| 摘要 |
The present invention relates to an improved demodulation system which improves the noise characteristics for the demodulation of a wideband television FM signal. An adapter for improving the C/N ratio is provided at the output of a bandpass filter, and the output of the adapter is applied to a discriminator which demodulates the FM signal through a limiter. The adapter is comprised of a variable bandpass filter, the center frequency and the bandwidth of which are controllable, and the variable bandpass filter is inserted in the FM signal path. The bandwidth of the variable bandpass filter is controlled so that it is wide when the input carrier level is high and is narrow when the input carrier level is low. The center frequency of the variable bandwidth filter is controlled so that it follows the instantaneous frequency change of the color sub carrier component (3.58 MHz) of the FM signal. A phase detector is coupled with the input and the output of the variable bandpass filter in order to derive the color sub-carrier component, and the output of the phase detector controls the variable bandpass filter through a narrow-band bandpass filter. This system is an improvement over the prior art in that it will follow the instantaneous frequency change of the FM input signal to the system, and improve the impulse noise characteristics which reduces the demodulated waveform deformation. |
