Problem regarding bitrate calculation [Archive]

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duedel

7th February 2016, 22:45

So I wanted to create a BD9 from one of my Blurays.
I know there are tools like BDtoAVCDH and the likes, but I wanted to do it "more manually" for learning purposes.
In the end, I wanted to have a bluray.iso that could be burnt on a dvd9.
I used Megui's bitrate calculator and I also added two audio tracks and a subtitle .sup file.
Then I chose "m2ts" in the calculator which, in my understanding has a larger overhead then mkv (ergo lower bitrate for the movie) but seemed necessary to get a bluray.iso.
I encoded the movie and then muxed everything to .iso format with tsmuxer, yet the final image was 190 mb too big.
Where did I go wrong?
Thanks for reading.

rik1138

9th February 2016, 05:02

I'm not familiar with the bitrate calculator you are using, but (and just taking a guess here) if you are choosing 'm2ts' instead of 'mkv' it sounds like it's just giving you a stream bitrate, not a disc bitrate. What I mean by that, is there's a lot more overhead created for a blu-ray disc than just what is in the .m2ts video file itself. There's HDMV code (even with no menu), PlayList files, ClipInfo files, and a few other little files... These are relatively tiny, but they do take up space.

If you want to get an _EXACT_ bit rate for a blu-ray ISO, you need a bitrate calculator specifically designed to do Blu-ray calculations (that can take into consideration all the overhead a blu-ray disc needs). And even then it's always safe to pad it a little... Doing a 90-minute feature? Tell the calculator it's 100 minutes...

Or, tell the bitrate calculator your 'DVD9' is about 5-10% smaller than a DVD9 actually is (if it lets you do that), and you should be safe...

Bitrates are rarely an exact science since every little detail of the final disc image has it's own amount of overhead (the amount of subtitles, the size of each individual subtitle event, the amount of audio streams, the TYPE of audio streams, the type of menus (if present), etc, etc). Bitrates are usually calculated to 90% disc capacity (and the calculators use generous estimates for things like subtitle streams), 95% if you are feeling brave. :) If you calculate the video bitrate (+ Aud/sub streams) to 100% of the disc capacity, your ISO will always be too large (but the .m2ts file might just fit by itself).

duedel

9th February 2016, 08:42

Thanks so much, that was quite informative!

Ghitulescu

9th February 2016, 09:48

The bitrate for a M2TS (a BD) is somehow complicated to compute.
In container formats, like MKV, the total bitrate is rather predictable. The overhead of the MKV internals is small and known.

Packet formats, like TS/M2TS are different as the streams are packetised, each packet is attached a header, and these are sent sequentially (hopefully) to the destination. A packet has to contain only a single type of information (it's not like RAR/ZIP multi-files). Imagine you move from one home to another and you have a collection of moving cartonboxes. You won't mix champagne glasses with shoes, or jewelries with chimney coals. A subtitle packet can contain less than its normal payload, like a truck that transports a single brick, because there are no more.

I could go into details but I don't think it's really necessary - what you need to know is that there is no simple way of computing this, and this is the reason why no precise BD bitrate calculator exists, like those for AVI, MKV etc.

duedel

9th February 2016, 10:07

I could go into details but I don't think it's really necessary - what you need to know is that there is no simple way of computing this, and this is the reason why no precise BD bitrate calculator exists, like those for AVI, MKV etc.

Good to know, so I can stop looking for one ;)
Thanks!

Groucho2004

9th February 2016, 10:28

Selur's Hybrid (http://forum.doom9.org/showthread.php?t=153035) has a decent bitrate calculator.
There is a thread here (http://forum.doom9.org/showthread.php?t=158828) on D9 where the required formulas for audio/container/subtitle/etc. overhead of m2ts were discussed.

duedel

9th February 2016, 13:50