HandBrake, bitrates, and huh!?

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In preparation for an upcoming Playlist column in Macworld , I’ve spent more time than I care to recount ripping DVDs to an iPod-friendly format. Although I’ve used a variety of tools to do the job, like many, I’m particularly keen on the free and open-source Mac and Linux utility, HandBrake.

Though I’m generally pleased with HandBrake’s results, something about those results stumped me. When ripping movies to my Mac I discovered that changing the bitrate of my movies didn’t always result in smaller files.

For those who react to this statement with a noncommittal “ And why should it? ” here’s a little background.

Video encoders allow you to muck with three primary factors—frame-rate (the number of frames displayed per second), resolution (the size of the frame measured in pixels—720-by-480, for example), and bitrate (the amount of data output in a second—2,000kbps, for instance). Changing any of these factors should change the size of the resulting file as that change either demands that more or less data is thrown into the mix.

I’ve found that while this is largely true of frame-rate and resolution, it’s not necessarily true of bitrate. Think of a bitrate setting as a pipe with data flowing through it. A small bitrate setting creates a small pipe and data must be constrained to go through it (this constraint contributes to smaller file size). Increase the bitrate setting and data can be less constrained (helping to create larger files).

At some point, you create a pipe so big that all the data can easily flow through it without being constrained. Continue to increase the size of the bitrate setting and you see no benefit—files get no bigger and data rates don’t increase because you’ve allowed all the data to pass through.

And this explains my initial confusion. When ripping a chapter from Master and Commander: The Far Side of the World at 640-by-272 I found that files ripped with a bitrate of 2,000- and 2,500kbps were exactly the same size (and offered the same data rate). I didn’t see a change in file size and data rate until I ripped at 1,750kbps (and the change was fairly negligible).

When I bumped up resolution to 720-by-304, the bitrate saturation point increased as well. In this case, there was a slight difference in size and data rate for files ripped at 2,000- and 2,500kbps, but no difference between 2,500- and 3,000kbps files.

So what does this mean for you?

The iPod supports two video formats—H.264 and MPEG-4. Apple suggests that you can encode MPEG-4 movies at an average bitrate as high as 2,500kbps. I’m simply suggesting that you may see no benefit from recording at bitrates that exceed 2,000kbps unless you’re also hitting the top end of the iPod’s resolution limits (which is 900 macroblocks —16-by-16 pixel blocks—for MPEG-4 videos). It doesn’t hurt to enter higher bitrate settings—as I suggested, you’re just making the pipe bigger—just don’t be disappointed when a setting of 2,200kbps doesn’t make your video look appreciably better than 1,750kbps.

While I have your attention I might also mention that as you increase bitrates when encoding with HandBrake, your video gets darker. In my tests a movie encoded at 1,000kbps was noticeably lighter than that same movie encoded at 2,000kbps. The 2,000kbps movie did have more detail, however.

This story, "HandBrake, bitrates, and huh!?" was originally published by PCWorld.

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