output within the band S<1> is applied via a suitable amplifier and a D.C. restoration circuit directly to the cathode-ray reproducing tube. The image detail represented by the high-frequency part S<1> of the original signal is reproduced normally whilst the low-frequency part S" is reproduced with interruptions at the frequency H. When the phase of oscillation H is reversed after every frame, the interruptions cancel visually due to the eye averaging the information presented over every two successive frames. Fig. 2 (not shown) illustrates a modified arrangement in which unit 4<1> is replaced by a stage in which the signal from filter 2<1> is interrupted by a square wave of frequency H. It is stated that any process, such as modulating, chopping or heterodyning, capable of converting component frequencies into another set differing from the original by a constant value may be applied to the invention. Figs. 6 and 7 (not shown) illustrate a transmitter and receiver employing the principles of the invention for the simultaneous transmission of blue, red and green colour video signals Sb, Sr and Sg of bandwidth respectively equal to Fb, Fr and Fg as shown in Fig. 8. The lower part of the green signal band is heterodyned by an oscillator of frequency Hg so as to be transferred into the upper part S<1>g, the lower part of the red signal band is heterodyned by an oscillator of frequency Hr so as to be transferred into the upper part S<1>r, and the blue signal is transmitted without modification. Synchronized switching devices may be employed at the transmitter and receiver to periodically change the colour signals occupied by the three bands. Where phase reversal of oscillations Hr and Hg after each frame is not employed, it is stated that smoothing circuits may be employed at the receiver to remove the interruptions introduced in the reproduced images.