If you’ve ever tried to copy a videocassette (for your own personal use, of course) that is encoded with the MacroVision brand of copy protection, you may have wondered how exactly it works. How do they encode a signal onto a tape that will not interfere with simple playback, but foils attempts to copy it to another machine? If you’ve ever contemplated the answer to this question, read on. I’ve got the answers.

All consumer VCR’s have a similar circuit built into them that is used for recording. This circuit makes sure that the video coming in is not too bright and not too dark by boosting or attenuating the video until it is "legal", or under 100%. This constant adjustment helps correct for small level variations that might be coming out of a camcorder or variations in different television stations’ signals. To fully understand how MacroVision works, you have to understand a few simple concepts about how NTSC (the television standard used in the United States and several other countries) video works. I’ll try to keep the techie jargon to a minimum.

Let’s look at a regular video signal. Below is a set of colorbars and a picture of what they would look like on a waveform monitor. A waveform monitor is a piece of equipment that enables you to measure the different attributes of a video signal. Labeled are the sync pulse, baseline, black and white level markers. This signal is "legal" in that the baseline is at 0 units, the sync pulse is at 40 units below the baseline, and the white level is at 100 units above the baseline. This signal is 1 volt from peak to peak, which would be from the bottom of the sync pulse (-40) to the white chip (100 units).

Now, back to MacroVision. When a VCR "sees" a signal coming in, it checks to make sure that it is 1 volt, peak to peak. If you were to run the above pattern into a VCR, it would make no adjustment to it. If the sync pulse was less than 40 units below the baseline, it would expand the signal to correct for it.

MacroVision preys on this automatic adjustment. In a MacroVision-encoded signal, the vertical interval contains several very bright white areas designed to confuse the auto-level circuit. These spikes are often in excess of 120 units; well over what would normally be the brightest white area in the video signal. Below is a picture of what these spikes look like if you offset your vertical hold control. (tech note: The vertical interval is the black bar that you may have seen drifting through the picture on a TV with a misadjusted vertical hold control. Normally the vertical interval is mostly black. It is often used to carry timecode, closed captioning, or other data. These appear as small flashing white lines directly above the picture and do not affect normal TV watching.)

When a VCR "sees" a signal with these spikes, it thinks that the spikes are part of the picture. When it measures the video from peak to peak, it sees well over the 1 volt that it can handle recording. So to compensate, it reduces the video level until the spikes are at 100 units. But by doing this, it reduces the brightness of the rest of the picture and shrinks the sync pulse as well. The visible result of this adjustment is a noticeably darker picture with too much color in it. The reason the color is more intense is because while the overall video level (luminance) is reduced, the color information is left untouched. This throws off the ratio of luminance to chroma; there is too much color for the amount of brightness present.

"So why does the picture sometimes pulsate from normal to dark?" you may ask. Excellent question. The reason is that the spikes will sometimes disappear for a short period, then pulsate on and off for a bit, then come back on. Whenever the spikes fade out, the VCR re-compensates for the drop in level by boosting it back up again. The pulsating is caused by the VCR attempting to keep a constant 1-volt signal.

"Do they also put this on DVD's?"  Another good question.  The answer is yes; most DVD's are MacroVision encoded to keep people from making high-quality tapes of their discs.  Although this does keep people from making copies this way, there are now many ways to digitally copy a DVD which completely bypass the MacroVision problem.

It’s really quite an ingenious little system that deters most "casual thieves" from making illegal copies. As you might have guessed, I in no way am encouraging people to make copies of copyrighted works. In fact, I’m not even offering a solution for getting around this type of copyguarding. This is for informational use only.