Advanced Codec
The Codec tab under the Advanced tab is divided into three areas. They are Codec Performance, Rate Control, and Frame Control described in the following sub-sections.
Codec Performance
The options available for Codec Performance are the same as the Quality options on the Basic page. Flip back to the Quality section of the User Guide to jog your memory.
Rate Control Based on Bitrate
Adjusting the Rate Control setting begins with choosing a Mode. Would you like 1 pass or 2 passes? The answer may depend on how much time you have. The 1 pass option works just fine, but if you insist on getting the best possible picture quality, then tell the doctor to make 2 passes through your input file. That ensures that Dr. DivX doesn't miss any opportunity to optimize the picture quality but it also means the whole process will take about twice as long.
After you flip a coin to decide how many passes to make, you need to choose a maximum Bitrate. The higher the bitrate, the better the picture but it also makes for larger file sizes. You have to make the tradeoff. In practical use, video codec bitrates over 1300 kbps look clear even on big screen TVs. You'll certainly find them better looking than your average television broadcast anyway. If you are encoding files for small screens like PDAs or phones with only 320 pixels across, you may want to run the video down to 130 kbps. The result looks pretty decent on 3 inch displays and an hour of video is only slightly more than 100 Meg in size.
Rate Control Based on Quality
If quality is more important to you than file size, then try out the 1-pass quality-based mode. Using this option you get to pick a target quantizer. Sound nifty? The quantizer is just a number between 1 and 31. Opting for a lower number will give you better picture quality but you'll also get larger files. Larger numbers reduce the file size but give you worse quality.
In practice, a quantizer of 4 is good quality with reasonable file size. You may want to move it down to 3, but anything less than that won't make much visible difference for most people.
Frame Control
The Frame Control box will allow you tinker with key frames and rates. Each setting is explained in the following sub-sections.
Bidirectional
While many of the settings available under the Advanced Codec tab will simply affect picture quality, the Bidirectional setting can affect compatibility so this is an important one to consider.
Without going into too much detail about how video compression works, you should understand that the output file is basically a mixture of partial images with a few complete pictures sprinkled around (keyframes). DivX will start with a complete picture then progressively apply changes to make the picture move. Every once in a while, a new, complete picture is displayed and the process starts over again.
Turning the Bidirectional setting off, reduces the number of complete pictures that a DivX player has to track while displaying your video. Changing the setting to Adaptive Single Consecutive, makes DivX players remember more pictures at one time.
The catch is, not all players are capable enough to handle Adaptive Single Consecutive encoded videos. Particularly, small devices like phones may have trouble playing those videos. However, any devices that are DivX certified for the Portable profile or better, will be able to handle those videos with no problem.
You certainly want to use Adaptive Single Consecutive if your DivX player can handle it. Most can and it will provide significantly better picture quality in almost every case.
Max Keyframe Interval
Fundamentally, video compression is achieved by not including every frame from the original video. Only a few key frames are included in the compressed video. The DivX player creates the frames in between by incrementally applying the necessary differences. Dr. DivX tries to figure out the best place for keyframes and will try to go as long as possible without using keyframes. This gives the highest compression but it's also a lot of work for the DivX player. The Max Keyframe Interval allows you to limit how long Dr. DivX will go without a keyframe.
If you are experimenting with low-bitrate encoding, you may want to increase the Max Keyframe Interval to improve the over-all quality assuming your player can handle it.
Framerate
The Framerate setting controls how many frames per. second will be displayed while your video is playing. The exact numbers available for you to choose from will be dependent on the framerate of your original video, but you'll usually have two options that amount to full-rate or half-rate. Typically, you'll want the highest framerate possible. However, if you are making videos for small, portable, slow devices like PDAs or cell phones, you may want to drop the framerate to make sure that the player will be able to keep up. The result might not look as smooth but at least it will play.