Minimum allowed bitrate

[mattias]

Why have a choice to set minimum allowed bitrate when the codec
uses less anyway?

When I'm encoding the 2nd pass the blue area goes much lower than
the white line on very low bitrate areas. The bitrate goes as low
as 130 kbps though I've set min to 400. This seems to make the
deviaton to go very much negative. Is this really correct?

[jeremymacmull]

Hey there,

The reply to your question is yes this is correct if the bit rate falls below your min setting its only cos nandub does not think it needs soo many bits for those scenes (ones with little movement or black bits) its perfectly alright cos nandub uses those bits it has saved in the next scenes if they need it and so increases the quality of the overall film!!

:cool:

[DigDub]

Correct me if I'm wrong, but I read somewhere on one of Doom 9's guides (Nandub Options Explained?) that Nandub will use the Min Allowed Bitrate even though the bitrate required for that frame falls below the Min Allowed Bitrate?

[LotionBoy]

Min bitrate is always honored by Nandub. It does not show up in the DeBug view or the output window, but nandub always bumps those values up to the min when encoding. So min. bitrate very much matters.

LotionBoy

[MxxCon]

so why always keep min bitrate at 270?
wouldn't it be better to set it to ~10?

[LotionBoy]

min bitrate should not be 270. Min. bitrate should never be lower than 300 or 350, because that will cause codec errors. High-pass is what is normally supposed to be set at 270. Why 270? I actually have no idea. this controls how the curve is scaled and sets a floor, so that all frames have at least a 270 as the bitrate in the curve. Setting this to the same value as the min bitrate (350) might actually work better, at least in terms of hitting file size dead on. If anyone knows why 270 is the value used for high-pass, please share.

LotionBoy

[mattias]

I've actually encoded several movies with perfect result using:

min kbps 0
high pass 0
low pass 9999

This helps me getting exact filesize which I found very hard
otherwise. Why not let the codec decide how much bits it needs
to encode a frame?

Example of movies which I got perfect using these settings are

The Matrix, Blade and more ...

[philippas]

What 9999!!!. There's no point go over 6000kbits for the low pass. Most of the time less than 4000 works fine. Going to high will result in an unplayable movie,and the low-mid motion scenes will look like shit.

If you use antiShit you can go as low as you want. I've never experinced any problems encoding with high pass = min bitRate= 150

This helps me getting exact filesize which I found very hard
@ mattias
I always use the above settings and always get +/-1mb in filesize prediction.
So you must doing something wrong

[Pikoa]

I've had a similar problem with file size previously. Assuming you've used full processing, set minQual=0. That will stop bumping the bitrate.:)

[LotionBoy]

First. Setting Min bitrate below 350 can, and usually will, introduce gross codec errors into your encode. It is a Bad idea. Which doesn't mean that it always will, but it is not something that is a good idea. High-pass should not affect file size. It just sets the floor on the bitrate curve that is then scaled to hit the target bitrate. 0 or 270 or 350 is not going to change your file size. It will affect how bits are distributed on the low-end. Setting the Min-bitrate setting above the high-pass can cause slight oversizing (though in my experience, never by more than a couple K). If you set min-bitrate and high-pass perfectly equal and setQuality to 0, your files will be perfectly on size, assuming you haven't screwed up somewhere else. Low-pass over 6000 is unnecessary, because the codec doesn't go higher. And 6000 is rather high, unless you are getting errors in your encode. 1500 will give you good quality in almost all cases. 3000 should be more than enough. Any more degrads the quality of other scenes by skewing the curve. Also, oversize errors can be caused if your bitrate calculator doesn't calc audio interleave overhead right.

LotionBoy

[mattias]

Thanks for giving me a very good answer to my questions.
Now I see the logic in the settings. Now one more question.
Will the option "Corrections on low-bitrate conditions" give
me better quality anyway or should I leave it unchecked?

[LotionBoy]

I use it. I think it is meant to keep really low bitrates from skewing the gauge and giving bad results. I always have it on, so I can't tell you what having it off does or if it is a recognizable difference. All the documentation I have read says leave it on, so I do. :-)

LotionBoy