View Full Version : Re: uncompressed video

Chris Kirtley
11-11-2003, 05:58 AM
Dear all,

This is an interesting discussion, but I think we would all agree that it is also quite confusing. I wonder if I might add some structure.

It seems to me there are three issues:

1. Method of recording - AVI or MPEG: MPEG 2 or 4
2. Method of storage - tape (analog or digital) or digital video file
3. Level of compression

In answer to Tomislav, I think there is a light compression applied to all recorded video, called YUV, which compresses shades of gray or luminance, called Y, and two chrominance values called U and V. The U vector points approximately in the red direction while the V vector points approximately in the blue direction. Any color can be converted from its RGB representation to its YUV values using a simple matrix transformation. The advantage to this approach is that the Y value can be digitized at full resolution while the U and V can be digitized at lower resolutions because the eye (and fortunately biomechanical analysis!) is not so sensitive to this information.

In my view, AVI (or QuickTime) is much preferable as he recording format because it can be easily edited. Due to the way it is compressed, MPEG files cannot easily (at all?) be pulled apart, making them difficult to edit. On the other hand, MPEG can be much more heavily compressed - mind you, I would suggest that we don't really want this for biomechanical purposes.

With regard to storage medium, I agree wholeheartedly with Rick Hinrichs that it is far better to have the video recorded directly to computer file, otherwise the downloading and conversion can be very tedious. I also think that it encourages the user to edit the data at the time of recording, whihc is much more efficent - most biomechanical movements are quite short. So, for me, the camera (or card) should record directly to disk or other digital storage medium. For this reason, I don't think FireWire is helpful. The same applies to using a card in the computer - OK for static setups, but I think most people are looking for a standalone solution.

I am personally always looking for a camera which is capable of recording short (up to, say 3 minutes or so) sequences at maximum digital frame rate (30 frames per second) with light compression to a SmartCard or mini-hard disk. I think this would be ideal for our purposes. To my knowledge, there's nothing yet out there which satisfies this spec, although the camera that I mentioned seems closest. I used to have a Hitachi MPEGCam 5 years ago (I think it is no longer on the market) which did this, but I had editing problems because of the MPEG format. Note that any camera will also have to have a fast, manual shutter speed and not suffer from the hibernation problem already mentioned. I carry a Fujifilm Finepix with me for still pictures which records short videos, but these are only 12 fps. I once tried the full "SLR" S7000 version which did seem to
record at 30 fps but was quite costly.

It is all very frustrating, but I really think that we are all waiting for Godot until such a camera comes along!


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