The principles for preparing technical videotapes are not very different from those for standard scientific papers. State the purpose of the research or system. Present the concepts and principles upon which the work is based. Explain how each principle is reflected in the system. Finally, integrate and summarize what was presented, perhaps with vignettes of previous scenes. Credit those who contributed but don't overdo it. Consider what copyright to include.

A common problem with technical videotapes is the difficulty the audience has in establishing context. Explain the who, what, where, why questions for this work. Identify the problem or issues being addressed. Clarify any unusual assumptions or terminology used in the presentation.

Realize that the video medium is different from either a lecture or demonstration. The pacing of a videotape presentation must be appropriate for concentrated presentation through a TV monitor. The audience will see only what the camera records and only at the pace determined by editing. Too slow a pace is as common as too fast. A recording of a live demo will appear too slow. A large number of jump cuts (abrupt change of image) tends to create too fast a pace.

The exposition style of your videotape presentation will greatly affect its impact. Utilize the multiple senses that are available simultaneously in videotape. The audio can describe what the video shows. Always explain (briefly perhaps) what is about to happen or what is most interesting. Try to combine explanations with something interesting in order to avoid alternating between talking head and action. Tell the viewer where to look and what to look for. Direct the camera operator as part of the change in screen image, such as ``if you zoom in on the top right corner of the display'' to help the viewer orient themselves. Display screens have few natural navigation aids! Make your point once effectively; avoid theme and variations that drag or are repetitious.

Narrate by speaking instead of reading; perhaps use professional actors. Keep the audio topic in synch with the visual activities. Avoid visual distractions, such as idly moving the mouse. Minimize the use of talking head. Consider a voice-over to avoid awkward pauses while you think what to do or say next. Seek variety of image; switch between face, screen, hands or text ``slides.'' If there is no audio, then supply background music (be cautious about copyright infringement by commercial use of your videotape).

Rehearse! Rehearse! Rehearse! Go find someone who doesn't understand what you do, sit them down and give your demo to them before the camera arrives. It's good practice in speaking and helps to clarify the delivery of your ideas. Very complex demos can be done in the 3-person style: after the demo is scripted, one person reads, one person drives the system and the third watches to confirm synchronicity.

Use the best quality camera possible. Computer displays, especially ones with 1,000 or more lines, are very high bandwidth. All video is not created equal. Use the highest bandwidth you can get: 1-inch, BetaCam, 3/4 inch U-matic, home video in decreasing bandwidth. Maintain the quality of the original recording medium throughout the editing process. Shoot black and white displays with a black and white camera or only one gun of a color camera, usually green.

Include color bars and tone at the beginning of the tape. Check that video levels are within standard settings. Some video projectors will not tolerate out of range levels or compensate for too low levels. Ensure that there is adequate lighting. If shooting from the display screen, darken the room to enhance contrast. Carefully set up camera geometry to avoid rhomboidal windows from the curved face of the display. Focus the camera for the closest zoom. Check that sound levels are adequate; avoid background noise, fans especially.

Ensure that the computer display refresh rate is compatible or synchronized with the video camera rate of 30 frames per second. Many displays run at odd rates to avoid flicker from fluorescent lighting.

Rehearse the camera choreography. Avoid sotto voce instructions to the camera operator. On occasion, tactfully zoom back to refresh the audience as to the new screen layout.

Special effects can contribute substantially to the communication. Subtitles can be effective as a summary, review or focus mechanism. Fade to black or cross fade rather than jump cuts between scenes. Multiple cameras and split screens permit several views of concurrent actions. For example, a scene might show the display screen with keyboard and mouse superimposed.

If you videotape with an NTSC encoder for color displays, then limit color saturation to 75 to 80 percent and choose hues carefully to avoid exceeding the NTSC color bandwidth.