Quantcast Bitrates: does Calcumatic (and Others) Calculate Correctly Average Bitrate - digitalFAQ.com Forums [Archives]
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08-24-2004, 01:48 AM
Prodater64 Prodater64 is offline
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Calcumatic, DIKO, Packshot? (I don't remember now), MencodeMe, KVCD/KSVCD Express and many others, and many people by hand calculates avg bitrate to fit with a movie a determined kind of media.
When we are encoding from 29.970 fps NTSC (interlaced or not) to 25 fps PAL (interlaced or not), and visceversa, there is a playing time change according with the shifted fps rate, as following:

29.970 interlaced to 25 (*)
TargetTime (TT) = SourceTime (ST) * 29970 / 25000

(*) This convertion bothers a lot. Some standalones supports (maybe many standalones, we all must to test it) the header trick: You can encode 29.970 interlaced to 29.970 interlaced or not with PAL resolution, and apply to target mpg DVDPatcher changing fps to 25 (DVDPatcher also can patch resolution if you encode to NTSC res, but I don't test if this kind of patched file plays in my standalone, but I'l try soon). Nor movietime neither voices pitch changed. MPlayer don't play fine this files but mplayer2 and bsplayer yes.

25 to 23.976
TT = ST * 25000 / 23976

23.976 to 25
TT = ST * 23976 / 25000

25 to 23.976
TT = ST * 25000 / 23976

25 to 29.970 -> It is a bad idea, better 25 to 23.976.
29.970 telecined to 23.976 no time change if we apply inverse telecine.
29.970 telecined to 25 is same as 23.976 to 25 if we apply inverse telecine.
29.970 interlaced to 23.976 -> It is a bad idea, better 29.970 to 29.970 (interlaced or not)

Now I want to know how calculates avg bitrates the previously mentioned programs and people that do that by hand, as if they take ST and do calcutations, and obtain a "x" avg bitrate for an "y" media, it means that ST * "x" will fit whole media (in a theoretical way).
BUT! if TT has changed related to that we are talking about here, it can result that ST * "x" would be (Im sorry, I don't use good verb tenses) bigger or smaller than whole media.

Does you, dear programs and friends, do correct calculations?
Or am I wrong?
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08-24-2004, 10:18 AM
kwag kwag is offline
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Quote:
Originally Posted by Prodater64

Does you, dear programs and friends, do correct calculations?
Or am I wrong?
Yes, you are wrong

Average bitrate has NOTHING to do with framerates.
It has to do with the BITS per frame, and then, averaged for the total number of frames.
As to your question: "Does Calcumatic (and others) calculate correctly avg bit"
Yes, it does, and it considers some "Extra" multiplexing space.
Is CalcuMatic 100% precise The answer is: No.
But it's 100% practical, and a "Perfect" precision is unnecessary, because no encoder is 100% accurate (to the byte) after encoding. There are just too many factors in the way.
Actually, CalcuMatic's precision is slightly below the perfect average bitrate, and that was done on purpose for "insurance"

-kwag
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08-24-2004, 12:32 PM
Prodater64 Prodater64 is offline
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Well, question about Calcumatic is answered but, I think (IMHO) that you are wrong when you say:
Quote:
Originally Posted by Kwag
Average bitrate has NOTHING to do with framerates.
Then this formula is wrong
avgbtr = (video required space Mb*1024) / (movietime min * 60) * 8

(Now I really don't understand * 8 in this previous formula, I taked it from Maurus scripts, and maybe him could explain me it when come back, or you if you understand it)

but this one would be correct:
avgbtr = ((video required space Kb) / Total frames) * fps
avgbtr = (video required space Kb) * (fps / Total frames)
but as Total frames / fps = Movietime sec
avgbtr = (video required space Kb) * (1 / Movietime sec)
avgbtr = (video required space Kb) / Movietime

and in this case 2 movies with same total frames for a same target media, will have different avg bitrate.
Surely you have tuned Calcumatic to compensate this (I think so according your previous answer), but avgbtr is related directly with framerates and movietime. Or what is your avgbtr calculation formula?
I mean that avg bitrate calculations must to be made taking in account target fps and target movitime (as I show it is correct in both ways)
And this is not theoretical, I compared results and if not use target fps or target time in calculations, final size has error.
I still ask, does other programs that calculates avg bitrates, take this in account?
I'm waiting mainly for Incredible, VMesquita and Maurus answers, but also anybody that want to participate in this thread.
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08-25-2004, 03:55 AM
Dialhot Dialhot is offline
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Quote:
Originally Posted by Prodater64
Then this formula is wrong
avgbtr = (video required space Mb*1024) / (movietime min * 60) * 8

(Now I really don't understand * 8 in this previous formula, I taked it from Maurus scripts, and maybi him could explain me it when come back, or you if you understand it)
This formula is not wrong, but as you suggest, it does not take in account any fps change. So the "movietime" must reflect the exact length of the target .

The 8 is simply to go from Bytes to bits
(bitrate are expressed in bit/s, not byte/s)
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