Patent GB-786236-A (GB)
Title: Electric wave-signal modifying apparatus
IPC: H04N11/14
Filed: 1955-12-05
Abstract: 786,236. Television. HAZELTINE CORPORATION. Dec. 5, 1955 [Dec. 14, 1954], No. 34806/55. Class 40 (3). [Also in Group XL (c)] A received NTSC chrominance sub-carrier comprising single side-band and double sideband modulation components representative respectively of different colour aspects (i.e. the I and Q signals respectively) is transformed into a resultant signal having double side-band modulation components for each colour aspect. This arrangement allows the Red, Green and Blue colour difference signals' to be derived directly from the resultant signal (modified sub-carrier) without the need for matrixing apparatus for the I and Q signals at the receiver whilst retaining the advantages of wide-band and narrow-band transmission of the I and Q signals, respectively. Figs. 1 and 2 illustrate one embodiment in which the modified sub-carrier is derived by separating from the received sub-carrier A (developed in amplifier 26) the double side-band modulation component B (by means of filter 27) which is combined in adder 29 with a signal E consisting of part of the single side-band component (i.e. part of the I signal) together with its upper side-band counterpart, E being produced in a balanced modulator 33 to which is fed a suitably phased wave of sub-carrier frequency together with part of the I signal which is separated out by filter 30 (producing signal C) and shifted in the frequency spectrum so as to occupy the range 0.5-1.5 Mc/s (signal D) by means of a synchronous demodulator 31. The output of adder 29 (signal F) which consists of a wave of sub-carrier frequency (3.6 Mc/s) having double side-band modulation components for both the I and Q signals is then detected in synchronous detector 16 and matrixed in known manner to produce signals for application to the colour picture reproducer 14. Fig. 3 shows a modification of the arrangement of Fig. 1, in which the whole of the received sub-carrier (signal A, Fig. 2) is supplied to adder 29 together with the upper side-band counterpart (occupying the range 4.1-5.1 Mc/s) of the signal C, this being derived in a balanced modulator 42 to which the signal C is supplied together with a suitablyphased wave of twice sub-carrier frequency. In a second embodiment (Figs. 4 and 5) the received sub-carrier is supplied to the anode of a diode 54 via the circuit 51, 52, 53, the passband of 51 and 53 having the form shown at B and that of delay line 52, the delay of which is equal to the delay introduced by the coupled circuits 51, 53, having the form shown at A. The signal then effective on the anode of 54 is shown at C (i.e. the single side-band I component in the range 2.1-3.1 Mc/s) and this signal is stated to be equivalent to a signal having two sets of (double) side-bands one of which comprises components in the ranges 2.1-3.1 Mc/s and 4.1-5.1 Me/s symmetrically disposed about the reference axis (the subcarrier frequency) and representing pure amplitude modulation of the I phase of the subcarrier and the other of which comprises components in the same ranges symmetrically disposed about an axis in quadrature with the sub-carrier (i.e. the Q modulation phase) and representing I cross-talk in the Q signal. The latter components are shunted to ground by the action of diode 54 which is rendered conductive in phase with the Q axis by means of a suitable phased signal of twice the sub-carrier frequency supplied to the diode cathode via 18, 55, 56, 58, 59 and the remaining components, comprising the double side-band I signals in the ranges 2.1-3.1 Mc/s and 4.1-5.1 Mc/s together with the double side-band Q signals, are then derived at the centre tap of circuit 53 and supplied to the synchronous detector 16 (Fig. 1), where they are utilized as described in connection with the embodiment of Fig. 1.