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Frame Size & Bit rate: which is the relation formula?

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federico betta
Frame Size & Bit rate: which is the relation formula?
on Jun 12, 2013 at 10:03:56 am

Dear ng,
there is a formula to understand the relation between the frame size dimentions and the amount of bps in data rate compression?

For example,
if my video looks well at 5000kbps for 1280/720 frame size, at which kbps I have to compress a 1920/1080 for the same result?

I conceive a formula like this:

1280/720 has 1280*720=921600 total pixel
so every pixel use 5000/921600=0,00542534722222 kbps

1920/1080 has 1980*1080=2073600 total pixel
so to have 0,00542534722222 kbps for every pixel I need 2073600*0,00542534722222= ca 11250kbps

It's right my formula?

Thanks to all of you.

Federico


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Craig Seeman
Re: Frame Size & Bit rate: which is the relation formula?
on Jun 12, 2013 at 11:38:18 am

This is a great Bits Per Pixel app and it's FREE
https://itunes.apple.com/us/app/bits-per-pixel/id346475050?mt=8

You have to take into account Frame Rate as you can see the app does.

The formula is
[bits/pixel] = (bitrate[in kb/s] * 1000) / (width * height * frames/s)

There's the Ben Waggoner Power of .75 rule. Basically as you target larger size, codecs get more efficient, you don't need as much bits per pixel to maintain quality. Jan Ozer mentions this in the article below.
Good info in this article although I'm pointing to Page 3.
http://www.streaminglearningcenter.com/articles/choosing-your-streaming-res...

One caveat is the bigger the video window, the lower the data rate necessary to support the same quality. In fact, Microsoft’s Ben Waggoner, a respected compressionist, calls this the power of .75 rule. Here’s a snippet of an email he sent to me explaining the rule.

Using the old "power of 0.75" rule, content that looks good with 500 Kbps at 640x360 would need (1280x720)/(640x360)^0.75*500=1414 Kbps at 1280x720 to achieve roughly the same quality.




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federico betta
Re: Frame Size & Bit rate: which is the relation formula?
on Jun 12, 2013 at 12:01:15 pm

wow Craig, it's a great answer!

I've only one matter more. I'm not math challenged so I don't understand what does mean the ^ symbol.
May you explain me?

thank you


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federico betta
Re: Frame Size & Bit rate: which is the relation formula?
on Jun 12, 2013 at 12:07:29 pm

sorry,
wikipedia is better than disturb cc forum users.
2^3 = 8

anyway, thank you very much for great answer.


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Craig Seeman
Re: Frame Size & Bit rate: which is the relation formula?
on Jun 12, 2013 at 12:50:48 pm

Yes, 2 to the 3rd power.
2x2x2=8



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