What is a GPI?

GPI stands for General Purpose Interface. There are two types of GPI connections - outputs and inputs...

1. A GPI output on a piece of video control equipment such as an edit controller. It usually takes the form of an open-circuit relay contact or possibly an open-collector transistor or a floating transistor in an opto-coupler. These types of output are known as "contact closure" and usually only activate for a brief period, say 1/100th to 1/10th of a second, from a single stimulus. In some equipment they can also be latching and then need two stimuli - one to turn on and one to turn off.

2. A GPI input on a piece of video effects equipment such as a vision mixer or a video title generator. In this case it will usually be an input to a logic gate or perhaps one end of a relay coil. Internally, the circuitry will commence some sort of process when the pulsed contact closure occurs. Depending on the equipment and the process to be controlled, it may perform an action then reset and wait for another stimulus, toggle (or reverse) each time it is pulsed, or it may latch.

It is reasonable to say that almost any GPI output will connect to almost any GPI input. I say almost because there are bound to be exceptions, but I haven't found one yet. That is not to say that the desired action will happen automatically, just that in most, if not all cases, no damage is likely. Generally, the voltage present at a GPI input or output is most commonly 5V at a short-circuit current of 20mA or less, but could be higher in older equipment.

Q. What's a GPI used for?
A. Almost anything! It's truly general purpose. Let's say you have an edit controller such as the VideoTech Designs VEC501, and one of their vision mixers, the VMX410. You have entered all the edit points for your two play VCR's and you want to do some slick cross-fades during the actual editing. BUT, the sound was less than perfect and you need to fade some music in and out to sweeten the sound track.

Well, unless you've got two brains and four hands you're going to find it impossible, especially if you have to do it more than once. Solution... use the GPI output of the edit controller to trigger the cross-fades on the vision mixer. This will leave your hands and ears free to do the sound mixing while your eyes can concentrate on the image.

The GPI output/s may be separately programmed to occur at exact times or may occur at the same time as the actual edit point. It depends what you want to use them for. They could trigger an effect, start a VCR, turn on a light, "press" a key on a computer keyboard, sound a tone, trigger a titler to start rolling the credits, etc, etc. The list is endless.

Where appropriate, all professional editing equipment has GPI's as does the better domestic editing gear.

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