Patent GB-1001383-A (GB)
Title: Improvements in and relating to indexing systems in colour television
Inventor: JUSTICE JAMES WILLIAM HENRY
IPC: H04N9/24
Filed: 1961-11-27
Abstract: 1,001,383. Colour television receivers. SYLVANIA - THORN COLOUR TELEVISION LABORATORIES Ltd. Nov. 22, 1962 [Nov. 27, 1961], No. 42362/61. Heading H4F. The invention relates to indexing systems in colour television receivers of the type in which the screen upon which the television image is reproduced includes parallel colour stripes in repeating groups each of m stripes, the stripes in each group emitting (that is generating or transmitting) light of different colours, and also indexing stripes, in which the screen is scanned by a beam in a raster of lines and in which the scanning beam generates from the scanning of the indexing stripes index signals which are used to generate a writing signal of angular frequency #, #/2# being the frequency with which the said groups are scanned by the beam and the index signals being disposed at intervals corresponding to the angular frequency #m - where n and m are different integers of which n neither is an integral multiple of the other. Apparatus of this type is described in Specification 895,772 and in the Proc. I.E.E. pages 511-523, Vol. 108, Part B, 1961. The invention is concerned with eliminating phase errors which are imparted to the writing signal, and in accordance with the invention the writing signal is derived by the subtraction of one of two frequencies from the other, oscillations of the two frequencies being fed through at least two partially separate loop circuits to a mixer in which the subtraction takes place, the frequencies which differ from one another by # being derived directly or indirectly from the scanning of the index stripes and at least one of the oscillations being derived by multiplying the frequency of a signal derived by scanning the index stripes. In Fig. 1 the writing signal is derived from a subtraction circuit 15 to which is applied two signals each derived through separate loop circuits from the indexing stripes, one of these circuits including a delay device 18 which compensates for the delay inherent in the amplifier 13 of the other circuit. The Specification derives expressions to show that, when the system is phase compensated, T 2 =T 1 /(r- 1), where T 1  is the inherent delay, T 2  is the introduced delay and r is a constant. The Specification further discloses that if k is the ratio of the index frequency to the writing frequency, the first circuit should include a   multiplication factor of r/k and the second circuit should have a similar factor of (r-1)/k. Thus, as shown, where the index signal has 3#3 an angular frequency -(r=-), the first loop 4 2 circuit comprises amplifier 13 and multiplier 14 and the second loop circuit comprises amplifier 13, divider 16, multiplier 17, delay line 18 and remodulator 21. If the remodulator 21 introduces a delay, the delay time T2 is correspondingly reduced. The divider 16 is regenerative when once started but is initially triggered by   a signal of #/2 derived from run-in stripes, the gate 20 operating to cut off this signal when the divider becomes regenerative. Other circuit arrangements utilizing different multiplier factors or index signals of different frequencies are disclosed with reference to Figs. 3-6 (not shown).