The Mac's monitor has
a refresh rate measured in Mz. This rate, or frequency,
is the number of times per second that the image on the monitor
If the image being drawn
is changed mid way through the monitor's refresh, the top half
of the monitor will have the old image, and the bottom half will
have the new image. This causes a noticable tearing effect.
To get around this problem
you must wait until the monitor has finished drawing the image
and then quickly alter it before it begins the next monitor refresh.
The pause between the montior refreshes is called the vertical
blank or VBL for short.
Synchronising to the VBL
can cause lower framerates as the TNT Basic's Draw
Frame command must wait for the monitor to finish drawing
before it can perform the copy. If the tearing is acceptable
then turning VBL syncing off allows higher frame rates to be
Digitally driven monitors
such as PowerBook screens and Apple Studio Displays do not have
a VBL and so VBL syncing is permanetly off when drawing to these.
VBL syncing defaults to