Video Bitrate Calculation Advice

NiVZ

Hello,

I'm pretty used to video converting but have a question about bitrates as I want to squeeze as much space as possible out of some videos I'm converting onto my kids InnoTab 2 and wondered if any of you can tell me if I've got this right.

I have to use MP3 96kbps audio so that's fine and the video is H.264 (the same as iPod)

Note: All bitrates quoted below are constant bit-rates.

So far I've been using 480x272@600kbps for my videos as most iPod settings use 600kbps (I know some use 768kbps). However, iPod videos are 640x480 but I'm only converting to 480x272.

So I was thinking:

480x272 = 130560 pixels
640x480 = 307200 pixels

Then divide 130560 by 307200 = 0.425

So that means the InnoTab has 42.5% less pixels to encode than the iPod format.

Can I apply that same percentage to the bitrate and still maintain similar quality as 640x480@600kbps?

eg 600kbps * 0.425 = 255kbps

I'd probably up this to [EMAIL="480x272@300kbps"]480x272@300kbps[/EMAIL] hopefully allowing me to fit twice as much videos as I currently have on it.

Any advice appreciated.

Thanks,

NiVZ

paddyrg

No, in a word.

H.264 is an implementation of MPEG4v10, it is a very clever encoding scheme which breaks the picture into chunks of varying sizes and tracks how those move between frames. There's different encodings for different frames, and the different codec implementations use those to differing efficiencies. In short, it is anything but a linear calculation!

Your best bet for the smallest possible videos at a certain bitrate is to get a good encoder. I don't know which one you're using, but the H.264 engine from MainConcept is probably the most efficient. It takes a long time to encode well as it takes multiple passes through the file. It also helps if you have a compression engine which can take advantage of your video card parallelism.

Might be easier to carry on as you are, and carry spare storage - MUCH cheaper and faster!

NiVZ

Thanks for the reply and explanation paddyrg

I've now gone and ran some test files (a 3min section of Ratatouille that has lots of fast movement/scene changes) at differing bitrates and have found that 350kbps to 400kbps give acceptable quality that is not notceably different to 600kbps when viewed on the InnoTab and will allow me to store around an extra 22 hours (15-18 DVD movies) on it

Thanks again,

NiVZ

paddyrg

If you're happy at 400kbps, brilliant :-)

Have fun!