Bob Currier, Synthetic Aperture
Give three different people access to the same video capture hardware and software and ask them to digitize the same original video, and you will probably end up with wildly different results.
Despite what some salespeople would like us to believe, there is more to digitizing video then simply plugging in a board and pressing a button on the screen. The manufacturers and software developers have given us marvelous tools to work with, but just as important as the tools themselves is understanding how to use them, and how the final use of the digital video influences the techniques that should be used.
This article covers some of the tricks and techniques that I have learned when doing video capture. Some of these I've learned from others, some I've read, but most were learned the hard way--doing it until it looks right. There is a lot of value in that last technique, so please do experiment, but I hope some of these ideas may help save you some time.
I work in a Macintosh environment so most of my references are to Mac-based tools. However, the techniques are platform- and software-independent and apply equally well to QuickTime or Video for Windows use.
EVEN THE LONGEST JOURNEY...
The best place to start any process is at the beginning. So it is with digitizing video: you need to start by getting the very best source material to work with. While you might think that the quality of the original video material would not matter by the time you reduce the image to a small window on the screen, reduce its frame rate, and limit the color resolution, exactly the opposite is true: the quality of the original video can make or break the final product.
You should start with the best video source available. You may not always have a choice if someone is handing you the video to be digitized, but you should always at least request the best format. In order of increasing quality, the common video formats you are likely to deal with are: VHS, 8mm, 3/4" U-Matic, S-VHS, Hi-8, Laser Disc, MII, and Betacam SP. Always try to use the best quality that is available. Just be sure you can handle the tape format, and that your digitizing hardware can handle the video signal format: composite, S-Video (Y/C), or component.
Because the original video will be greatly reduced in size, and because of the wide differences in computer monitors that will be used for playback, the original video should be well lit, with good contrast ratios. A muddy original will only get more so after digitizing.
AVOIDING GENERATION LOSS
One of the great promises of digital video--as with digital audio--is the prospect of limitless generations without degradation. Strictly speaking this is true, but only when making an exact copy. As soon as you throw some processing of the image into the equation, then we quickly get back to generation loss problems just as with analog video--only things can degrade much more quickly if you aren't careful.
The culprit is the compression that is used to reduce the size of the video data, starting with the video capture hardware. Most of the common hardware makes use of compression during capture, typically motion JPEG or proprietary schemes. This is most likely not the compression scheme that the final product will use, which guarantees at least one generation will be lost when you convert from the original compression method to the final.
And each time the data is de-compressed and re-compressed during the editing and effects process, you will suffer generation loss and degradation of the image. Because of the type of artifact that digital video compression creates, the degradation caused by repeated de-compression/re-compression very quickly becomes objectionable.
To reduce this problem--we can't entirely eliminate it until our tools are up to dealing with uncompressed video at every step--switch to uncompressed video at the earliest opportunity. When capturing original video using motion JPEG-based hardware, immediately create a "sub-master" without any compression at all. This can be accomplished using any of the standard tools: Adobe Premiere, ConvertToMovie, etc. by specifying the codec to be "None."
Use this uncompressed sub-master file for all subsequent editing steps, and don't use any further compression until you are ready to generate the final output file. Uncompressed video takes a lot of disk space and processing time, but the elimination of repetitive compression significantly improves the quality of the final product.
You can further improve the quality of this uncompressed sub-master by capturing the original video at twice the size you want for the final product, and scaling the size when you convert it to uncompressed format. This helps hide some of the original compression artifacts by averaging them out during the scaling process.
For example, if you plan on delivering the final product at 160 x 120 pixels, then capture the video at 320 x 240. Then create an uncompressed sub-master at the 160 x 120 size. The quality is improved over simply capturing at 160 x 120 and then converting to uncompressed.
KEEPING YOUR FRAMES CLEAN
Video capture hardware will digitize the entire video frame, including areas that are hidden by overscan during normal playback. This is particularly pronounced when the source material is from consumer level formats and equipment, such as VHS and Hi-8, which rely on overscan to hide their sins.
