| 摘要 |
A black and white detector circuit monitors a burst signal separated from a composite video signal in order to determine when the composite video signal includes color video information or black and white video information. An output signal is generated which is active when color information is included and inactive when black and white information is included. The burst signal will oscillate above and below a first threshold value during a burst period if color information is included, and will remain constant if black and white information is included. When the burst signal oscillates above and below the first threshold value for a predetermined period of time, an output signal is activated and will remain active until the burst signal remains constant during a burst period. A current source is enabled when the burst signal rises above the first threshold value. The current source builds up a first level of charge on a first capacitor during the burst period. A detecting circuit monitors the first level of charge to determine when it rises above a second threshold value. A second level of charge is built up on a second capacitor when the first level of charge is above the second threshold value. An output signal is activated when the second level of charge rises above a third threshold value, signalling that the composite video signal includes color video information and deactivated when the second level of charge falls below a fourth threshold level, signalling that the composite video signal includes black and white video information.
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