Encode at same QF or at maximum bitrate, advice please.

[madhatter300871]

Hi all

I have finally moved over to blu-ray this year and am now backing up some of my movies.

I resize to 720p and encode with x264. I just set the level, the preset and the tune and let the codec take care of the other values. So far I'm very pleased and will probably at some point in the future delve into setting more of the settings manually, but for now its fine.

And so to my question : Is there any merit in encoding the resized 720p video with a bits/pixel/frame (QF) value higher then the original 1080p value? I will be burning to DVD-5 or DVD-9 so I have a total finel file size of 7.9GB (ish) so I can easily set a higher QF.

Just wondering if it's worth it. Is anything higher than the original QF just wasted in that a higher bitrate will not magically introduce higher detail. Is the QF relative ?

Hope this makes sense and look forward to a reply or two.

Thanks.

[nurbs]

QF is mostly useless because whether or not you get good quality at a certain QF depends on
1) format used
2) encoder used
3) source complexity
4) resolution
5) encoding settings
and probably other stuff I can't think of right now. If you change one of those you won't get the same quality at the same QF.

Yes, there is a point where it makes sense to downscale in order to get better quality, but that point is usually at a lower bitrate (higher quantizer) then most people expect (at least with x264) and QF doesn't really help deciding when. Most content will look good at 720p on a DVD-5. For some stuff (very long, very noisy, ect.) you'd be better off with a DVD-9, but DVD 5 will probably still be acceptable.

[madhatter300871]

Thanks for the reply.

OK. I have been scouring the net and looking at other posts here and bitrates for resized video are not a simple factor of bitrates used on the original. So if you scale a video by half it does not mean that you simply reduce the bitrate by half and anything else is wasted.

Would I be right in saying, then, I may as well always use the maximum bitrate that I can based on the filesize I want, or encode using a CRF (general consensus seems to be CFF 16 - 18 gives almost transparant quality) if I dont care on file size ? Anything else is simply a matter of perceived visual quality as opposed to some easily measureable quantity and simple mathmatics just dont work here.

Thanks.

[nurbs]

Yes, just use the bitrate to fit your medium or, if you don't care about the final size too much, do a CRF encode. Most people here would say CRF 18 to 26 is the range you should pick from, but you can go lower if you want to be on the safe side. Generally you can pick a higher CRF for higher resolutions without there being noticeable quality loss, because the artifacts are smaller (compared to the whole picture). Filesize will vary a lot depending on source though.