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The Lock Source allows selection of the input used as the basis for genlocking, picture in picture, overlay and underlay.

You can choose any input connector, completely independently of Program source or Preview source - however it only becomes active when the Lock method is set to anything other than 'Off', at which point the scaler's Output image and type are locked to the Lock source (and thus the Output image is no longer adjustable).

In general: the Lock source is the background when in PIP or Overlay mode; and the foreground when in Underlay mode.