file size guidelines?

[henhao]

I'm wondering if anybody knows or could provide a link to guidelines that correlate output file size to number of minutes of video, if one is aiming for a good quality encoding.

(In other words, don't need perfection, but also don't want anything noticeably shoddy.)

I am using AutoGK, with the xvid codec. and I have been using the predefined size option when encoding, in hopes of fitting a number of videos on one DVD.

So far I have encoded a few 50 minute documentaries, each time choosing the 746MB option. Is that overkill?

If anybody could help me get a better sense for this, I'd appreciate it.

[guns1inger]

There is no absolute answer. Noisy video or video with a lot of action needs more bitrate (and therefore a larger file size) than very clean video of a talking head. Higher resolution often needs greater bitrate just to cover the extra real estate needed.

Also, what will you be watching the videos back on ? You can get away with a lower bitrate for CRT viewing that you will if you are upscaling to an LCD TV.

I recently encoded a set of DVDs for series shot in the late 80's. The DVDs had been mastered from broadcast quality tape masters, but had been badly filtered during the original encoding, making them overly clean. Because of this I was able to encode happily to 700 MB per 48 minute episode without losing quality. Had these been higher quality DVDs to begin with, I would have taken a different approach.

It is also worth doing test encodes at various bitrates and also using quality based settings. Start at around 70% in AutoGK and work up. The higher the quality, the bigger the file, but you know that each file will get what it needs, rather than being squeezed into an arbitrary filesize.

[jagabo]

Use quality based encoding. You always get exactly the quality you specify. The file turns out whatever size is necessary to deliver that quality.