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Old 26th August 2003, 15:05   #1  |  Link
r6d2
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RoBa: Binary search for Q or Newton's Method?

Hi,

I've done some research on finding a suitable Q for a RoBa 2 pass encoding with DVD2SVCD.

(Outlined in http://forum.doom9.org/showthread.ph...071#post361071)

As you may be aware, Tylo's plugin (D2Sroba) uses the binary search approach to find the best Q for both a single pass (OPV) and multipass (VBR).

Tylo's tool offers to calculate several Qs for as many CDs the movie will fit in keeping at least the expected quality. Binary search needs 6 or 7 sample tries to find the optimum Q for each category, depending on the range you defined for the search. Tylo has optimized his tool to skip some searches and guess the result. This actually can lead to significant differences with real data, since the slope of the curve is not known.

Alternatively, I implemented two variatons of Newton's Method to find the Q, as well as Binary search. I used Excel to have side by side comparison of the three methods. The final spreadsheet also contains sample data (sample sizes of a movie for all Qs).

I found that the Newton's standard version outsmarts Binary search in about half of situations. The improved Newton version is found to outsmart Binary in most cases (actually, outsmarts the standard too). A suitable Q for 2 pass RoBa can be found in 2 or 3 tries!

Please feel free to download the sheet file from:

http://www.angelfire.com/droid/r6d2/

and play with the sample data. The sheet is self-documented so you should have no trouble using it. Also, it documents RoBa method and you can get a clear understanding of how it works when quality and edge-filled CDs is what it's all about. The sample data will also give you a glipmse of how the Q function behaves like on a typical source.

Also, the magic of the improved method is based on a magic number. If through experimentation you find another magic number which converges even faster, please share the knowledge. It's likely DVD2SVCD will implement this feature in a future version, so we need all the help we can get.

Regards,
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Last edited by r6d2; 27th August 2003 at 20:03.
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