Keene Retail Hints & Tips Library

GPI Trigger - what is it?

Many (but not all) edit controllers, titlers and video mixers are equipped with a socket labelled 'GPI', or 'control'. GPI stands for 'General Purpose Interface'. It is a commonly adopted standard which provides the means for one piece of equipment to control another, albeit in a rather limited fashion.

Most pieces of equipment will, when they receive a GPI trigger pulse, behave exactly as if the 'play' or 'take' button has been pressed. For example, a titler could be programmed to automatically display a particular graphic upon receipt of a trigger from a compatible edit controller. Similarly a video mixer could perform a particular A/B dissolve or effect when triggered.

The socket used for the GPI trigger is usually (but not always) a 3.5mm stereo jack socket. If both pieces of equipment use the same socket then a simple 3.5mm jack to jack cable will provide the connection. If one of the connections is different it usually means contacting the manufacturer to obtain a dedicated cable.

If you're reasonably handy with a soldering iron and your equipment uses a 3.5mm jack socket then you can experiment with GPI quite easily. Take a 3.5mm stereo jack plug and simply connect one end of a piece of wire to the centre tip and another wire to the ground terminals. Do NOT connect anything to the middle (sleeve) connection. Plug the jack into the GPI socket of your equipment. Now, momentarily shorting the other ends of the wires should cause the equipment to behave exactly as if it has received a trigger pulse from another piece of equipment. For ease of use you could connect the other ends of the wires to a controller switch, either a push button type or even a footswitch, so long as the switch contacts are normally 'open'.

Important - don't apply any voltage to the wires and don't leave them permanently shorted. Not only could it invalidate any warranty, it could all go up in smoke, and remember - if it does - we really don't want to know!

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More about edit controllers

Normal video editing is the process of selecting clips or scenes you want from your source tape(s), and re-recording these scenes (in your chosen sequence) on a destination tape.

Edit controllers help you do this by controlling the functions of the machine that plays the source tape (the source machine, a camcorder or one or more VCRs) and the machine that records your edited video (the destination machine, a VCR). Edit controllers enable control of play, FF, rewind, record, pause etc of both source and destination machines from a single console, or keyboard in the case of PC based versions.

Edit controllers may also be capable of performing other tasks, but it is the control functions we are interested in here. How do they do it?

Dealing with the destination machine (VCR) first, all(?) VCRs come with an IR remote with which you can control its operation. Similarly, the edit controller comes with some sort of IR wand through which it sends the appropriate signals to the VCR.

Now the source machine, which is where the problems seem to arise.

If the source machine is a camcorder

(1) It must have 'controllability' built in:
The most common methods of control are hard wire LANC, hard wire 5-pin, or IR
* If the camcorder has a small jack-type socket marked LANC or CONTROL L, then it is LANC controllable.
* If the camcorder has a small multi (usually 5) pin socket marked EDIT, then it is 5-pin controllable. Do not confuse this with the button marked 'edit' or the jack-type socket marked 'remote' on many JVC/Ferguson camcorders - these cannot usually be used by an edit controller.
* If the camcorder came with an IR remote control, then it is IR controllable.
* If the camcorder has none of these, then forget about using an edit controller - choose another method of editing.
(2) The edit controller must be compatible with the camcorder:
Not all editors offer the full range of source machine control protocols. Most come with LANC and 5-pin, fewer offer IR

If the source machine is a VCR:

(1) The same issues of controllability arise as with camcorders. Most VCRs are IR controllable.
(2) The same issues of edit controller compatibility also arise. In addition, it should be noted that some edit controllers will only work satisfactorily with a camcorder as source device.

To summarize: To use an edit controller, both the source and destination machines must be controllable, and the edit controller must be compatible with both.

Notes on different methods of camcorder control

Camcorder control protocol	Notes
Infra-red (IR)	High compatibility - almost all camcorders have IR control, but overall accuracy is poor
LANC	Found on most Sony, Canon and compatible camcorders. Accuracy is good
5 pin (or 11 pin)	Found on most Panasonic and compatible camcorders. Accuracy is good
LANC plus RCTC timecode	Excellent accuracy
5 pin plus VITC timecode	Excellent accuracy, but some Panasonic camcorders need an external box to decode VITC

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The PROs and CONs of computer-based editing systems

If you're thinking of getting into video editing, and you don't know whether to go for a system based on your computer or to get a stand alone editor, the following brief rundown of computer-based systems might help:

Pros Cheap - basic editing package starts at £49.99 - much less than stand alone editors. Expandability - you can add genlock & overlay cards at a later date. Storage - you can save your edit decision lists on disk and come back to them at any time in the future (ideal for wedding videographers etc.) It looks clever - a good way to impress your friends (if you know what you're doing!)

Cons Physical problems - you need your camcorder, VCR, TV & computer to be near to each other. Can get expensive with add-ons if you get bitten by the bug Complexity - MS-DOS and Windows have been known to make grown men cry, even without a camcorder connected to the computer. Depreciation - computers lose resale value at an alarming rate

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About video editing

Editing is the process of removing unwanted scenes and arranging footage into the desired order. The dramatic impact of a film depends largely on the arrangement of scenes, and this is why there is a Hollywood 'Oscar' specifically for editing. Proper editing is just as important for your home videos.

