Control circuit for mixing two video signals

摘要

A mixer control circuit generates content control signals which are used by a mixer circuit to control the content of an output signal. The output signal will include either an analog signal, a digital signal or a mixture of the analog and digital signals. The level of a digital content control signal corresponds to the percentage of the output signal which includes the digital signal. The level of an analog content control signal corresponds to the percentage of the output signal which includes the analog signal. When the output signal includes a mixture of the analog and digital signals, a differential pair and an external control voltage are used to specify the percentage of each signal to be included within the output signal. During a horizontal blanking period, when the signals are being mixed, the differential pair and the external control voltage are bypassed and only the analog signal is included within the output signal. When the output signal is to include only one of the signals, either analog or digital, the differential pair and the external control voltage are also bypassed. A switching network is used to control the generation of the content control signals. The switching network is controlled by a control logic circuit in response to the levels of two control signals and a horizontal blanking signal.