[Devinda Fernando]

So you don’t actually know the methodology of how Adobe Premiere reduces the file size when the Bitrate setting is reduced for the Export?

[Devinda Fernando]

*** Bitrate is just giving more or less data per sec to the footage. ***

But in order to reduce file size, data must be reduced and the only way to logically do that, would be to compress it by reducing detail in some way? There has to come a point when the file size reduces to a certain level where the individual frames or Group of Pictures are also further compressed. Does that make sense?

*** if you output the original to a higher quality codec it will not magically get better and have the qualities of the new codec.***

Makes complete sense. I did not doubt that for a second.

*** If you want to really wrap your head around this you should read The MPEG Handbook by John Watkinson.***

Thanks for the recommendation, I will try to get hold of a copy.

[Devinda Fernando]

Hi Tim,

I’m starting to dive into the details of Codecs and compression and video formats, and saw your very detailed explanation. I use Premiere a lot, and had a similar question to that which you just answered: Do you know the specific mechanism by which Adobe Premiere compresses the footage when you change the VBR or CBR? I’ll give a specific example of shooting on a Canon 5D and having native footage of H.264 in the .Mov container. I believe this footage is 1080p 8 Bit Color, and 4:2:0 chroma subsampling. So when you export this to the typical H.264 and .Mp4 container, the highest quality of the output can’t be more than the original footage was taken at? I believe Adobe’s default setting for export is a VBR of Target:32 Mbps and Max:40 Mbps. Now if these were reduced to save file size, how exactly does premiere do this? For example, would the software further compress the chroma subsampling (i.e. it ‘jpegs’ each frame further?) or is there some other mechanism it employs.

[Devinda Fernando]

*** You don’t get RAW of course because the original is mpeg codec so there is no way to change colour temp, ISO etc. ***

True… I used the word “RAW” slightly incorrectly here. I was really referring to uncompressed Chroma Subsampling; you are right as it cannot logically be possible to create detail data from thin air, so if the 4K footage is only 8 Bit Color, then the resulting 2K cannot be any greater. So I guess the best I can hope to get from a pro-sumer 4K camera is 2K (actually 1080p) at 4:4:4 8 Bit.

[Devinda Fernando]

*** There are varieties of H.264 that are not ideal and will bog down the works due to overhead. 8 bits is a bigger problem ***

I guess its also must be how you Export the final output too? I mean if you import a 4K 4:2:0 8 Bit to your Editing software, then export it at 2K at also 4:2:0 8 Bit, then you are losing the additional Chroma Data from the downscaled 4K image. You would have to Export the final Output with a Codec that preserves the 4:4:4 Chroma data like Apple ProRes 4444.

[Devinda Fernando]

I totally agree with you on the keying and VFX… Obviously going from 4K to 2K would not give accurate pixel chroma data when you merge them together. I was mostly interested in being able to match footage taken from good cameras (Sony Fs7) that record in Apple ProRes or even 2K Raw with footage taken from a GoPro Hero 4 and the DJI Phantom 3. Both the action cams only take 4K 4:2:0 8 Bit, whereas the Sony Fs7 does 4:2:2 or 4:4:4 10 bit.

[Devinda Fernando]

I actually haven’t noticed any visible difference using either workflow in Premiere. At this point it seems like a pointless extra step.

I did infact buy Davinci Resolve Lite, but have not gotten round to learning it yet,… I still occasionally use Apple Color since I am most familiar with that for those special occasions that require some sort of custom Color grade, but for the most part I just color correct in Premiere for most of my work since its just quicker.

[Devinda Fernando]

I’m so glad I work in a PAL environment.

[Devinda Fernando]

I’m not familiar with the methodology for the audio. But the theaters are such low quality here that everyone is used to it. Half of them don’t even have AC, and the projectors are really old in the village areas so picture quality is not the utmost concern. In fact as of 2009 when the war ended, 40% of all the TVs in the country were still Black and White, – that has probably changed a lot but I don’t know the new figures, being that this is the fastest developing country in Asia in the past 5 years.

[Devinda Fernando]

So does 25fps(PAL) actually record at 25Fps or is it also 24.9 something like the rest?