How to know if an AVI is encoded in 1 pass ?

[Alain_French]

Hi all,

Is there a way to know if an AVI has been encoded in 1 passe or 2 passes ?
Something in the header or something else ?

Thank you
Alain

[Sharktooth]

no.

[stephanV]

well... you could look if it has a constant quant throughout the file ;)

but... such info would pretty much be useless anyway... whyd you wanna know?

[Alain_French]

I want to know because somebody tells me that it is possible and i dont know how :)

Constant quantizer for 1pass at Q2 and constant bitrate for 1pass at constant bitrate ?
And finally how analysing an AVI to know that ?

[Sharktooth]

Originally posted by stephanV
well... you could look if it has a constant quant throughout the file ;)
That does not mean it was done in 1 pass..
i can make a 2 pass encode with minq and maxq at 2 and it's still a constant quant encode... but 2 passes were used.

[Alain_French]

And moreover, I believe xvid makes target bitrate or quantizer... so it is not really constant !

I tested on a sample and the mean was ok but that s all, I have a lot of frame with lower quantizer and upper...

So Sharktooth seems to be true :)

[stephanV]

Originally posted by Alain_French
I want to know because somebody tells me that it is possible and i dont know how :)


well its not :p

[SeeMoreDigital]

Yep...

It's a shame some kind of "VBR" or "# PASS" flag can't be incorporated into the stream header!

By-the-way, how are VBR .MP3 files identified?


Cheers

[stephanV]

Originally posted by SeeMoreDigital
Yep...

It's a shame some kind of "VBR" or "# PASS" flag can't be incorporated into the stream header!
Why? That kind of information is completely useless...


By-the-way, how are VBR .MP3 files identified?

I don't think MP3 really has a specified way for that (besides parsing the file)... i believe lame pastes its own header in front of the file to give that kind of info.

[Kagura]

Well, I believe that it is possible to determine the # of passes used for a decent length (> 10 minutes) or for a file with significant variations in bitrate requirements.

1 pass is a guessing game of allocating bitrate, and it is sure to screw up a lot, so there's huge fluctuation in quality. For example, a still scene might look fine, but then if it immediately shifts to an action sequence, there's bound to be huge blocking/smearing/edge noise. This is because the codec is slowly fluctuating to adjust to the increased demand for bitrate instead of anticipating it as it does in 2 pass mode.

In contrast, 2 pass produces a more consistent quality. If there's blocking and/or edge noise, it'll be pretty much evenly spread throughout the entire file.

Of course, things like zones can throw out my whole argument hehe.