Improvements in colour television transmitters

摘要

1,011,706. Colour television; transistor limiter circuits. COMPAGNIE FRANCAISE DE TELEVISION. May 2, 1963 [May 2, 1962], No. 17457/63. Headings H3T and H4F. In a colour television transmitter for use in a system, such as the SECAM system, in which a picture signal (i.e. chrominance signal), frequency modulates a wave and creates a signal frequency band which lies within the band of another picture signal (i.e. the luminance signal), there is provided means responsive to the components of said another signal within said band for establishing the relative amplitude of the frequency modulated wave and said components at such a level that the components do not introduce distortions when the wave is demodulated at a receiver. In one way of carrying out the invention, Fig. 2, the luminance signal, before combination with the frequency modulated wave, is passed through a transistor circuit 11 associated with a diode limiter 16. The emitter circuit includes a network 14, 15 series resonant at the frequency of the frequency modulated wave and functions to provide frequency emphasis over the band of the luminance signal which is common with the frequency modulated wave. The collector output circuit includes a network 141, 151 also resonant at the frequency of the wave and provides de-emphasis over the common band. For low amplitudes of luminance signal, where diode 16 is conducting, the gain of the circuit is constant throughout the luminance band, but for higher amplitudes of signal, such as may cause interference and distortion of the frequency modulated wave, diode 16 cuts off to limit the signals over the common band. In another way of carrying out the invention, Fig. 3, a band of the luminance signal 20 corresponding to the frequency modulated wave is selected by a filter 21 and applied via an amplifier 22 to a rectifier 23 feeding a low-pass filter 28. The resulting signal controls an amplitude modulator 24 through which the frequency modulated wave 26 is passed before it is combined with the luminance signal in a mixer 25. The modulation factor is selected so that the frequency modulated wave is always of sufficient amplitude relative to the luminance signal. A threshold circuit may be included between filter 28 and modulator 26 so that modulation operates only above a predetermined luminance level.