With the release of the EOS-1D X and the 5D Mark III, Canon has introduced two new compression formats. Interframe (IPB) and Intraframe (ALL-I) are essentially different methods of compressing movie files. There are varied benefits to using either IPB or ALL-I compression configurations. Both the EOS-1D X and 5D Mark III cameras record movies using the H.264 codec in the .mov container; however, it is important to note that the compression type within this codec has changed for these two cameras.
Even though a digital video camera may record 24 or 30 frames per second, most video compression methods don't simply take every single frame as a single entity, and compress them individually – the resulting video file sizes would be huge. Instead, what often happens is that one frame for every half-second of video recording is maintained, and in the subsequent frames, only visible changes are retained. The compression system therefore looks at one frame, and "predicts" what data the next approximately 15 frames would have.
The individual video frames that are fully retained are called key frames (also called intraframes); the subsequent (roughly 12 to 15 frames) are predicted frames. Intraframes are used as reference frames to assist in the compression process.
The IPP compression method is used in many existing EOS models like the Canon EOS 5D Mark II, EOS 7D, 60D, and Rebel series models. This compresses video files in a way similar to that described immediately above. At 30 fps, the first frame in a group of 15 is a key frame, and the next 14 are predicted, based entirely on the data from the previous key frame and preceding predicted frames. With this compression method, each group of 15 frames (at 30 fps) is stored in what is known as a Group of Pictures (GOP). The drawback to this method is that frame-by-frame editing often results in slightly lower image quality.
The Interframe (IPB) method is now an option with Canon's high-end EOS cameras launched in 2012, such as the EOS 5D Mark III. The IPB method similarly breaks video frames into a key frame and subsequent "predicted" frames. But unlike the previous IPP method, the newer IPB compression type has the ability to predict the content of future frames by referencing both previously captured frames and subsequent frames. In this setup, only the data that changes in the following frames is recorded.
Incidentally, the "B" in IPB stands for Bidirectional compression. If the IPB method is used for an interview where the camera is locked in place on a tripod, the background may stay the same in each frame, with only the speaker's mouth and body showing movement from frame-to-frame. In this case, data from the last frame of the background can be used (because the data has not changed), and doesn't need to be recorded again for the next few frames. The moving data that changes in each frame – the speaker's mouth and body – would be extrapolated for each successive and compressed predicted frame, using info from the frames before and after to fill in the information needed.
If you are using the IPB compression method and you wish to shorten the beginning or end of clips in-camera, it is very important to know that in-camera editing can only be done in one-second increments. This is one of those little details than can make a big difference in your workflow.
On the other hand, the Intraframe (ALL-I) method is designed for users working in high-end editing systems or those looking for the highest quality possible. ALL-I stands for "Intra-coded Frame." This method differs from IPB and IPP in that all of the frames that are captured are treated as key frames. In other words, each frame is compressed, but treated as a separate, single image.
Since each frame within a 24fps or 30fps sequence is compressed as an independent, separate frame, instead of the extremely compressed method used in IPB or IPP for "predicted" frames, it stands to reason that ALL-I file sizes will be larger. In fact, you can expect ALL-I files to be about 3x the file size of the same scene shot in the IPB method. The ALL-I compression option was introduced in the EOS 5D Mark III and EOS-1D X cameras, which were launched in 2012.