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Please inhale deeply, put your extremities in some strange positions, close your eyes and explore your inner light... Umm, no, that's Yoga or something. What I meant, is that you should try to envision the meaning of the words "RAM preview".
Naturally it, by and large, means that your frames will be rendered to your system's physical memory. Regardless of how small these may appear, they will still consume RAM (or memory, to use layman's terms), ultimately filling it until no more fits into it. Picture it like a bowl, where your frames are drops of liquid that fill it until no more fit and it runs over. The running over could be symbolic of the disk cache (if you use it), but regardless of this, you will understand that each new drop that enters the bowl, pushes out another one - the overall available volume does not change. Now there are a few things to consider in addition: The size of the drops (the resolution of your footage) and their density (their color depth). If you increase both, the bowl will fill more quickly and the amount of drops being pushed out also increases.
So in short: depending on your system configuration, you will never be able to RAM preview a clip or composition in its full length. Some rough figures of RAM preview lengths for orientation (all based on my own daily use of PAL resolution footage, 720x576, 25fps, 2GB RAM)
Full res, 8bpc --> ca. 16 secs
Half res, 8bpc --> ca. 65 secs
Full res, 32bpc --> ca. 3 secs
Half res, 32bpc --> ca. 17 secs
As you can clearly see, even though the progression is not linear (more like a square function, actually), you may not be able to preview more than 2.5 minutes at any given time even at quarter res.
Therefore you should look into previewing by other ways. If you e.g. use the disk cache and do multiple RAM previews of consecutive segments, at some point you will have buffered the entire timeline and could be able to play it back in realtime using the spacebar. That hinges however on the speed of your disks and possibly any lag that your graphics card may introduce. It also requires your composition to be halfway "heavy", or otherwise AE will assume it doesn't need to cache on disk because it thinks it can calculate frames quicker than load them from disk.
Another alternative along the same lines might be generating preview clips. Nowhere near as intuitive and practical, but at least that way you get guaranteed framerates and durations. so it might be smart to create a quick preset based on Quicktime Sorenson 3 or Windows Media just for that purpose...
Thank you ever so much!! Very pedagogic explenation! So I was actually looking for something that i won't find. Really good to know that and to have tips on what to do! Thank you!