Maybe we ought to have a thread where we draft up and refine a good FAQ on the subject. Here's a first stab at it, and obviously, changes, amendations, and clarifications are needed.

When video is recorded, it is usually compressed; otherwise, it'd require writing massive amounts of data to the memory card, which may not be large enough or fast enough to handle it. This requires the camera to perform some very intensive processing to compress the data, so that it can be written to the memory card.

Because this processing takes place during shooting, the camera has to do this very quickly. So the camera relies on shortcuts, which will be described later in in this FAQ. As a result, the camera uses “lossy” data compression, and the image and motion quality can be compromised. (For now, we recommend reading Wikipedia on http://en.wikipedia.org/wiki/Video_coding#Video.)

Vitaly's hack enables users to reconfigure the way the camera encodes the video, in order to improve the quality of its recording images and motion.

This reconfiguration is complex, and sometimes, the settings may not work with each other. (For example, some patches with high image quality may have trouble recording acceptable sound, or “spanning” across multiple files. More about this below.)) So some users have spent a lot of time and effort experimenting with the settings, and they have developed “patches” with various strengths and weaknesses. These include the Sanity patch, the FlowMotion patch, and the many patches developed by Driftwood.)

New or less-techy users may find it daunting to read through Personal View's lengthy forum discussions, which can be very technical. This FAQ is intended to give such users a rough idea of what's going on.

If the GH2 is shooting a lot of action, it can’t decide to simply shoot constant I-frames, because that’d create too much data for the system. And if it’s shooting a lot of placid, low-movement images, it can’t compress it down to a still image. It can’t decide to use macroblocks that are too big, because the image would start to look blocky, and it can’t use small macroblocks too often because, again, it’d create too much data.

So the camera has to have limits. The camera is creating data that falls within a range, a sweet spot between good compression and high detail. This range is specified by the bit rates.

A unhacked GH2’s highest bit rate is 24mbps. So the GH2 is constantly adjusting its compression techniques, as described above, so that the data stream doesn’t exceed 24mbps.

But if you have a hacked GH2, you can specify a higher bit rate—say, 40mbps, 80mbps, or in the case of some of Driftwood’s patches, more than 100 mbps. Now, the camera has the leeway to use compression techniques that are less lossy, and preserve movement and detail more effectively. That is the advantage of higher bit rates.

This does not always work. You can’t simply use Vitaly’s hack to specify a 100 mbps bit rate and the camera to work flawlessly. Your memory card may not be fast enough or large enough to handle a flood of uncompressed data. The CPU can overload. Or, the other settings, like the GOP values, create conflicts or don’t work right. Happily, we don’t have to go into why this doesn’t always work. That’s for the people who design and test the patches, and who read all of the forum posts about these things. They try various combinations of GOP sizes, bit rates, specs for the macroblocking and motion prediction, until they find combinations that do work.

The rest of us don’t need to understand this at that level. All we need to know for now is what the “bit rate” really means for us. The bit rate is a number that shows how much range the camera has in compressing video; the higher the number, less loss in compression.

These are two techniques by which video is compressed. The simpler technique is “intraframing.” For each frame of video, the camera compresses it down in the same manner as a JPEG picture file. This can be very lossy, or even lossless, but it also requires a lot of processing work by the camera.

A more complicated method is “interframing.” Here, the camera records an initial frame as a full photo. The camera then records the subsequent frames as only the changes that occur from the previous frame. (See http://en.wikipedia.org/wiki/Video_coding#Video) for a discussion.) So the video file may consist of the initial Frame 1, and then data that contains only changes between Frame 1 and 2, then between frame 2 and 3, etc.

Since most frames aren't stored as full frames, but are calculated, this technique requires the camera to make a lot of calculations before writing the data to the memory card. It is also a lossy technique. It can be made less lossy by sampling full frames more frequently.

Refinements to interframe Not written yet: see http://en.wikipedia.org/wiki/Inter-frame#H.264_Inter_frame_prediction_improvements for now.