VBL Sync

Enables or disables vertical blank synchronisation

Syntax

VBL Sync On
VBL Sync Off

Description

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 is redrawn.

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 achieved.

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 on.


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