Understanding EOS HD Compression Options

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.

Let's take a look at the two ways to access the "Movie Recording Size" menus on your camera. The following steps use a Canon EOS 5D Mark III that is running firmware version 1.1.3.

1. In the movie mode, press the "Menu" button.
2. Use the joystick to navigate to the fourth shoot menu (Set of red menus) labeled SHOOT4:Movie.
3. Navigate to Movie rec. size (third item down).
4. Press the "set" button and turn the quick control dial to select the desired option.

1. In the movie mode, press the "Q" (Quick Menu) button.
2. Use the joystick to navigate down to the third item from the top (which displays the current resolution).
3. The resolution can now be changed by turning the main dial or quick control dial. Pressing the "set" button will also access all the resolution options.

A typical memory card configuration with dual card slot cameras such as the 5D Mark III and EOS-1D X cameras is to use both slots. Once Card Slot 1 or 2 is designated as the primary card, enable the "Auto switch card" function. *Note: that the EOS-1D X offers dual CF card slots, while the EOS 5D Mark III include one CF card and one SDXC card slot. To access this function in the camera, simply press the "Menu" button and go to SET UP1 > Record function+card/folder sel. (the yellow menu section). Choose Record function > Auto switch cards.

Be aware that for video recording, current Canon EOS SLRs with dual card slots cannot record video simultaneously to two cards, so "Record separately" and "Record to multiple" menu options are ignored – they are used for still-image shooting only.

Notice the difference in available recording time for both compression methods. Regarding file size, an IPB 30-second clip shot at 1920x1080 24fps is 85MB and an ALL-I 30-second clip also shot at 1920x1080 24fps is 106MB. Typically, however, you will see a difference in recording times per 4GB file on your memory card. According to standardized tests performed by Canon:

• IPB compression, 1080p – approx. 13 minutes (for 4GB file)
• ALL-I compression, 1080p – approx. 5 minutes (for each 4GB file)

Note that the newer Canon EOS SLRs that provide the choice of ALL-I or IPB compression also now allow continuous recording up to 29 minutes, 59 seconds – the maximum size of each file remains at 4GB like similar Canon EOS SLR predecessors, but if you fill up a file, a new one is created immediately on the memory card, and recording continues without interruption until 29:59 has elapsed.

Here's an interesting fact: Even though shooting with ALL-I compression generates significantly larger file sizes than IPB, ALL-I footage not only requires less computer processing power than IPB or IPP, but the playback is generally smoother on lower specification computers, and it is easier to edit individual frames without degradation of the image quality.

Counter-intuitive as this may seem, it actually makes sense, because Intraframe (ALL-I) compression works by separating each frame and compressing it individually. Therefore, each frame is seen as an individual image, and there is no rendering needed to extrapolate data from the GOPs used in IPP and IPB. In other words, with ALL-I the compression algorithm does not have to put in extra work in comparing data between frames.

With the introduction of these compression methods in new Canon cameras, we now have the option to choose what kind of compression we want to apply to our video files. If you are in need of smaller file sizes – remember, at the potential risk of decreased image quality – go with IPB. IPB works especially well where long, continuous recordings are likely (such as interviews, concerts or press conferences, for example), and is especially well suited in situations where frequent, precisely-timed edits and cuts are not likely.

However, if you are in need of the highest quality video and file size is not an issue, use ALL-I compression. The main benefits to ALL-I include less computer processing power which results in smoother playback and easier, faster editing.