As the other posters have mentioned, the playback device will determine what the best output format is. If you are using a computer, then the available software will also factor in.
I don’t generally encourage people to use computer playback for live events unless they have specifically set up their system for video playback. Power management, virus scanners, wireless network notifications, etc. all have tendencies to pop up at the worst possible moment. If you are giving your client a file for them to use on their laptop, it gets even scarier. There are still a lot of laptops in the field that would struggle with smooth playback of 1920×1080 h.264-encoded HD.
If you must go this route, let me add to Alan’s advice: test the final media file on the show machine with a second monitor (matching the projector’s resolution) attached. If the media will be played from within a Powerpoint presentation, use WMV and test it from within PPT and make sure that the video driver doesn’t try to “help” by switching resolutions for video playback.
Also, it’s standard practice in the events industry to always run live backup with a switcher for any media playback. We’ll run the same video roll on two Beta decks, two DDRs, two computers, etc. This prevents a simple hardware or media failure from ruining your event and client relationship.
Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
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