| 摘要 |
<p>A signal matching signal substitution system includes a tuner (24A, 24B) for selectively receiving signals in respective television channels with an input gain control (42) for controlling the signal output level of the tuner (24A, 24B). The system substitutes television signals in a substitute channel in lieu of normal signals in a normal channel selected by a viewer. The system has a component (228) for determining the input signal level of currently received signals in a currently received channel and generating a current amplitude signal. A set point generator (56, 232A) is responsive to the current amplitude signal corresponding to the normal channel most currently received for providing a set point for the output signal level. A gain contol signal generator (63, 86) is responsive to the set point and the signal output level for generating a gain control signal maintaining the output level at the set point. The set point may be set at a predetermined fixed level when the current amplitude signal corresponding to the most current normal channel is relatively high and is set at progressively lower levels when the current amplitude signal corresponding to the most current normal channel is below a transition level. The set point at the progressively lower levels is noise limited to provide substantially equal signal to noise ratios in the signal output of the tunner (24A, 24B) in both the normal and the substitute channels upon signal substitution. The set point is gain limited to the maximum signal output achievable by the gain control (42) for the normal channel.</p> |
