In recent years, television receivers having semiconductor memories and using the digital signal processing technique for reproducing high-quality pictures have been developed. This TV receiver has a delay element for delaying TV signals by one frame period, for example, and a comb filter. Using the correlation between TV signals of two adjacent frames, the TV receiver separates a luminance signal and a chrominance signal from the TV signal. Further, the TV receiver has an interpolation circuit for producing a video signal to be inserted between adjacent scan lines using the correlation between two video signals of adjacent fields. In this TV receiver, the number of scan lines is increased to twice in one field period. This TV receiver is described in "Nikkei Electronics", July 1, 1985, P. 195 to 218, and "A Motion-Adaptive High-Definition Converter for NTSC Color TV Signals", SMPTE Journal, May 1984 writter by M. Achiha, et al., for example. Although the comb filter and the interpolation circuit of this TV receiver properly generate a video signal and a chrominance signal for a stationary picture signal as well known, the comb filter and the interpolation circuit generate a disturbance signal for a moving picture signal. Therefore, a movement detection circuit is used for detecting the difference signal of two video signals between frames to detect the movement of the picture. And the stationary picture signal undergoes temporal and spatial processing in a frame comb filter and an inter-field line interpolation circuit. The moving picture signal undergoes spatial processing of the signal between scan lines within one field.
The above described comb filter and interpolation circuit properly function with respect to a TV signal (hereafter referred to as a standard signal) whose chrominance subcarrier signal frequency f.sub.SC, horizontal, scan signal frequency f.sub.H and vertical scan signal frequency f.sub.V accurately agree with predetermined frequency relationship. However, the comb filter and the interpolation circuit do not function properly with respect to a TV signal (hereafter referred to as a nonstandard signal) such as the signal of the home VTR or the personal computer whose chrominance subcarrier signal frequency f.sub.SC, horizontal scan signal frequency f.sub.H and vertical scan signal frequency f.sub.V do not agree with the predetermined frequency relationship.
For example, the chrominance subcarrier signal frequency f.sub.SC and the horizontal scan signal frequency f.sub.H are so defined as to satisfy the relation: ##EQU1## And the horizontal scan signal frequency f.sub.H and the vertical scan signal frequency f.sub.V are so defined as to satisfy the relation: ##EQU2## The frequency interleave relationship holds true between the luminance signal Y and the chrominance signal C. The phase of the chrominance subcarrier signal is inverted between signals one frame period apart. Using this phase difference in the frame comb circuit, the luminance signal is derived from the sum of two TV signals between frames and the chrominance signal is derived from the difference of two signals.
In the nonstandard signal whose frequencies f.sub.SC, f.sub.H and f.sub.V do not satisfy the equations (1) and (2), however, the frequency interleave relation does not hold true. Even in a stationary picture signal, therefore, the luminance signal and the chrominance signal are not accurately separated from the TV signal. When it is judged that the picture is a stationary picture, the picture quality is significantly degraded. In the conventional TV receiver, it is thus difficult to apply suitable processing to a nonstandard signal.