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# Bit rate in video rendering and converting softwares

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 Bit rate in video rendering and converting softwares on Nov 14, 2011 at 12:04:25 pm

Hello, I want to find out more about bit rate-size of a video.

Well, concretely, I want to convert (from mkv to H.264/MPEG4-AVC) a video or a movie from 720p to 320x240. But I have one big problem: I want the output video to be maximum (highest) quality, and also I want it to be smallest by size.

exactly, I mean, for example a video in 320x240 and H.264/MPEG4-AVC, has highest quality at 800kbps and I don't know about it, then I convert the video at 1000 kbps, so the distinction of the bit rates: 1000-800=200 just increases size of the video, and quality is the same as in 800kbps. So, I mean that I want to know what is the maximum bitrate of a 320x240 video quality (in H.264/MPEG4-AVC) to vindicate smaller size. Or, is there any formula or something like that to calculate maximum quality bitrates of different kinds of resolutions of videos, for example 1280x720 or 1920x1080.
Thanks a lot.....

 Re: Bit rate in video rendering and converting softwareson Nov 14, 2011 at 3:35:40 pm

Quality is subjective. Compression is lossy. General you can use Bits Per Pixel as the closest to an objective measure of quality.

http://www.streaminglearningcenter.com/blogs/h264-encoding-parameters-of-th...

http://www.streaminglearningcenter.com/blogs/the-essential-key-to-producing...

and this might help in the above article
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.

 Re: Bit rate in video rendering and converting softwareson Nov 15, 2011 at 6:22:38 pm

I wrote on this years ago (very simplistic and meant for beginners):

http://www.polymathsunited.com/2009/11/video-bitrate-what-bitrate-should-i-...

...but the real answer is test, test, test. 720p should look great at 2.5Mbs. If your content does then try it again at 2.2Mbs and 1.8Mbs. If you're worried about speed vs. quality, see how low you can go with single pass, then double pass, change the variability percentage, key frame sensitivity, etc.

Bitrate is becoming less and less an issue, almost a settled question.

William Caulfield
Video Encoding Manager
http://metroencoding.com

 Re: Bit rate in video rendering and converting softwareson Nov 18, 2011 at 3:22:12 am

01. There is a quality gain but it's imperceptible to the eye. You don't see it but the noise, sharp edges and others picture details are reproduced more accurately. That's why the file is bigger.

02. Never, never, never use bitrate as a quality measure, it's not. At the same resolution, some content will need more bitrate than others to achieve the same quality level. The best example is a sport game vs a news interview. If the static news interviews is okay at 500Kbps, the sport game with lots of action, fast moving camera and scenes cuts will be horrible.

What I understand is that you want to encode each of your files, whatever the content or resolution, to the maximum perceptible quality. Instead using an average bitrate for video encoding, you should use a constant quality index.

02. In the video panel, encode it using "Constant Quality" with an RF value around 19 to begin with.

03. In the advanced panel, max out all the options and disable mb-tree.

04. Review the encoded video and lower the RF value to 18 to increase quality or increase it to 20 to lower the quality until you find the right spot that please you.

05. Don't look at the bitrate, ignore it.

You don't need to go insane with the encoding options because at a point you will get about no quality increase or file size decrease. Some good options to use are :