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Tom Yanowitz

Sony a7s II video compression

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Hi there,

I'm asking myself some questions about compression in the sony a7s II's various recording formats and maybe some of you know the answer.


I tried to calculate the mean (file) size of a pixel in the different formats to compare the amount of compression:


4k

30p 100Mbps : 3.33 Mb per image = 0.40 bit per pixel (on average, obviously)

24p 100Mbps : 4.17 Mb per image = 0.50 bit per pixel


HD

24p 50Mbps : 2.08 Mb per image = 1.00 bit per pixel

30p 50Mbps : 1.67 Mb per image = 0.80 bit per pixel

60p 50Mbps : 0.83 Mb per image = 0.40 bit per pixel

120p 100Mbps : 0.83 Mb per image = 0.40 bit per pixel


Is 4K 24p actually 80Mbps to have the same image quality as 4K 30p, or is it true 100Mbps an thus produce a less compressed image ?

Same question for 24 and 30p 50Mbps HD.

If 24p 50Mbps HD is actually 50Mbps, it means it's the format with the least compression with over 1bit per pixel on average.


If the limiting factor of the camera is a data stream of 100Mbps max, why are there no HD 24/30/60p 100Mbps modes for better IQ ? Is it to keep consistency over compression levels across the various framerates even if it means sub optimal IQ ? But then the 4K 24p should be 80Mbps to fit the 30p compression ?

100Mps for all the different res/framerate combinations would allow the 60p to be better quality than the 120p, as it should, and also allow an even lower compression 24p mode (with over 2 bits per pixel)

This would come in handy for specific shots where you'd want low compression over high resolution.


Or am I just talking nonsense ?


Cheers.

Edited by Tom Yanowitz

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I think I may be looking at it from a different angle, although I follow your logic through and through.

 

What I believe it means is when 100 Mbps is recorded per second, that means the camera is approximately writing at that speed per second, no matter the frame rate of 24fps or 30fps.

 

So with that logic, I would suppose that it is recording more information for 24fps vs 30fps, but the quality is negligible.

 

Also, remember that the camera is writing approx. 100 Mbps, not exactly 100.00 Mbps. Therefore, with those six additional frames, you aren't getting a huge bump in quality if you stuck with 24fps instead.

 

That's my view on things. Although you did the math and may be correct, in the end I don't think it actually matters. :) You are still going to have the same quality whether you're in 24 or 30fps.

 

Cheers,

 

Dylan

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Remember the A7SMKII records in Long GOP MPEG, which isn't a "frame" based format.

 

It takes one key frame and then only captures the changes from that key frame, every X amount of frames. So bit rate is hard to calculate per frame as the frame data is calculated off the first frame of a group + only the changes. The system is also NOT VBR, so it always records X data rate, no more and no less.

 

Usually data rate fluctuates based on frame rate. Most companies list bit rates at 30fps, so at 24fps, the rate would be lower.

 

The reason why there isn't a 100Mbps HD stream is simple... Sony doesn't want the camera to be "professional" in any way, so they limit it. They're also sticking to very specific MPEG specs, not wanting to waiver from the "norm".

 

This is one of the reasons so many people have dumped these still cameras that shoot video, they're so limiting. It's really frustrating to have a camera you like and it be super limiting in the codec. The only solution is to record the HDMI output with a decent recorder, but even that is still 8 bit 4:2:2, instead of 10 and 12 bit 444 like some of it's near-priced competitors.

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Thanks for your answers !

Yes I know their is inter-frame Gop compression so the pixel weight I gave were more useful as relative values to be compared against each other than absolute values.

 

Yes I agree those codec limitations are frustrating.

But the a7s II performs surprisingly well in post for an 8bit files camera I found.

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