Quantization levels: original vs backup-questions

[rapscallion]

Hi all,

Currently using DVDRB Pro 1.05.2, however, below comments apply to previous versions as well.

I have read many posts re quantization and still am a little fuzzy on the subject. So If someone could please shed some light on the DVDRB quantization levels/results described below I would very much appreciate it.

First, the following from the FAQ about Transcoding Technique in the One Click Solutions forum :

"Q. What is the Q-Level in Bitrate Viewer and how does it relate to my transcoded (or re-encoded) stream's quality?

A. This is the quantization level of the stream. I believe Bitrate Viewer is looking at the quantization level used and performing some statistical analysis to find out an average used. This number should be slightly higher than the original's stream because of how the transcoding process works (increasing quantization level to decrease size). In a re-encoded stream (i.e. CCE, TMPGEnc, etc.) it should be nearly the same.. maybe a little lower because you're starting with a lower data accuracy to begin with, so you can use a lower amount of compression to represent it.

I did a comparison of the Jaws DVD first movie VOB (VTS_01_1.vob) versus a movie only mode of 86% (modest) compression level DVDRB backup, cce2.66+, default settings f/VBR and Qual_prec, 5 passes. (tried BitRateViewer for the first time)

Differences in bitrate were reasonable and expected :
9402peak/6349avg f/the dvd and 8881 peak/5369 avg f/the rebuild.

However, the "q" levels were quite a bit different and unexpected:
>13.5 peak/7.7 avg f/the dvd
>5.62 peak/3.76 avg f/the rebuild. A 50+% difference/reduction !

Additionally, Bitrate Viewer graph shows the original dvd "q" level often higher than the bitrate level. The DVDRB backup "q" levels were always lower than the bitrate level.

I also checked several previous DVDRB backups I had done w/the same settings and the results were similiar. Is this normal and why so differnt than described in the above FAQ? (and other posts in this forum)

BTW, using a transcoder on the same Jaws VOB, the "q" values were consistent w/the FAQ description.

Thanks for your comments/experience/observations !

[apfraats]

I don't get the whole point here to be honest, but 1 thing I take for granted:

BITRATEVIEWER is a tool NOT TO TRUST.


You cannot trust reported bitrates.
You cannot trust reported Q-values probably also.

I still look for a simple tool to correctly report max/min/avg bitrate.....

All tools used now, expect some mentioned in the thread about this topic, are reporting different values.

Also I heard that BITRATE VIEWERS Q isn't the Q people often refering too. They refer to some 'other' Q.

So I hope someone can explain all these Q's, but don't be blind trusting on BITRATE VIEWER, or you ganno have problems....

I even payed for that .... programm.

[jdobbs]

The relationship between original quantization levels and those you get in a recode is a difficult thing to predict. Essentially what happens is you start over again -- so they can't really be compared. The original quants are calculated using a baseline that is the original film as it was digitized. The reencoded baseline (or starting point) is the output of the MPEG stream as it was stored (from the first encode).

[rapscallion]

jdobbs...are you saying that the results I observed are normal/expected and thus, the FAQ quoted, is incorrect/mistaken ?

apfraats...re read carefully and you will get the point. (ie "q" level comparison, regardless of the prog, is apples to apples)

edit f/spelling

[apfraats]

I know, just as bitrate should be apples to apples, but some programms seems to use red apples and others green ones......

By the way, youre statement is not completely true, I get as often higher q graphic line then bitrate as the other way arround....

Totally depending on source...

And probably depending on VBR_BIAS, MIN_BITRATE, MAX_BITRATE etc.

[jdobbs]

jdobbs...are you saying that the results I observed are normal/expected and thus, the FAQ quoted, is incorrect/mistaken ?

apfraats...re read carefully and you will get the point. (ie "q" level comparison, regardless of the prog, is apples to apples)

edit f/spellingSure they're normal. But that doesn't mean the FAQ is incorrect. There are lots of variables involved. It depends of the complexity of the sources and the available bitrate for the encoding. If you are making only a small reduction in size -- the output quantization may be exceptionally low. Yours was 85% -- which explains the low quants you see. But the two q values (before and after) are not being applied against the same source -- so they definitely aren't apples to apples.

[manono]

A transcoder keeps the same matrix, and it's to be expected that the Q-Level rises. If you reencode using a different and lower bitrate matrix, even though you're reencoding for a smaller filesize, it's entirely possible for the Q-Level to wind up smaller, and sometimes much smaller as you've observed. I think for the FAQ about transcoding vs reencoding to be correct, it expects the matrices and other settings to be the same for both the original DVD and the reencode. As jdobbs said, there are lots of variables involved.

[rapscallion]

Many thanks to the three of you ! Lot's to learn here and I do appreciate the
the responses. Very interesting info, which sometimes seems conflicting.

BTW, I should mention that I have been very happy with the DVDRB results.
The BRV program, in my case anyway, probably lends credence to the old saying "a little knowledge is a dangerous thing" -:)