Edit Suite: TBC Trivia

Many people don't quite understand what a TBC does. Even some of the video know-it-alls can't completely explain what happens to video inside a TBC. You've probably even asked one or two questions yourself about how a TBC works.

With the answers to the following common TBC questions, you can be a fount of TBC trivia the next time someone asks. More importantly, you'll understand what effects this crucial piece of videomaking hardware can have on your own productions.

What does the acronym TBC stand for?
TBC stands for Time Base Corrector.

What does a TBC do to a video image?

Video frames are made of stacks of horizontal scan lines. Each line contains an electronic representation of a thin slice of the image in the frame. When all the lines in a frame appear together on screen, the whole image is visible.

For a video frame to look as crisp and clear as possible, each scan line must begin at the same horizontal point just off the left side of the screen. If they don't, the picture will look fuzzy or "soft" because the details in each scan line don't line up with details in the adjacent scan lines. Severe distortions or variations in line position can even cause the frame to "break up" or jitter during playback.

Unfortunately, videotape recording and playback introduces a slight drifting of the scan lines--time base error--into nearly every video frame. Tape stretches easily, especially when it's wrapped around the hot mechanical parts inside your VCR for long periods of time. That causes the VCR to read a slightly different "start point" for each scan line recorded on the tape. The error is inevitable, and all videomakers must deal with it, regardless of the chosen tape format.

TBCs, however, can eliminate this problem by correcting the video output from a VCR. A TBC can realign the scan lines by digitizing each video frame and storing it in a digital "buffer." It then redraws each scan line in the proper position and sends the corrected frame back out.

If you connect the TBC between the video out jack of your playback VCR and the video in jack of your record VCR, you'll get sharper video images on dubbed tapes and edited master tapes.

While its main job is to separate the video frame into its individual scan lines and reassemble it again, most TBCs can do much more than just align the scans into a sharper picture. Those with a processing amplifier (proc amp) built in can correct color errors and adjust picture brightness or contrast. Some TBCs will even freeze and hold a frame of video.

Only more capable TBCs offer these extra features. Basic ones just correct the time base error.

What are the most common TBC controls, and what do they do?

Basic TBCs have little more than an on/off switch. More advanced units, like those with a built-in proc amp and digital effects, will have any number of the following controls.

• Video: Sometimes called luminance, this control adjusts the overall brightness of a picture. If you have footage that's a bit too dark for your taste, this control can brighten things up.
  
• Setup: Also called black level, this control adjusts how shadows and other dark areas of the image appear on screen. If subjects in your video appear too shadowed, adjusting the setup control may add more detail to those dark areas. Too much adjustment, however, can turn shadows into large gray blobs that look unnatural.
  By combining setup and video level adjustments, you can increase or decrease the contrast in a video image. Adjusting video level down and setup level up reduces contrast. Raising the video level and lowering the setup level increases contrast. Though these controls will never fully make up for poorly-lit video, they can help salvage an otherwise unusable shot.
  
• Chroma: This control adjusts how much color you see in an image. Some models call it saturation level or chrominance. If you notice that color on your dubbed tapes looks weak or non-existent, this control can boost the color from your playback VCR so your dubs look colorful again.
• Hue: Sometimes called tint, this control adjusts the colors themselves. If a red car looks too orange on your video, adjusting this control can help make it look red again. Hue may be able to partially compensate for incorrect white balance during the shoot.
  
• Field or Frame Freeze: Each video frame actually has two separate video fields. Each field carries half of the scan lines in the picture. The two fields interlace together on the screen to form the complete frame.
  TBCs that support freeze or still frames can often freeze the entire frame (both fields), or just one field alone. To freeze a subject in motion, you'll generally want to use the field freeze. Freezing the complete frame gives you all the available resolution, but you may see an annoying oscillation between two frozen video fields if the subject was moving quickly when recorded. For slow or static scenes, use frame freeze.
• Strobe: A variation on the freeze feature, strobe grabs and holds a still video field or frame for a brief moment, releases it, and then grabs another. The result is a simulated strobe lighting effect that looks good on video with lots of movement or action. TBCs with strobe usually include a control to adjust how long the freeze is held before it's released.
  As with the freeze feature, select field strobe if the action moves quickly, and use frame strobe if it moves slowly or not at all.

Some TBCs offer dropout compensation. What does this do?

When you repeatedly play a videotape, you run the risk of damaging the metal coating or putting tiny creases on the tape surface. This tape abuse causes tiny bits of the magnetic material to flake off the tape, taking your signal from that area with it. On screen, dropouts appear as thin white lines that flash for a brief moment. Dropouts won't harm your VCR, but they do take away from your video's impact.
A dropout compensator eliminates the problem by detecting the dropout and replacing the damaged scan line with a portion of the line scanned just before it. On screen, you'll see a cleaner, less distracting picture if you use a TBC with a dropout compensator. The amount of dropout compensation available varies in rough proportion to cost, from a small portion of a single line to several scan lines.