A video data transfer control system allowing secure protection of video
data from unauthorized copying is disclosed. A scrambling key used to
scramble video data is stored in a key register and a new scrambling key
is stored in a register. At the timing of a vertical sync signal of the
video data, the scrambling key is changed to the new one. As a result,
the scrambling key can be frequently changed while transferring the video
data, achieving enhanced protection of the video data from unauthorized
copying.