It is impossible to edit video by splicing the tape in the same manner as cine film. Rather, it is done by producing another copy of the original, but removing or changing scenes in the process. If desired, titles and other effects can be added at this stage.

There are several ways of achieving the same end result:

• It is possible to shoot your footage very carefully so that the end result when played back is exactly how you want it. This is called 'in camera editing'. It is very difficult to do, as it allows for no corrections or re-takes and means very careful alignment before each section is shot.
• Editing can be done manually, simply by connecting your play VCR (or camcorder) to a record VCR and changing from record to record pause at the scenes you want to remove. This is assembly editing at its most basic. Problems usually occur with the timing of each sequence, and sometimes you get a 'snowstorm' in between each scene.
• Editing can be done manually with the help of a processor. This will enable you to fade the footage out at the end of one sequence, and fade back in at the start of another. This gives a smooth transition from one scene to another, and is often more pleasing to the viewer than the 'jump cut' approach of the previous method. Some video mixers will even make it possible to fade and dissolve moving footage.
• You can use an editing machine. Such a machine lets you run through your tape and select the scenes you want, then copies only these scenes to the destination machine. It also makes life much easier by operating both play and record VCRs for you. The end results will be more accurate, and if you want to produce more than one copy, you can do so using the original (master) tape each time.
  Processors are sometimes referred to as editors, but this is incorrect. Unlike an editing machine, a processor does not 'control' any of the equipment, but needs you to manually operate it.
  The method of connecting an edit controller is worth mentioning here, as not all camcorders and VCRs are compatible. The editing machine usually controls the source machine (camcorder or VCR) by a single cable. For most 8mm/Hi-8 machines the connection point is labelled as 'control L' or 'LANC', and for Panasonic VHS/VHS-C machines it is a small circular socket labelled as 'edit'. With this connection made, the editing machine can control all of the source machines functions: play, pause, ffwd, rewind etc.
  If your camcorder does not have any edit control connection or an infrared remote control, then editing is best done in one of the other ways described previously. The destination machine is usually controlled by infrared remote, and so almost any VCR can be used without any problems.

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Video editing - What is timecode?

A timecode is a unique electronically generated number that is assigned to each individual frame of the film. Editing becomes easier with equipment that can make use of timecode because the process becomes one of 'go from frame number x to frame number y'.
For camcorders there are two main timecode systems in use: Sony's RCTC commonly called 'arctic', and Panasonic's VITC, commonly called 'vitsee'. Each system is unique and not usually compatible with any other manufacturers equipment.
It doesn't matter if your equipment doesn't use timecode, you can still use an edit controller to give accurate results. All VCRs identify the start of each frame with a sync pulse. Many edit controllers can count these sync pulses and display them alongside the normal hours, minutes and seconds display to give an additional 0 to 25 frame count as the tape is running. It is not as 'intelligent' as timecode in that it can be fooled by stretched or damaged tape, or tape slip. That said, if both tape and equipment are OK then it can be used to give very accurate results.
A word on accuracy - the holy grail of all video editors appears to be 'single frame accurate'. Production of an edit that is accurate to one frame is not wholly down to the edit controller used, but is also down to the equipment used for source and destination. Different VCRs have different response times to instructions they receive. A good edit controller will allow you to compensate for these response times, but not all equipment will operate with 100% consistency. When considering the accuracy of your editing, remember that just one second of film contains 25 frames.

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More about timecode

In order for an edit controller to do its job effectively it must
a) be able to control the camcorder (as discussed in a previous 'tip')
b) know whereabouts on the tape each scene to be edited is located.

It is normal when commencing an edit to rewind the tape to the start and zero the tape counter. This gives a known start point and, as the tape progresses, the edit controller can count the sync pulses and calculate the elapsed time. The problem with this method is that it is prone to tape slip. Try it yourself. If you rewind your tape, zero the counter, then fast forward the tape all the way to the end and then rewind it again, does the counter still read zero? Not usually. This is because the tape has slipped slightly during the process and the counter will have missed a few sync pulses along the way. It follows that this is a compound error; ie an edit sequence which has scene one at the start of the tape, scene two at the end, scene three from the start again, and so on will have a greater degree of error than one in which the scenes are sequential.

Enter timecode - this is essentially an electronic tagging system which assigns a unique number to each frame on the tape. This improves accuracy because the edit controller is no longer counting sequentially, but simply reading the number that is already there.

There are two common domestic types of timecode: RCTC and VITC.
VITC (pronounced 'vitsee') stands for Vertical Interval Time Code and is found mostly on Panasonic equipment. It is recorded in digital code as part of the overall video signal by making use of some of the unused 'space' available within each frame. Because it is an integral part of the video signal it cannot be dubbed on to a tape at a later stage but must be introduced at time of recording.
RCTC (pronounced 'arctic') stands for Rewriteable Consumer Time Code and is found mostly on Sony and Canon equipment. This timecode is written on to a spare part of the tape itself and is not integral to the picture. This makes it easy to add timecode to previously recorded material.

A word of warning for users of vision mixers - these usually remove all the non-visible parts of the video signals, including the timecode, before mixing the signals together.