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AN009 Application Note

Understanding Stop Clock High Speed Timing
?? J. Styer, Oct 22, 2001, revised Sep 14, 2005


Applies to products: VSC300, VSC400, VT300, VT400

The models VSC300, VSC400, VT300 and VT400 Video Stop Clocks and Video Timers have high resolution timing capability, as precise as 1/10,000th of a second, however, the video display in North America is only updated at about 1/30 sec (1/25 sec in most of Europe). This 1/30 sec limitation is due to the fact that the video system only has 30 frames per second (30 full pictures are drawn each second). When the counter is operating at 1/100, 1/1,000 or 1/10,000 sec, this means that there will be a number of counts between each frame of video when recorded on tape or disk.

Each unit provides 3 remote input lines that can be used to control high speed timing operations. Due to de-bouncing (noise filtering) and human imprecision, the front panel switches cannot be used for extremely precise timing. The front panel switches can only provide frame (1/30 sec) resolution timing. The 3 remote inputs can have the noise filtering set to one of several speeds or turned off completely to match the display precision to the application. In addition, high performance external filtering could be provided in extremely noisy environments with long cable runs.

Since the video screen only updates once every 1/30 sec, the only way to effectively use the high precision of these units is when the count is stopped or frozen. Once the count is stopped or frozen, you can easily read the full resolution value. Numerous modes and options have been included to make this as flexible as possible to match the application. The units have the ability to stop, start or freeze the count at any time. Special modes which can freeze the display, reset the counter, and continue counting without losing any time make the job much easier. Once the display is read, it can be once again updated to the current count. This is just one simple example of the extensive programmable nature of the these products.

They can also be used as an event counter as well with quite high speed capability. As always, if any of these products don’t quite meet your requirements, let us know, we may already have the solution available. We would be happy to quote on a customized version as well. The VSC300, VSC400, VT300 and VT400 are also available as board level products.