Patent GB-1090848-A (GB)
Title: Colour television systems
IPC: H04N11/14
Filed: 1965-08-23
Abstract: 1,090,848. Colour television. V. E. TESLER. Aug. 23, 1965, No. 36010/65. Heading H4F. Colour television signals are transmitted in the form of a luminance component and a chrominance component which is produced in a manner generally similar to an N.T.S.C. subcarrier (i.e. by balanced modulation of red and blue colour-difference components E R-Y  and E B-Y  on quadrature phase subcarrier signals), but with the modulation characteristics changed during successive lines in such manner that the subcarrier signal received during one line may be utilized as the reference signal to demodulate the subcarrier signal received during the preceding line. In effect the hue information is conveyed by the phase difference between the subcarrier is successive lines, the two subcarrier signals being brought into time coincidence at a receiver with the aid of a one-line-duration delay line to permit phase comparison. In a first arrangement, Fig. 1, the subcarrier E S1  is formed in the usual way during the first and subsequent odd numbered lines as shown at a, but during the even lines, c, the phase component of the modulation is omitted and the subcarrier E s2  is modulated in amplitude only at a fixed phase corresponding to the E B-Y  axis. Since the colour-difference signals are bipolar signals, the subcarrier vector may occupy the second and third quadrants, and b and d illustrate the operation under such conditions. In each case the phase angle # denoting hue is obtainable by demodulating the subcarrier with the signal from the next line. The result of this, however, is to cause the detected phase angle # to be alternately plus and minus with respect to reference phase. In terms of signals this means that E R-Y  alternates in polarity and suitable compensation is introduced in the receiver. In a transmitter for this first arrangement, Fig. 5 (not shown), a luminance signal Ey and colour-difference signals E R-Y  and E B-Y  are derived from red, green, blue signals E R , E G  and E B  by a conventional matrix (1) and a chrominance subcarrier E s1  is formed in the usual manner in quadrature modulator (3) which is supplied with a subcarrier oscillation sin # 0 t. The subcarrier is applied to one input of a change-over switch (4) and is also envelope detected in a stage (5). The resulting signal then amplitude modulates a subcarrier oscillation of E B-Y  reference phase in a modulator (6) to obtain a subcarrier signal E s2  which is applied to the second input of switch (4). The switch is operated back and forth at line rate to obtain E S1 and E s2  alternately at the output which is connected to a circuit (2) for combination with the luminance signal. In contrast to N.T.S.C. practice the subcarrier signal is applied through an amplitude distorting stage whereby the subcarrier emplitude equals #A, where A is the normal N.T.S.C. amplitude, i.e. #E<SP>2</SP> R-Y +E<SP>2</SP> B-Y . At a receiver, Fig. 11 (not shown), demodulation of the subcarrier is effected by two demodulators (29) and (32), the received signal being applied directly to (29) and via a 90 degrees phase shifter (30) to (32). Furthermore, the signal from the preceding line is applied in the same phase to both demodulators via one-lineduration delay (28). E B-Y  appears from (29) and E R-Y  from (32). To compensate for the polarity change in E R-Y  (see above) the input to (32) is inverted in phase at line rate by a circuit (31) controlled by line pulses from the horizontal scanning generator (not shown). Correct phase of operation of circuit (31) is achieved by pulses included in the E R-Y  signal in the frame blanking interval. The green colour difference signal is obtained from E B-Y  and E R-Y  in the conventional manner with a matrix circuit (33). A second arrangement, Fig. 2, modulates the subcarrier such that only half the phase modulation # is imparted in each line, with the direction of phase change being in opposite directions in succeeding lines as shown at a and c. With this arrangement, however, it is necessary to restrict the signal so as to be unipolar (i.e. the modulus of the signal is used) in alternate lines as indicated at d, since otherwise, when the vector is in the second or third quandrant as at b, the phase difference between succeeding lines would be # instead of the correct value # + 180. The transmitter for this arrangement, Fig. 6 (not shown), utilizes two pairs of quadrature modulators (3) and (8) which receive signals at twice the subcarrier frequency. E R-Y  and E B-Y  signals are applied to modulator (3) directly and to modulator (9) via circuit (8) which converts the signals to monopolar form. In addition E R-Y  is reversed in polarity so that the direction of phase modulation imparted by the two modulators is in opposite directions. The outputs of the modulators are selected alternately by a switch (4) operating at line rate and then combined with the luminance signal E y  in a mixer (2) after first passing through a circuit where the subcarrier amplitude is converted to #A and the frequency is divided by two, so obtaining the correct frequency # 0  but half the phase shift only in each line. The receiver for this second arrangement is identical with that described above for the first arrangement. A third arrangement, Fig. 3, uses the phase angle of one line as the reference phase from which the modulation is imparted in the succeeding line. Furthermore, the direction of phase modulation is in the opposite direction in succeeding lines. The result is indicated at a, c, e, g and i for five successive lines where # 1 =# 2 , # 2 ## 3 , # 3 ## 4  and # 4 =# 5 . The need to restrict the signals to monopolar form arises as in the second arrangement, the operation proceeding as indicated at b, d, f, h and j. A transmitter for this arrangement, Fig. 7 (not shown), uses two modulators (12) and (13) which operate in quadrature, the first receives E B-Y  and the second receives E R-Y  which is reversed in polarity in alternate lines so as to cause the direction of phase modulation to reverse. The modulator outputs are combined at (14) and added to the luminance signal E y  in mixer (2) after first passing through square-root circuit (7). In order that the subcarrier signal for the modulators may have the phase of the preceding line, the subcarrier output from (14) is detected at (15) with a reference phase signal sin # 0 t to obtain an oscillation at the instantaneous subcarrier phase. The oscillation is then delayed for one line duration in circuit (16) and applied to the modulators with 0 and 90 degrees phase shifts from circuit (17). A receiver for this arrangement is identical with that described for the first arrangement. A fourth arrangement, Fig. 4, is similar to the third but dispenses with the reversal of the phase modulation direction in alternate lines. The phase angles in succeeding lines thus add continuously as shown at a, c, e, g and i. The signals in alternate lines are restricted to monopolar form so that vector operation in the second and third quadrants is as shown at b, d, f, h and j. The transmitter for the system may be similar to that described for the third arrangement but with the polarity reverser (II) omitted. With this arrangement the sign of the phase angle is always indicated correctly from the difference between successive lines. The signal E R-Y  derived at a receiver is not therefore subject to reversal of polarity on alternate lines. A receiver for the arrangement, Fig. 12 (not shown), is thus similar to that for the first, second and third arrangements but with inverter omitted.