| 摘要 |
A known form of SECAM subcarrier demodulator employs a multi-stage limiter-amplifier (15) to supply two inputs (L, P) to a signal multiplier (19), one input (L) being supplied directly and the other input (P) being supplied via a phase shifter (17) which imparts a phase shift of 90 DEG C at one of the SECAM subcarrier resting frequencies. One of the amplifier stages, preferably the final amplifier stage (15C), of the limiter-amplifier (15) is provided according to the invention, such as by the provision of positive feedback (via 68), with a hysteresis characteristic ensuring development of a limiter output having a rectangular waveshape with rise and fall times of substantial constancy, independent of input signal amplitude variations. Negative DC feedback (through 71) from the penultimate amplifier stage (15B) to the input amplifier stage (15A) of the limiter-amplifier (15) can also be provided to assure desired symmetry of limiting action. Color difference signal output is obtained by low pass filtering (21) of the products of multiplication of the respective signal inputs. |
