Two Pass Encoding Worth it With High Bitrates?

vbimport

#1

So I’m ripping DVD’s and have a target output of 1.4 to 1.7 Gig file size. these are DivX encoded with Ultra DivX settings of “Insane” quality (a name DivX uses in their DivX Author to refer to quality vs speed ratio of encoding *Insane takes the longest but is highest quality) and I’m setting the bitrates to about 1,700 to 1,800. The resolutions are source at 720 x 480. Additionally, (I don’t know how this is relevant, but it may be with regard to file size) I’m also using 320kbps MP3 or 320kbps AC3 depending on the source audio.

Where I’m confused is the 2 pass/multipass encoding. I know this takes twice as long, but reading briefly about it I inferred that 2 pass was supposed to be better ultimately, but reading somewhere else just a few minutes ago, it seemed that someone was explaining that if you have high bitrates then multipass is a waste and doesn’t matter.

As is, it now takes about 4 to 5 hours to encode each movie. This is in part because of 2 pass encoding. My question is: is 1,700 bitrate high, thus justifying only 1 pass? And do I even need to be using 2 pass encoding? Will the difference be profound? Or even noticeable?

Any light shed on this for me would be cool.


#2

The 1st pass does some analysis as to how to distribute the bits. It’s a good idea to do 2-pass.

Regards


#3

[quote=blutach;2080442]The 1st pass does some analysis as to how to distribute the bits. It’s a good idea to do 2-pass.

Regards[/quote]

What I’m wondering is, why would it be a “bad idea” to use single pass with bitrates of 1,700-2000?


#4

[QUOTE=Kung-Fu Theater;2080445]What I’m wondering is, why would it be a “bad idea” to use single pass with bitrates of 1,700-2000?[/QUOTE]

Because it won’t distribute the bits as efficiently and you could end up with fast moving scenes looking bad because they haven’t been allocated a high enough bitrate while other scenes have a higher bitrate than they need.

Two pass optimises the bitrate more effectively and allocates more to where it’s needed.

[B]Wombler[/B]


#5

[quote=Wombler;2080476]Because it won’t distribute the bits as efficiently and you could end up with fast moving scenes looking bad because they haven’t been allocated a high enough bitrate while other scenes have a higher bitrate than they need.

Two pass optimises the bitrate more effectively and allocates more to where it’s needed.

[B]Wombler[/B][/quote]

Ah ok. So really, 2 pass is essential if I don’t want to encode crap basically is what it sounds like. Thanks for explaining. I actually need to explain this to a buddy of mine as well, who’s been trying to convince me otherwise and has slacked off with his own encodes.

I’m guessing there must be quite a pony race between the codec heavyweights looking to improve this area. Would be nice to have a single pass that did it all someday.


#6

I’ve been using Acidrip in Linux to encode at ~2067 bitrate (Bits/Px is 0.200) usually at 720 x whatever (varies, mostly widescreen formats), with a single pass to Xvid, and the results are usually really good on a standard-def 31" TV. I may have to try 2-pass sometime to see if the extra couple hours is worth it…

As is, my file size is usually between 1.2-1.7gig.


#7

[QUOTE=anjilslaire;2080683]I’ve been using Acidrip in Linux to encode at ~2067 bitrate (Bits/Px is 0.200) usually at 720 x whatever (varies, mostly widescreen formats), with a single pass to Xvid, and the results are usually really good on a standard-def 31" TV. I may have to try 2-pass sometime to see if the extra couple hours is worth it…

As is, my file size is usually between 1.2-1.7gig.[/QUOTE]

It depends so much on the source material that it’s difficult to establish any sort of general principles.

The two pass encode will always be better but it depends on the source as to how much better.

I always recommend trying something known to be difficult like ‘Saving Private Ryan’ if you want to make a meaningful judgement. The landing scene at Omaha Beach is a good one to compare for relative quality.

There are even a couple of very exceptionally difficult discs I’ve come across that in my mind produce unacceptable results regardless of the methods used. But that’s beyond the scope of normal tests.

[B]Wombler[/B]


#8

[QUOTE=Kung-Fu Theater;2080484]Ah ok. So really, 2 pass is essential if I don’t want to encode crap basically is what it sounds like.[/QUOTE]

It’s not exactly forbidden to do single pass encodes and the world isn’t going to end as a consequence but in my view two pass is a must if you want satisfactory results on more difficult than average source material.

Some guys go overboard and do anything up to six passes but it’s the law of diminishing returns and beyond two passes it becomes increasingly difficult to tell the difference.

I’m guessing there must be quite a pony race between the codec heavyweights looking to improve this area. Would be nice to have a single pass that did it all someday.

All we need are faster PCs and then encoding times won’t be an issue.

I reckon anything under 20 minutes for an entire DVD would be the point beyond which I wouldn’t even think about it anymore.

[B]Wombler[/B]


#9

I just completed a single-pass rip on Batman Begins. I think I’ll rerun the process as a 2-pass to see if there’s any difference. I’ll post later.


#10

[QUOTE=anjilslaire;2080746]I just completed a single-pass rip on Batman Begins. I think I’ll rerun the process as a 2-pass to see if there’s any difference. I’ll post later.[/QUOTE]

Fast action scenes or scenes with smoke or fog or fast changing lighting etc. are the ones that will show the most difference.

It depends on a lot of factors though as to how much difference you’ll see.

The worst case scenario is if a movie is long, has lots of fast changing scenes and the bitrate is already fairly well optimised.

[B]Wombler[/B]


#11

Well, I can say for my own purposes, I can see a bit of a difference in a 2-pass, and the file remains the same (Batman Begins is ~1.8gig after conversion), so I’ll probablly start going with 2-pass from here out, as I’m not generally crunched for time, and can let it encode overnight If I’m really lazy.

Thanks for pointing out exactly what 2-pass does, folks.


#12

No problem.

Glad you found it useful.

[B]Wombler[/B]