Video compression of DVDs and personal MP3 players

I’m assuming the next generation of MP3 players will have 60GB+ of hard disk space as well as an actual LCD screen capable of playing movies. IPod recently came out with one and I’m guessing this kind of MP3 player will be more and more common.

So what kind of compression technology do you use if you want to put DVDs onto your MP3 player? Do you use MP4 or something else? What kind of compression rates do you get with various video compression methods if you start with a 4.7 or 9.4GB DVD? Would an MP4 file be under 1GB if it starts out as a 9 GB DVD?

MP4 is a common format for portable media players; 9GB of DVD content won’t typically compress down as small as 1GB without noticeable loss of quality, but MPEG can usually be encoded at any bit rate you like, so it’s technically possible to make a really small file out of a DVD - it just won’t be pleasant to watch.

However, these handheld video devices usually have a small screen with a resolution like 320 x 240 - so unless you intend to take the footage off the device to play on something bigger later, you can resample down to a smaller resolution before compressing - this often results in a considerable size saving, simply because the size of each frame (in pixels) is proportional to the square of the perceived ‘screen size’.

Mangetout has it right. Reauthoring the DVD to a smaller image is the way to go. If you don’t mind a movie that plays like you’re watching a webcam on dialup, then you could get the file size way down, but even with the best compression, you always eventually hit the file size = quality problem. Get yourself some video editing/DVD authoring software and play with it a bit and you will quickly get a feel for what you can do.

Video Help has lots of good info.

A datapoint for you - I have an iRivier Clix 2GB flash device, with a 320x240 screen. Using the iRiviter software ( I can take a full length DVD movie and compress it down to 320x240x15fps MP4 200kbps video, 64kbps audio, with a resulting file size of around 200MB.

The video quality is perfectly acceptable on such a small screen.

Keep in mind that even on a 9 GB DVD, not all of that 9 GB is the main movie, and not all the bitrate given to the main movie is strictly necessary - authors tend to just set the bitrate high enough to fill up the disc, but you can still drop the bitrate significantly without noticable loss of quality, even if you stick with MPEG 2. DVD Shrink uses a quick hack instead of actually reencoding the video, but it can still drop the bitrate by up to 50% with very watchable results.

If you’re transcoding to MPEG 4, you can often get the main movie to fit on one CD (700 MB) at near-DVD quality, just by using multi-pass encoding and reducing the frame size by 100 pixels or so in each direction.

Oh, Diddly mentioned something I meant to, but forgot; resampling at a lower frame rate also saves space, as does increasing the keyframe interval, but both of these can quite noticeably affect the quality.

I’ve had a 40-GB iRiver player since Spring 2005. It handles AVI video and I regularly watch 30 and 60-minute TV programs. I’ve also uploaded 90 and 120-minute movies onto it with a lot of disk space remaining. There is a third-party application, iRiverter, that handles the conversion/compression of the files.