Standard television signals such as NTSC, PAL for example, are usually formed of image signals to produce 60 or 50 fields per second. When the image signals of the television signal systems are displayed on a screen, there are some flickers observed on the displayed picture. So called flickerless television receivers with a provision of making the flickers less conspicuous have been on the market. In a typical conventional flickerless television receiver in FIG. 6 a tuning/detecting circuit 20 amplifies and detects the broadcasting signal. A double-speed signal generating circuit 21 converts the detected signal to image signals having the horizontal and vertical scanning frequencies thereof increased to two times the horizontal and vertical scanning frequencies of the inputted image signals. The converted image signals are referred to as double-speed image signals and the image displayed on a television receiver is referred to as double-speed image.
In the so called flickerless television receiver, the same image is displayed twice, in 1/60 sec or 1/50 sec repeatedly and then the number of fields per second is converted to 100 or 120 and vertical frequency is 100 Hz or 120 Hz, which is twice the ordinary frequency. When the double-speed image signals are displayed on the television receiver, flickers are reduced.
However, flickers of a television receiver inherently tend to be difficult to notice in certain environments. Therefore, in order to show clearly the advantages of flickerless television receivers, it would be desirable to make a direct comparison of displayed images by placing a flickerless television receiver and an ordinary television receiver side by side.
Regretfully, however, ordinary TV viewers do not have opportunities to watch a standard television receiver and a flickerless television receiver at the same time and to compare pictures displayed on respective screens to see the difference in picture quality. Rather, TV viewers usually tend to recognize the excellent picture quality or not so good picture quality just by watching the screen of a flickerless television receiver. Therefore, it is difficult for TV viewers to truly appreciate the excellent picture quality of the flickerless television receiver.