If you simply digitize and display the full frame, you will see head switch noise and other artifacts. Using a TBC to clean up the signal will certainly reduce the problem by eliminating flagging and other timing related problems, but some cropping of the video image will most likely be necessary. Besides simply giving you better looking video, you don't want the compressor to spend its bandwidth compressing noise instead of the real image. If you can, it is a good idea to also reset the video levels at the same time so that the capture board is seeing a clean video signal.
In order to allow for cropping, it is best to digitize the original image at a slightly larger size than the final product. Depending on the size of the final image, the amount you will want to crop will be anything from a few pixels to ten or more. Whatever software you use to render the final video should have cropping capability.
Keep in mind that some video compression methods work significantly better if the frame size of the video is kept to certain multiples or ratios. For example, the Cinepak compression codec is based on examining 4x4 pixel regions of the video image. This means that when using Cinepak it is vital that your final image size be a multiple of four in both height and width, or the playback performance of the final product will be reduced.
DEALING WITH GAMMA
If your final product is to be used across multiple platforms--such as on both the Macintosh and the PC--then you have some more decisions to make. One difference between the two systems that is often overlooked is the different way that the gamma of the display monitor is handled. If you don't take this into account, the appearance of your digitized video will suffer greatly when you move it from one platform to the other.
On the Macintosh, all monitors are assumed to have a gamma of 1.0. Built-in to the display hardware is a gamma lookup table that automatically adjusts the data that is sent to it. With Apple-brand monitors, this is fairly accurate, and even with other third-party monitors the results are fairly close to a straight-line 1.0 gamma. System extensions, such as the Gamma extension that comes with Adobe Photoshop, can fine tune the gamma to be even more accurate.
On the PC, things are quite different. The hardware makes no adjustments for gamma, so things are left up to the monitor manufacturers. This means that the gamma of a PC display will be somewhere in the 1.8 to 2.5 range, depending on the monitor, adjustment of controls, etc.
So if you create your video masterpiece on a Macintosh, and then move it over to a PC for display, you will find that the image is very dark and constrasty.
Fortunately, you can adjust for this when you render the final video. Premiere, for example, provides a Gamma Adjustment filter which allows you to correct gamma during rendering. Ideally, you should create two separate movies, the first with a gamma of 1.0 for use on the Mac, and the other adjusted for use on the PC. More often, you will need to choose a compromise setting and render a single movie for use on both systems.
People seem to be more tolerant of a slightly washed-out look on the Mac than they are of a dark, contrasty look on the PC. So when creating the scaled, cropped, uncompressed sub-master, use the Gamma Adjustment filter set to a value of 0.6 to give an image suitable for playback on both platforms. (A setting of 0.6 will adjust the video for a monitor with a gamma of 1.67, the reciprocal of 0.6.) Any lower than this and too much noise appears in the dark area of the images on the Mac; much higher and the image is still too dark on the PC.
Unfortunately, the Gamma Adjustment filter that comes with Premiere is rather limited in its range of selections, moving only in steps of 0.1. To find the best value for your material, be sure to try the resulting movie on a number of different monitors. This is a compromise solution at best, and you need the widest range of samples possible.
If you are used to viewing D1 video on studio monitors, you are going to be just a bit disappointed by even the best computer-based video. But if you use care in your choice of techniques, you can create results that are the best that the current state-of-the-art allows.
Bob Currier is President of Synthetic Aperture, a multimedia production company specializing in digital video and QuickTime VR. He also serves as Sysop of the Macintosh Multimedia Forum on CompuServe.
He can be reached at email@example.com. Be sure to visit the Synthetic Aperture web site at <http://www.synthetic-ap.com/> for more tutorial information, sample content, and information on new media services.
This article orignally appeared in a slightly different form in Computer Video magazine.
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