Is this statement about Single-Pass True

[weaver4]

This is a statement from StaxRip's DVD Guide. Since I always thought that multipass would give you a better quality picture for the same file size I did not think the following statement was true. Is this statement true? I don't think it agrees with the autogk guide.

"Before reading the next sections it's important to understand one thing. If you think multipass is superior to single pass with fixed quantizer, forget about it, it's not true, you won't gain higher quality, a smaller file size or generally more compression, the quantization is different but not more efficient.

The assistant reminds to make a compressibility check. The compressibility check makes only sense for multipass encodes and multipass encodes make only sense to hit a exact file size so since there is a fixed file size and a fixed movie length which result in a fixed bitrate the only left variable is the image size. The compressibility check helps to find a ideal image size for the targeted bitrate. Movies differ greatly in compressibility, a movie with a lot action like many explosions need a much higher bitrate for the same image size.

A image size around 200000 pixel (e.g. 528x384) is a good compromise. A smaller image size requires a smaller bitrate which will result in a smaller file size but a smaller image size means of course also less quality.

A better solution then aiming for a file size is aiming for constant quality. It can be achieved by setting up the codec to use single pass quality mode encoding at a fixed quantizer or a similar quality metric. StaxRip offers profiles for constant quality. Since movies have a different compressibility the resulting file size will always be different.

Using single pass with constant quality is the prefered way if you use your hard drive to archieve your movies.

It's recommend to use the following codec settings:

DivX in Quality Mode using quantizer 4 or XviD single pass using quantizer 3. This will produce equiqualent results than multipass at 60% quality (100% quality is quantizer 2). This is very high compression and requires post processing while decoding. DivX has excellent post processing which is enabled by default as most MPEG-4 use (and should use) high compression. The DivX decoder is also excellent to decode XviD, another very popular decoding filter with post processing support is ffdshow."

[xyloy]

If you think multipass is superior to single pass with fixed quantizer, forget about it, it's not true, you won't gain higher quality, a smaller file size or generally more compression, the quantization is different but not more efficient.
Whoever did this statement is someone who doesn't understand that aiming a specific filesize cannot be done with a one-pass encode with fixed quantizer(or fixed rate factor):
It's impossible to tell the finale filesize with a fixed quantizer/quality because each content(and each scene of ths content) needs a different amount of bitrate to achieve the mean quantizer/quality specified by the user.

The only one-pass mode wich respect a given filesize(or bitrate) is one-pass constant bitrate, but a constant bitrate cannot use higher bitrate for high motion/detail scene and less for low motion/detail scenes. But why do so ?
Because when you aim for a specific fiesize, you'd better use your bitrate wisely among the different scenes of the movie to have an overall constant quality.

To summary:
two-pass(or multipass) is good to achieve an overall constant quality with limited bitrate or filesize(1CD for example)
One-pass with a fixed quantizer/quality is better only when you aim for a specific quality and not for a specific filesie(or bitrate).
One-pass with constant bitrate shall burn to Hell( :D )

Simple. ;)

[Audionut]

A better solution then aiming for a file size is aiming for constant quality.

Correct.

No where in that statement does it mention 1 pass bitrate.

In a nut shell it is saying.

Forget compressability check.
Forget multiple passes.
Go for quality at a quick contant quantizer/quality.

[weaver4]

I don't think his statement was about trying to make a specific filesize. He is saying that if you make a single pass file and it comes out to be 900MB then a 900MB file made with two pass will not look any better.

Thanks for your bullets I had always thought that a one-pass with a fixed quantizer was the same thing as one-pass with a fixed data rate. I think that is where my confusion came from.

[xyloy]

He is saying that if you make a single pass file and it comes out to be 900MB then a 900MB file made with two pass will not look any better.
That is correct, of course. The two first lines I quoted shocked me, that's all. :D

[Audionut]

always thought that a one-pass with a fixed quantizer was the same thing as one-pass with a fixed data rate.

1 is a very fast way of encoding to a specific filesize/bitrate.
The other is a fast way of hitting a known constant quality.

[stax76]

Seems you are using a old StaxRip version because the guide was updated seval times. Currently it looks like this:


Before reading the next sections it's important to understand that multipass is not necessarily better than single pass in quality mode. Some codecs are tweaked better for multipass but the difference is as far as I know not huge.

The assistant reminds to make a compressibility check. The compressibility check is for multipass encodes and multipass encodes are done hit a exact file size so since there is a fixed file size and a fixed movie length which result in a fixed bitrate the only left variable is the image size. The compressibility check helps to find a ideal image size for the targeted bitrate. Movies differ greatly in compressibility, a movie with a lot action like many explosions need a much higher bitrate for the same image size.

A image size around 200000 pixel (e.g. 528x384) is a good compromise. A smaller image size requires a smaller bitrate which will result in a smaller file size but a smaller image size means of course also less quality.

Another solution then aiming for a file size is aiming for constant quality. It can be achieved by setting up the codec to use single pass quality mode encoding at a fixed quantizer or a similar quality metric. StaxRip offers profiles for constant quality. Since movies have a different compressibility the resulting file size will always be different.

Using single pass with constant quality is a good encoding method if you use your hard drive to archieve your movies.

It's recommend to use the following codec settings:

DivX in Quality Mode using quantizer 4 or XviD single pass using quantizer 3. This will produce similar results than multipass at 60-80% quality (100% quality is quantizer 2). This is very high compression and might require post processing while decoding. DivX has excellent post processing which is enabled by default as most MPEG-4 encodes use (and should use) high compression. The DivX decoder is also excellent to decode XviD.

Not using post processing requires encoding with a lower quantizer or higher bitrate which means a bigger file size so it's recommend to stick to the values explained above.

[weaver4]

Thanks, that cleared it up.

Yes, I was using version 0.9.9.1, so it was not the latest help file.

So it sounds like you are recommending Single pass if the customer cares about getting specific quality, not size.