GH2 Where does the 'extra' data come from?

[zm15]

This is something I've never quite understood, but have always wondered. Forgive my basic knowledge here, but I've tried to research this without a specific answer as to what is going on.


When I convert from an .MTS file to ProRes422 HQ, the file get's huge. Why is that? Where is that extra data coming from if it's not there to begin with? i.e. higher bitrate. I understand the codec has a standard bitrate it will always maintain, but why do it if the original file bitrate is so much lower?


For example, a stock GH2 mts for me streams at about 22 Mbits/s, I convert it to ProRes422 HQ, that same file is now playing at 216 Mbits/s. - With no discernible image quality difference.


What is going on in the codec that makes the file so huge? Other than workflow issues, why would you want to transcode from MTS? Does converting it to ProRes (or another codec) 'Unpack' the data in a more efficient way?


Why not use something like ProRes422 LT if you're starting from a much lower data rate like a native MTS file provides?

 

[mcbob]

Pre-apologies if I'm munging up the science of it...

Uncompressed codecs store the complete data for each pixel of each frame in a sequence.

Compressed codecs, when possible, group picture data... that is, if in, say, a 7-frame group of pictures (GOP), pixel #921,43 doesn't change significantly, it only stores its data once and only refers to it in later frames of that 7-frame GOP.

In cases where the compressed codec can't fit all the information on what pixels change, it averages areas around blocks of pixels - leading to oft-decried "macroblocking," big blocky messes often occurring in areas of high detail and motion.

Transcoding a clip from a compressed codec to a less compressed codec writes out more information for each frame based on the averages and references of the original compressed codec.

 

[TheDingo]

Essentially you're comparing advanced Long-GOP compression ( where most of the data that is stored is made up of image changes within each Group-Of-Pictures ) with either non-compressed or low-compressed video formats.

Long-GOP compression is very efficient but takes a lot of processing power to re-assemble all the GOP pieces into the original frames from the data stored within the Long-GOP file, while the non-compressed or low-compressed formats are almost ready to be viewed as individual frames so very little processing has to take place.

If you remove compression from the equation most of these video formats would produce files that are identical in size. ( obviously storing 8-bits of data in a 10-bit or 12-bit container is less efficient from a file size perspective )

 

[mmaller]

Leaving the technical specifics aside, To credit Larry Jordan's analogy.. Think of the mts file as a one gallon pail of water. When you convert that into for Example, ProRes422 you've poured that pail into a 5 gallon bucket. (you now have much more room to add things to that file) The advantage to having the better codec (less compression) besides better computer performance is when you begin to add effects, composites, color corrections, etc. Granted the image will look no better than the original mts file, but now the processing of the additional renders, effects and composites will look better because you are working in a better codec that was designed for editing.