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The ICC5 was unusual in this (amongst other) department/s, as to enable the text (and OSG) or external RGB inputs via the scart to be brightness and contrast adjusted by the video processor IC, the RGB was first converted to YUV internally (matrix 1), then after internal processing was converted back to RGB in matrix 2 to drive the tube base output stages in the usual way and consequently the cathodes of the crt itself.
this IC could handle only two RGB input channels, ie. a two into one switch, the scart RGB input took priority, there was no capability to output RGB to the scart.