Clean Copies, One Generation to the Next

Jim Stinson
June 1999

One of the most compelling claims that people make for digital video is no generation loss when you make copies. This means the 9th level copy-of-a-copy-of-a-copy is 100% as good as the 1st generation original. This claim of digital superiority is just a tad overrated, (see Digital's Dirty Secrets sidebar); but there's no denying that traditional analog video markedly decays from parent to child to grandchild. In the worlds of VHS and 8mm, generation loss is heartbreakingly fast.

How fast? Your critical eye can see that your second generation edit master is not quite as sharp and clean as your camera original. Even your mom can tell that the 3rd generation copy you sent her doesn't equal what she's used to watching on her satellite TV system. Anything 4th generation or later is irritatingly bad.

Like death and taxes, analog generation loss is universal. But it is possible to postpone or partly avoid video degeneration. We'll show you how to protect picture quality from one tape level to the next. First, let's take a look at why video copies degenerate in the first place.

Briefly, video quality declines from one generation to the next for three principal reasons: high frequency loss, luminance/chrominance split and noise.

• High frequency loss: Image sharpness is most visible in the edges of on-screen objects: the borders between one thing and another. Edges are recorded in the high frequencies of the video signal and those frequencies are easily degraded in the copying process. Each generation appears "softer" than its parent because all the edges in the picture are less well-defined.
• Luminance/chrominance split: Signal split happens because video signals carry four streams of image information: brightness, color hue, color intensity and a sync signal. Brightness (and, with it, contrast) is governed by the part of the signal called "luminance," while color hue and intensity are combined in the part labeled "chrominance." The fourth signal component contains sync information to display the images correctly.

In an RF (antenna jack) or composite video (yellow RCA plug) signal, the four parts of the video signal are combined; but as soon as they reach the internal circuits of a VCR, DVE/switcher, titler, or color processor, they split into separate components. On the way out again, the signal components re-merge into one. If you've daisy-chained a source VCR, source monitor, digital switcher, titler, color processor and record VCR, the signal may split and recombine five times or more.

The problem is that inside each editing box, the different signal components take separate internal routes from input to output. Where luminance may drive straight from New York to Los Angeles, so to speak, chrominance may go via Tallahassee, Dallas and Tombstone, arriving later in LA because of its longer journey. So when luminance and chrominance reunite at the exit point, they're no longer in perfect sync.

Each time the split/merge sequence repeats, the discrepancy increases. That's what accounts for the smeared and mis-registered colors that make fourth generation and lower dupe tapes look like badly printed newspaper photos.

Noise Signal noise shows up as the speckles, graininess and blahed-out color of multi-generation copies. Just as anything in your garden that you didn't plant is a weed, anything in a waveform that isn't signal (information) is noise. Every recording contains some noise and every copy adds more of it, so that a sixth-generation dupe has six times as many electronic weeds as the original. (Noise got its odd name from the audio world, where you can actually hear the non-signal parts of a recording as noises.)

Okay, so high frequency loss, signal splitting and noise cause your videos to degenerate as you duplicate them in the editing process. What can you do about these problems? Nothing dramatic, alas, but you can minimize generation loss by systematically optimizing every part of your video recording process, beginning with the quality of your original camera tapes.

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