Chrominance carrier modifiers are needed, for example, for reproduction of still pictures by PAL-color video signals recorded on magnetic tape in helical scan systems. In reproduction of still pictures, the tape advance is stopped, and the signal track is scanned by a magnetic head at 50 or 60 times a second. In a still picture reproduction of recorded PAL-color video signals, it is necessary to maintain the PAL-standard monotone alternation of the switching phase frequency which is lost when scanning a stopped tape.
Chrominance sub-carrier modifiers are known, see for example, the periodical "Telefunken-Zeitschrift", series 37, 1964, No. 2, pages 115-135. In such a known chrominance sub-carrier modifier, a reference carrier with double the frequency is modulated with a color signal F by means of a balanced modulator. The upper sideband is filtered out of the resulting heterodyne products. The remaining lower sideband F* corresponds to the conjugated complex form of the color signal F. An electronic switch thereafter selects out the proper sequence of the chrominance signal types F and F*.
Known chrominance sub-carrier modifiers with a double balanced or push-pull modulator have some disadvantages when used to reconstitute the phase relationships of PAL-video signals. Aside from the expense of the double balanced modulator and the frequency doubler, as well as the tolerance problems relating to the necessary phase adjustments, a shift from zero line offset to half line offset arises in the luminance component in the frequency range of the chrominance sub-carrier frequency if the original subcarrier is in a quarter line relationship. A reduction of horizontal resolution in one field is also observed. Disturbances due to color cros-talk arise as well.
It has previously been proposed--see British Pat. No. 1,015,179, Telefunken, to change signals following the standards of the PAL-color television system to those of a non-alternating system like the NTSC system but which is not adopted to the standardized NTSC system, in which special circuitry is provided to improve the color rendition even if phase errors may occur. As described, the color sub-carrier signal of one line, separated from the brightness signal, and the delayed color sub-carrier signal of the preceding line of substantially the same amplitude and phase--and also separated from the brightness signal--are added in a first combining stage and subtracted in another combining stage. The outputs from the two combining stages are added in an adder to the brightness signal from which the color sub-carrier signal has been removed. The negative polarity of the output signal in each alternate line is phase-shifted by 180.degree. by a phase shifter included in the circuit after the second combining stage, and operating at line frequency. The line-by-line alternation of the I-signal of the NTSC PAL-signal is thus eliminated. This signal--with reference to line alternation--is actually an NTSC signal.