I2C-bus controlled YUV/RGB switch
TDA8443A When the synchronization pulse is over, the current source is switched off and the voltage at pin 24 will rise to its higher level. Because of the time constant at pin 24, the restoration will take some microseconds. The voltage at pin 24 is also sensed internally and at the time it is between 0.456VP and 0.544VP, a time pulse is generated and used for the clamping action.
Input clamps The R, G, B respectively (R−Y), Y and (B−Y) video signals are AC-coupled to the IC where they are clamped on the black level. The timing information for this clamping action is derived from the associated synchronization signal SYNC, which could also consist of the composite video information signal CVBS. The syncsignal is AC-coupled to the IC where it is clamped on top-sync level, information obtained from this action is used to generate the clamp pulses.
2. Using a sandcastle pulse (external clamping) If an associated sandcastle pulse is available, it can also be used as a clamping pulse. In this event the sandcastle pulse should be connected to pin 24, the top of the clamping pulse should be between 0.544VP and 0.456VP. The timing of the internal clamping pulse will be equal to the timing of the higher part of the sandcastle pulse. If the sync signal is also connected, the current sink will also become active during the synchronization pulses. This means that the sandcastle pulse should be connected to pin 24 via a 1 kΩ dropping resistor. In this event only the sandcastle pulse at pin 24 will be influenced during sync pulses, but the sandcastle pulse at the sandcastle source will be unchanged.
The clamp pulses can be generated in two ways: 1. Using the sync information (internal clamping) The sync information is clamped on top-sync and the information obtained from this action is used to switch an internal current source at pin 24. Pin 24 should be connected to VP via a 4.7 kΩ resistor, and a 1 nF capacitor to ground. During video scan the voltage at pin 24 will be HIGH (equals positive supply voltage). During the synchronization pulses the voltage at pin 24 will drop to zero because of the current sink (2.5 mA).
V1 = 0.544 V P V2 = 0.456 VP
9.4 kΩ V1 1.8 kΩ V2
clamp pulse signal
pin 24 ON when current sync = 2.5 mA OFF when current sync = 0 mA MEA623
Tolerance on V1 and V2 is given by ∆R/R and ∆VP/VP. The diffusion process gives ∆R/R (max) = 1.5%.
Fig.7 Clamping circuit.
1995 Mar 07