Patent GB-1435954-A (GB)
Title: Compatible stereoscopic colouer television system
IPC: H04N13/00
Filed: 1973-08-01
Abstract: 1435954 Stereoscopic colour television V E TESLER 1 Aug 1973 36649/73 Heading H4F A stereoscopic colour television signal is derived by modifying the amplitude of the chrominance signal of an NIR signal derived from the first image of the stereo pair with an additional signal relating to the second image of the stereo pair (or the difference between the first and second images), this additional signal having an amplitude less than 10% of the maximum amplitude of the NTSC chrominance signal derived at 7, Fig. 1, from the first image, the amplitude of the resultant signal being equal to the sum of the square root of the amplitude of the NTSC chrominance signal from 7 and the additional signal in alternate lines and the difference of these signals in the intervening lines. The additional signal may be a luminance signal, or an NTSC, PAL or NIR composite colour television signal derived from signals from the second image (or the differences between these signals and the corresponding signals from the first image). The NTSC chrominance signal is passed via a square rooter 8 to an amplitude detector 9 and an insert switch 10, in which an insert of the reference subcarrier is included in the chrominance signal when detector 9 detects zero amplitude, and then to an amplitude limiter 13 and switch 14. Switch 14 is operated at half-line frequency to pass alternately lines of the reference subcarrier and lines of the output of 13, which has a constant amplitude but is phasemodulated, to a balanced modulator 15. The other input of modulator 15 receives a signal from an adder 11 receiving the output from from amplitude detector 9 and the additional signal, derived at 16, via a 0/180 degree switch 17 operated at half line frequency. Circuit 16 may derive from the luminance signal of the second image a bi-polar luminance signal by adding to the luminance signal in each line blanking interval a pulse which has an amplitude equal to half the maximum luminance amplitude, and clamping the luminance signal to this level (Figs. 2, 5, not shown). A pedestal voltage equal to 10% of the maximum amplitude of the output of 7 may be added to the additional signal when the amplitude of the output of 7 is zero to ensure that a positive signal is always derived at 11. Circuit 16 may include conventional circuitry for deriving an NTSC chrominance signal (Fig. 6, not shown), a PAL chrominance signal (Fig. 7, not shown) or an NIR chrominance signal (Fig. 8, not shown). The NTSC chrominance signal may include reference bursts as may the PAL chrominance signal, but the bursts in the PAL chrominance signal may be omitted if the frequency of this subcarrier is made equal to half the frequency of the subcarrier of the first image. In a receiver the luminance signal of the first image is derived by a filter 33, Fig. 11, the subcarrier of the first image being separated by a filter 36 and passed directly and via a one-line delay line 38 to a switch 39 operated at halfline frequency. The outputs of the switch 39 are passed to two envelope detectors 41, 42 connected to an adder 43 which separates the additional signal (by effective subtraction of the signals of adjacent lines). For monochrome stereoscopic reception the luminance signals from 35 and 43 are used to produce the display. For stereoscopic reception with only the first image reproduced in colour the outputs of switch 39 are passed to multipliers 44, 45 to derive the colour difference signals of the first image. For stereoscopic reception with both images reproduced in colour the output of adder 43 is passed to further filters and demodulators which are conventional for NTSC (Fig. 12, not shown) and PAL (Fig. 13, not shown) except that if the additional subcarrier is at half the frequency of the subcarrier of the first image, the reference signal for demodulating the additional subcarrier is derived by frequency dividing the reference phase output of switch 39. For demodulation of an NIR additional subcarrier, this subcarrier, separated by a filter from the output of adder 43, is passed directly and via a one line delay line (56, Fig. 14, not shown) to an adder and subtractor, the outputs of which are fed via a double switch operated at half-line frequency to two multipliers.