Problems solved

If I had a dollar for every time someone asked me why the DVD they've just created is jerky and unwatchable even when they've encoded it at a high bit rate, I'd have about, let me see, about four bucks. Possibly five. Not quite the amount I need for the new chroma key setup I'm after (see www.reflecmedia.com and click on Products/Chromatte), but enough to suggest that perhaps this is a problem that more than a few people, including myself, have encountered.

The symptoms are a relatively clean picture during static moments, which turns into a blurry, stuttering mess when things start moving about. To confuse things further, it may look just fine when you play it on your PC, preferring to wait until after you've spent four hours encoding your project before revealing itself during TV playback. No amount of tweaking the compression settings of your software seems to change anything, other than wasting a shockingly large chunk of your life, so what's the problem?

Restoring order

The answer can be found in the Field Order setting. Unless you're working with progressive scan video (like clips you've downloaded from the Web), each frame in a PAL signal is made up of a pair of interlaced fields. In both your recording and your MPEG-2 encode, these fields are combined before being compressed, then unpacked by the signal processor in your DVD player and squirted out to your telly. If you use the wrong field order, then your TV is showing each field out of sequence, in effect showing the last one first, and the first one last - hence the unwatchable movie. The reason that it looks okay on your PC is because your playback software is more flexible than your DVD player, and your monitor is a progressive scan display that shows both fields at the same time. To confuse the issue further, the two field order options are often described in different ways by different packages (Field Order A/B, Field Order 1/2 or Upper Field First/Lower Field First).

The good news is that none of this matters. If you're getting what looks like field order nastiness on your MPEG-2 encodes, just pick the field order option that you're not currently using and it should all go away (see here). If it doesn't, you might want to consider an eye test. Or a new TV.

Code breaker

Any videographer worth their salt will tell you that the first thing to do when you buy a new blank MiniDV tape is to stripe it before use. This means popping the tape into your camcorder, leaving the lens cap on (optional) and pressing record. By letting it run from the start to the end, your camera creates a single, continuous time code across the entire tape - not unlike formatting a hard disk for the first time. Frankly, it's a hassle, and I would dearly love to see tapes sold "pre-striped", but unfortunately, this can't be done for a variety of reasons.

The reason we're supposed to do this is to avoid time code breaks, which crop up when you start recording at a point on your tape that's a little bit too far from where you previously stopped - usually caused by you checking to see if the footage you just shot is okay then cueing the tape a little too far forward afterwards. Because your camcorder doesn't find an existing time code value when you press record again - which it would if your new recording overlapped the previous clip even slightly - it assumes that you're recording from scratch, and starts from a base time code of 00:00:00:00.

This may not seem like a problem when you're shooting. After all, it doesn't stop the camera from recording video, right? But the real problem with a time code break only reveals itself when you get to the capture stage. Most automatic capture tools (with the exception of Pinnacle's Liquid Edition) get a bit dazed and confused when they come across one and respond by simply stopping the capture process there and then, leaving you to resort to manual tape capture. It also makes clip logging and batch capture virtually impossible and you may find that you have to abandon footage to get your capture tool back on track, which is just the kind of thing that annoys me more than Channel Ten's "Home of Event Bigness" strapline.

But let's face it, how many of us have the patience and forethought to stripe a tape before use? I thought so. Well the solution is a handy, but generally overlooked camcorder function called End Search (sometimes abbreviated to End Sch or Rec Search), see this image. In your camcorder's Play (or VCR) mode select End Search at a point before the end of your footage and it'll fast scan to find the end of recorded area and then search frame by frame until it hits the very last frame in the sequence. Now, when you switch to Record (or Camera) mode, your tape's time code will be preserved and you'll be saved the aggravation of having to babysit your capture tool until it's finished.

Video grab

If you've ever tried to use the key to capture a frame of video from Windows Media Player, or a screen grab of a video application in use, you'll know that most of the time the video tends to get lost somewhere along the way, leaving you with a black square.

This is because the print screen function only copies the contents of your desktop to the clipboard, and video normally resides in something called an "overlay" - a separate panel that's handled by your graphics card rather than your CPU.

To get around this, you need to (temporarily) switch this form of acceleration off. In Windows Media Player 10, you do this by clicking on Tools-Options, then the Performance tab, and then the Advanced button at the bottom (as shown here). Uncheck the radio button next to "Use Overlays" and click OK.

Earlier versions of Media Player don't have the Advanced section, just a performance slider - move the slider to about halfway and it'll achieve the same end. You should now find that the &lt Print Screen &gt function now includes the video frame you're trying to grab. Just bear in mind that some editing packages depend on video overlays to work, so don't forget to undo these changes when you've obtained the screen shot you need.

If you've come up with solutions to any video problems that you've experienced, don't keep them to yourself, feel free to send them to me at laurence_grayson@idg.com.au so I can take the credit for your hard work.


HIGH-SPEED RANDOMNESS

It's pretty unlikely that you'll learn much from this Web site, but it's a pretty interesting way to while away a coffee break. Using a high-speed Kodak digital camcorder, students at Colorado's State University have put together a collection of random video clips in extreme slow motion. To see what I mean, go to http://www.engr.colostate.edu/~dga/high_speed_video/.

They're not the prettiest actors in the world, but some of the clips are definitely worth a look. The subject matter was all inoffensive at time of writing - but I make no guarantees regarding future content. They are students, after all.

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Laurence Grayson

PC World
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