Patent GB-744028-A (GB)
Title: Television receiving system
IPC: H04N9/64
Filed: 1953-10-22
Abstract: 744,028. Colour television. PHILCO CORPORATION. Oct. 22, 1953 [Oct. 23, 1952], No. 29212/53. Class 40 (3). [Also in Group XL (c)] Relates to colour television receivers for use with signals of the type comprising (1) a picture carrier modulated with a wide band brightness signal, (2) a colour carrier adjacent the end of brightness signal and modulated with narrow band chromaticity information, and (3) a sound carrier spaced a predetermined amount from the picture carrier and located adjacent the colour carrier components at the end remote from the picture carrier. According to the invention, the received signal is divided into a first portion containing the picture carrier and brightness information to the exclusion of the colour and sound carrier components, and a second portion which contains the colour and sound carrier components. The first portion is then detected to derive the brightness information whilst the second portion is heterodyned with a signal at picture carrier frequency to derive modulated colour and sound carriers. By the division of the signal into two portions which are detected separately the invention prevents a beat signal at the difference between the colour and sound carriers from appearing in the brightness signal channel. In a first embodiment, Fig. 1, the received signal after conversion to an intermediate frequency by stages 10 and 12 passes through amplifiers 16 and 18 having broad-band symmetrical characteristics as indicated at 20 where PC denotes the picture carrier, CC the colour carrier and SC the sound carrier. The signal is then divided into a first portion and detected to obtain the brightness information by amplifier 22 and detector 24 having passband characteristics as indicated at 26 and 28 respectively. The second portion of the signal comprising the colour and sound carrier components is derived by amplifier 30 having a pass band as shown at 32 and applied as a first input to a heterodyne detector 34. A further amplifier 40 having a pass band as shown at 42 selects only the picture carrier from the output of amplifier 22 and applies this as a second input to detector 34. The difference signals in the anode and screen outputs are then selected by band-pass filters 66 and 64 to derive the modulated colour and sound carriers respectively. In a second embodiment, Fig. 2 (not shown), the use of a separate amplifier for the picture carrier is dispensed with and the colour and sound carrier components together with the picture carrier are derived by means of an amplifier having a characteristic such as to attenuate only the picture carrier modulation components. In yet a third embodiment, Fig. 3 (not shown), the band-pass characteristics of the channel conveying the picture carrier and brightness signal are obtained by a band-pass filter preceding the detector, a low-pass filter covering the range of the brightness signal is connected to the output of the detector, and the whole of the received signal is applied to the channel for deriving the colour and sound carrier signal. The effect of this last feature is, in some instances, to allow the brightness information to modulate the colour carrier output and means are provided for combining part of the output of the brightness channel with the colour carrier output so as to compensate the effect. Specification 715,506 is referred to.