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 HandBrake adjusts the bitrate (that is, the size) to meet it.
Benefits over Average Bitrate
- Faster Encoding - No need to do 2-Pass encoding (thus almost halving the encode time.)
Constant Quality can produce the same quality as a 2 pass encode.
- Reduced file sizes
Videos usually have a mix of complex and less complex frames. (The latter requiring less bitrate to achieve a set quality level)
If less complex frames do not have bits wasted on them, your overall output filesize is reduced without any loss of quality.
Output File Sizes
Because it takes a different bitrate to reach a given quality level for any given part of any movie, the output size is unpredictable and varies from source to source.
Example: Given 2 sources of the same length, one could turn out to be 1.2GB, the other 1.9GB. The difference can be more or less depending on the source.
If your source material is consistent, you will get roughly the same file sizes out for each.
In particular, grainy sources tend to come out larger as they require a higher bit-rate to maintain all that extra detail in the video.
In this case, you may try turning on the "Denoise" filter or reduce the RF value a few points.
You should also note that even if a file size is significantly smaller than a counterpart encoded with target filesize, it does not mean it is any lower quality.
It's simply encoded more efficiently.
x264 Recommended Quality RF Values
The Quality slider ranges from (Low Quality) 51 to 0 (High Quality) For good quality output at reasonable file sizes, the following values are recommended:
- Standard Definition (e.g DVD's)
Use an RF value of around 20 +/- 1
As an example using the AppleTV2 preset at RF20, with 20 different sources, the average size was 925MB per hour of video. (Min: 625MB/h Max:1,503MB/hr)
- High Definition (e.g Blurays 720/1080)
Use an RF value of 22 +/- 1
Since HD sources are typically quality, you can get away with a slightly higher RF value than SD content without any perceived difference in quality.
You should note that the quality slider is not linear for x264. It is logarithmic (like the Richter or pH scales.) This means small movements in the numbers can have large results.
You can control the granularity of the slider in the preferences / options.
You can encode a few chapters of a source to experiment with different quality values if the above recommendations are not suitable. Encoding a 10minute sample in the middle of your source should give you a good idea of what to expect.
x264 and RF 0
RF 0 applies no compression. It is lossless: it compresses the source without throwing away practically any detail.
So should you use RF 0 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 RF 20. It throws away detail. When HandBrake uncompresses the video prior to conversion, the quality lost when the DVD was made is still gone. When you use RF 0 quality with x264, you're telling it to losslessly preserve the decoded, 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 an RF of ~19, which is roughly equivalent to how heavily the DVD is compressed. If you do go higher, your output will be larger than your input!