| 摘要 |
A backporch soft-clamp circuit using a servo loop clamps the blank or DC level of a composite video signal to a known value without altering the other components of the signal. The values of the components of the composite video signal are determined by determining their amplitude with respect to the blank level. The backporch soft-clamp circuit sets the blank level to a known value for determining the true value of the components. An output composite video signal is generated which represents the input composite video signal with the blank or pedestal level set to a known DC level. Preferably, the DC level is set to two volts. A burst gate pulse representing the presence of a burst signal within the composite video signal is received by the circuit. During the burst period, the circuit soft clamps the blank level of the output signal to the appropriate level without altering the content of the burst signal. The DC level of the output signal is compared to the appropriate level by a comparator circuit. During the burst period, if the DC level of the output signal is less than the appropriate level, it is raised by a charging circuit. During the burst period, if the DC level of the output signal is greater than the appropriate level, it is lowered by a discharging circuit. During non-burst periods, when the burst signal is not present within the composite video signal, the charging and discharging circuits are disabled and the DC level of the output signal is maintained at a constant level.
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