A TBC Tale

Now that the editing is finished, it's late at night. You've had too much coffee to sleep; you see visions of yourself accepting that coveted award. You flip on the TV. There's that gawd-awful infomercial you've endured countless times. This time, you watch from the director's perspective. The content stinks, but the lighting is good; camerawork--good; picture quality--great! In fact, the image looks a hundred times better than yours. It's not only sharper, but smoother, less jumpy, with crisper edges.

As you nod off in front of the television, images jump in your head--the infomercial, the jagged edges in your video, the security guard at the Academy Awards kicking you out. You cross a snowy bridge ready to throw your camcorder into the chilly water.

Then your guardian angel appears and whisks you away to a lovely video store with giant monitors showing your video as smooth and clear as the infomercial. Below some of the monitors are sleek and narrow boxes with the initials "TBC" on them. Some monitors receive signals from Macs, PCs and Amigas.

The angel then shows you the largest monitor where you see yourself receiving the Academy Award for greatest videomaker. "How did you get there?" asks the angel.

And then he answers himself. "Before editing your video, you purchased one of these." He points to the sleek boxes and computers near the monitors. "You bought one of these magic boxes, the digital creations that turn camcorder tapes into broadcast-quality images. You bought a TBC!"

You beam with delight as your angel continues with the explanation. He tells you how the copies of your previous videotapes had jagged edges in the picture, and how a time base corrector fixes this by redrawing video scan lines for perfect alignment. The gentle angel reminds you that videotape stretches easily and creases when it runs through worn-out VCR components. That's what causes the scan lines to get out of alignment, resulting in jagged edges and sometimes even complete breakup of the picture.

"But what about that shot I had to use from Uncle Charlie's camera? That scene was jumpy; it's stable now?" you ask.

The kind angel replies that yes, the TBC stripped off the old sync and added new sync information to stabilize the picture and eliminate the jumpiness.

"And that shot was a little green," you continue in astonishment; "now it has perfect color!"

The patient angel adds that your particular TBC had a built-in proc amp (processing amplifier) that let you adjust your colors on your videotape copy much the way you do with a TV monitor. The angel reminds you too look for the proc amp feature when you're shopping for a TBC, because not all of them have it. You beam with delight and wake up with a grin on your face.

A time base corrector is a device that digitizes each scan line of video, stores it for a very brief period of time (microseconds) and then releases the line of video at a precise time. Since videotape is flexible and, as we said, subject to stretching and shrinking, it may not display each scan line at the precise moment. This "time base error" shows up as jagged edges on the images and a subtle shaking of each scan line. When the time base error is really bad, it can cause a complete loss of sync and an unwatchable picture.

The more sophisticated TBCs also have digital video noise correction. Some even have comb filters that maintain the image's original sharpness. TBCs that include a processing amplifier (proc amp) allow control of color, brightness and contrast. TBCs with a built-in synchronizer can lock onto virtually any video signal, including those from consumer VCRs.