Patent GB-796640-A (GB)
Title: Signal-matrixing apparatus
IPC: H04N9/66
Filed: 1956-01-30
Abstract: 796,640. Colour television. HAZELTINE CORPORATION. Jan. 30, 1956 [March 2, 1955], No. 2921/56. Class 40 (3). A subcarrier signal modulated in quadrature with I and Q signals in accordance with the American N.T.S.C. standards is modified in accordance with the equations R-Y=0.62Q+ 0.96I and -0.87(B-Y)= -1.47Q+0.96I to derive two further subearrier signals which may be demodulated to derive R-Y and B-Y directly. The invention avoids the production of colour crosstalk which is produced (due to the assymmetrical side-band modulation of the I signal) when the original subcarrier is demodulated to derive R-Y and B-Y. The general procedure is to divide the subcarrier into a first signal having a narrow bandwidth corresponding to the Q signal and a second signal having a wider bandwidth corresponding to the I signal. The two signals are then combined with a relative phase shift of 90 degrees in accordance with the equations above to produce the two further signals. In a first embodiment, Fig. 2, the 3.58 Mc. subcarrier signal bearing quadrature modulations I and Q as shown in Figs. 3a and 4a, is applied from chrominance amplifier 15 (not shown), to two paths, the first comprising tuned transformer 32 and the second comprising delay line 34, cathode-follower 35 and high-pass filter 36. The transformer has a resonant response extending over the range 3.1-4.1 Mc., corresponding to the side-band range of the Q signal, and has a secondary tapped in the impedance ratio 0.62 : 1.47 whereby the output voltages, which are arranged to be shifted 90 degrees with respect to the input voltage, are as indicated in Figs. 3b and 3c for the upper and lower secondary terminals respectively. The delay line path is given an overall delay equal to that of the tuned transformer circuit together with an extra delay of 90 degrees and has an amplitude response of 0.96 whereby the output from filter 36 is as shown in Fig. 4b and 4c. In practice, the delay line circuit is given a delay of 900 degrees (90 + 90 + 2 x 360 degrees) since with this delay the slope of frequency-phase characteristic is the same as that of the transformer and the envelope delays in the two paths are equal. Filter 36 has a lower cut-off frequency of 2 Mc. so that the path has a frequency response corresponding to the range of the I signal. The filter output is then connected to the transformer tap so that the signals from the two paths add to produce outputs at the upper and lower terminals, as shown in Figs. 5b and 5c respectively and bearing modulations corresponding to R-Y and - 0.87 (B-Y) as defined by the equations above. These signals may be synchronously demodulated at 17 and 18 with 180 degree phase-displaced reference signals to derive R-Y and B-Y, suitable gain or attenuation being included in one demodulator to remove the 0.87 difference in amplitude. The quadrature components which are also produced are not indicated in Figs. 5b and 5c, since they are eliminated in the demodulation process. As a result of the increase of voltage which appears across resistor 33 over the range 2-3.1 Mc., when the input impedance of transformer 32 falls, the circuit provides a 3-6 db boost in the I signal to compensate for the reduction in output due to the single side-band operation over this range. A second embodiment, Fig. 7, operates in a generally similar manner but isolates the two signal paths by input stage 41. Delay line 34 provides a delay differing by 180 degrees from the transformer output so that a Q band-width signal coupled into the path from a tertiary winding on the transformer 43 reduces the level over the band 3.1-4.1 Mc. and thereby effectively boosts the 2-3.1 Mc. range to compensate for a single side-band modulation of the I signal. The phase of the signal is reversed by amplifier 46 and tuned network 36 or a delay line introduces the extra 90 degree phase-shift before the signals from the two paths are added. Specification 790,407 is referred to.