Hello Javier
By 'edit' do you mean 'mark an in and out and put in the timeline', or do you mean 'apply filter or motion'?
If it's the former, you should not need to render the clip unless the sequence settings are not the same as the clip. Check your capture settings and sequence settings.
If the render is needed because of a filter or re-sizing, sometimes a render will be required, depending on your system specs, codec used and realtime setup. With RT on, and the framerate and resolution set to 'dynamic' in the timeline, you can achieve a lot of realtime previews depending on the above.
This is a pretty big question - you can find a lot of the answers in the manual (it's pretty good) and you may also find the 'basics' forum on the Cow helps.
Ben
Editec Broadcast Editing Ltd
EVS & FCP specialists for live broadcast.
OB Server 1 HD - Mobile FCP editing done right.
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