On screen display processor

摘要

A decoder of an encoded video signal representing a video image includes an apparatus for scrolling an on screen display of closed caption information that is included in the encoded video signal. The apparatus decodes the closed caption data from the encoded video signal as a sequence of lines of on screen display data. An on screen display processor alternately stores successive groups of lines of the sequence of lines in a first memory buffer and in a second memory buffer where each successive group of lines is shifted by at least one line with respect to the previous group of lines. A display processor alternately displays the on screen display data from the first memory buffer and the second memory buffer synchronized with the storage into the buffers whereby the most recently stored group of lines is displayed. Rather than using first and second buffers, lines of on screen display data may be stored in a linked list of memory storage regions. On screen display information stored in the linked list is transferred to the display processor to produce the on screen display. To scroll the on screen display, a memory region is added to one end of the linked list and a memory region is dropped from the other end of the linked list where the added memory region stores a successive line of on screen display data from the sequence of lines of on screen display data.