KVideoPod Pro supports three different methods of controlling the rate of encoded video. The default method is average bitrate. The other methods are target size and constant quality.

Average Bitrate and Target Size

Bitrate is the number of bits dedicated to the video in a second. And remember, that gets split between 24-30 frames. When you set an average bitrate, the encoder will vary the number of bits given to any one portion of the video, but try to keep everything at the average you set. By default, this average is 1500kbps.

The way the encoder varies the bitrate is that it dedicates more bits to the complicated parts and fewer to the simple parts. But how can the encoder know how much to vary the bitrate? That is, how does it know how close or far away it is from the average? Well...it doesn't. It guesses. And, usually, its guesses aren't quite right.

That's why there's 2-pass mode. When you use average bitrate in 2-pass mode, the encoding will take twice as long. On the first pass, it won't write your movie to the hard drive. Instead, it will record a log file with statistics about the video. Then, on the second pass, when it does write your movie to disk, the encoder has a log file to use, so it doesn't have to guess. This means it can do a better job doling out bitrate, and you get a better quality video that's closer to the average bitrate you told it use.

If taking twice as long doesn't sound like a good idea to you, look towards the Turbo checkbox. By enabling this, when encoding 2-pass with x264, you can cut down the time of the first pass by 50%-75%. There will be a drop in quality, but so slight as to be imperceptible to the eye.

But what about target size? Target size, really, is just the flip-side of average bitrate.

The size of a video is the bitrate times the length. So when you set a target size, KVideoPod Pro just divides it by the movie's length to get a bitrate the encoder should use as an average. It's really the exact same thing.

Constant Quality

With the average bitrate or target size methods, you control the size of the output file but give up control over the video's quality.

Constant quality mode does the opposite; you specify a quality level and KVideoPod Pro adjusts the bitrate (that is, the size) to meet it.

Constant quality usually means CQP: "constant quantizer parameter." A quantizer parameter, or QP, is the level by which something is compressed. A QP of 15 means less compression (higher quality) than a QP of 20. By setting a constant quantizer, you force a certain quality level. Because it takes a different bitrate to reach a given quality level in any given part of a movie, the output size is unpredictable and varies from source to source.

x264 Quality Quirks

While the other encoders use a linear quality scale, x264's is logarithmic, like the Richter or pH scales. What this means is that 0% is the strongest compression and lowest quality, while 100% losslessly compresses the source without throwing away practically any detail.

So should you use 100% to perfectly preserve the source? Nope. Not at all. In fact, you'll end up with video that's way larger than the DVD, but doesn't look any better.

See, DVDs use lossy compression to squeeze down the raw video the studios use to make them -- sort of like a quality level of 80%. It throws away detail. When KVideoPod Pro uncompresses the video prior to conversion, the quality lost when the DVD was made is still gone. When you use 100% quality with x264, you're telling it to losslessly preserve the uncompressed video feed, not to losslessly preserve the DVD. Both have the same picture quality, but the uncompressed feed takes up a lot more space.

To sum up: when converting from a DVD source, there is no reason to go above 80% quality, which is roughly equivalent to how heavily the DVD is compressed. If you do go higher, your output will be larger than your input!