Means to Synchronize Multiple Cathode Ray Tube Controllers

Means to Synchronize Multiple Cathode Ray Tube Controllers

       Disclosed is a means of automatically synchronizing
multiple Cathode Ray Tube Controllers (CRTCs) and holding them
synchronized as is necessary in some multimedia display systems.

      Synchronization may be needed typically where one CRTC is
controlling the display of graphics data and another is controlling
the display of image data or another is controlling the display of
something else.  Some switching device controls what is actually
being displayed on each part of the screen.  In order for these
different pictures to line up correctly and be positioned stably, it
is essential that all the CRTCs are synchronized so that they all
start the display of each line and frame at the same time.

      The basic new technique is to designate one CRTC to be the
master CRTC.  The blanking signals of the other CRTCs, called slave
CRTCs, are compared with the blanking generated by the master CRTC
and, if they are different, the clock into that slave CRTC is
stopped.  It runs only when its blanking signal is in the same state
as that of the master. It is clear that once the CRTCs are
synchronized, the slave CRTC's clock will never be stopped.  This is
a stable situation and the CRTCs will then stay synchronized.

      The technique also guarantees that synchronization will be
achieved from all starting points.  The situation is analogous to
trying to line up two identical cyclically repeating patterns which
are initially not in phase. Clearly they can be brought into phase by
shifting one pattern relative to the other.  This is exactly what
stopping the clock of one of the CRTCs will do.  Because the blanking
signal is active for several lines during vertical flyback, vertical
synchronization will also be achieved. Any misalignment of vertical
blanki...