MACROVISION FAQ

Contents:

HISTORIC, not applicable to non-VCR media!

[Document Version: 1.2] [Last Updated: 11/12/96]

HISTORIC, not applicable to non-VCR media!


1. About the Author

Author: Antti Paarlahti
E-Mail: ap105053@cs.tut.fi
Version: 1.1

Copyright (c) 1992-1995 by Antti Paarlahti

This file may be freely distributed in electronic form as long as it stays complete.


2. Introduction

This FAQ has been built upon my "Macrovision explained" article that I originally posted to rec.video (22-Oct-1993). That contained only a raw version of the technical section.

Version 1.0 (3-Dec-1993) of the FAQ introduced a "reader-friendly front-end", i.e. an explanation for general audiences without technical background. Also some modifications and editions were done to the technical section.

This version, v1.1 (15-dec-1994), has additional questions and general information, also minor modifications to the technical section.

Some material has been acquired from the Net and some (most) from my own experiments.

New contributions AND new questions are most welcome. Grammar corrections are accepted, because I am not a native speaker of English.

Information presented here should be correct as far as I know, but utter rubbish may have crept in, so proceed carefully.


3. What is it???


  3.1) What is Macrovision?

Macrovision is a videotape copy protection method for VHS video cassette recorders. It is used on pre-recorded videotapes, and it seems to be more common in North America than in Europe. If you want a tape that will have Macrovision, get a Disney one.

Macrovision comes from a company that is surprisingly called Macrovision, formed by the inventors of the process. They also make other scrambling systems.


  3.2) What does it look like?

When dubbing a protected tape, the picture that has gone through the recording VCR will get dark and then normal again periodically. The picture may also become unstable when it is at its darkest.

Some televisions do not like Macrovision either; the top of the picture might be unstable all the time and the colors may flicker. It resembles very much a mistracked tape.

If you have a TV that has an adjustement for picture height or vertical hold, you can play with those. Macrovision signals can be seen as very bright and very dark regions (vertical stripes) near the top of the picture.


  3.3) Is there an easy and economical way to get rid of it?

Not really. There are three main alternatives:

There have been observations that there are less and less commercial boxes available. That is because Macrovision holds patents for the Macrovision process AND the most obvious ways to defeat it, so they can sue the companies that manufacture the eliminators. In fact, they have been doing just that, so many of the companies are now out of business.

Of course you could always buy a time-base corrector or other legal device that recreates the sync signals. The downside is that they aren't usually known for their small price....


4. A non-technical explanation

Here is a simple explanation of how the method works. It exploits the automatic gain control (AGC) circuit in the recorder. The purpose of the AGC is to adjust the level of the video signal in such a way that the recording capabilities of the tape are fully used. This means that weak signals are amplified and strong ones are attenuated.

In the Macrovision method, some new signals are inserted in the non-visible portion of the picture. These signals can make the VCR think that a perfectly normal picture is suddenly way, way too bright. The AGC circuit therefore darkens it until it thinks the brightness is normal. Of course, now the REAL picture is very dark. The picture is varied between bright and dark periodically in order to defeat simple eliminators that would just amplify the dark and murky signal back to almost normal.

In fact, there was an early version of the method which had a static level of brightness, but it was easy to readjust the AGC gain setting inside the VCR. The level shifting was obviously added fairly soon after that discovery.

Why isn't the TV affected? Well, most TV sets do not have any AGC-circuits at all, and the rest behave differently from VHS ones.


  4.1) My friend has a VHS VCR that is not affected. How is this possible?

The proper behaviour of the AGC-circuit is very important in order to achieve good protection. Apparently the specifications were somewhat "loose" in the pre-Macrovision days, so the old (how old?) machines are not affected.

JVC, the VHS license holder, has tightened the spec and the control, so it is difficult to get an "immune" VCR, but there are some machines that are conveniently "out of spec". No, I don't know which ones.


  4.2) I have heard that 8mm video is not affected by it. Is this true?

8mm video is not affected by Macrovision, because it is totally separate from VHS. There is no need to have AGC circuits that bear any resemblance to JVC ones.

Of course, 8mm recorders do not remove the protection, so any subsequent VHS dub will again be unwatchable.


  4.3) Can Macrovision be defeated by copying via the aerial inputs/outputs?


  4.4) Can it be defeated by routing the signal throgh a TV with video inputs/outputs?

No, it can't. There were some ancient methods that could be eliminated with some of the above methods, but they are now long dead. Macrovision is so integrated in the video signal that these simple tricks will not work.


  4.5) Will USA mail-ordered boxes work in Europe?

There are many mail-order companies in the USA selling those boxes. Can I buy one and use it in Europe?

No, not really. You might get some improvement, but the protection timings are sufficiently different to keep it from working properly.


  4.6) Is unstable picture on rental tapes due to MacroVision?

My TV does not get a stable picture, when I watch rental tapes. Could this be a fault of the copy protection?

Yes, very likely. Macrovision signals resemble false synchronization pulses, and some sets mistake them for the real ones. Also colors may vanish from the top of the picture. This seems to affect the newer TVs most; the old "steam engines" have the electronic equivalent of heavy flywheels in their synchronization circuits, so they don't get distracted so easily.


  4.7) Can laserdiscs have Macrovision?

Apparently no. First of all, it is technically difficult, because the region used by Macrovision is also used to hold control data for the laserdisc players. Secondly, because of backward compatibility, the laserdisc specs cannot be easily changed. Thirdly, as long as the laserdisc market stays as a niche market, the distributors will not pressure the manufacturers to change the specs. And if they did, the sales would surely drop.


  4.8) Can Macrovision damage my equipment? Can I attenuate it?

I've heard that the protection consists of very bright pulses, some 2-3 times the maximum allowable brightness. Can it damage my equipment? Can attenuation remove the protection?

The brightness is actually only about 15% over the normal white level, so no harm will be done. Simply attenuating the signal won't work, it's not the brightness alone that does the trick.


5. A technical explanation

In this part, I represent the empirical data about "Macrovision in action" that I have gathered with my trusty 20+ year old oscilloscope.

The main topics are:

Tapes that I used were Disney's Beauty and the Beast (PAL) and 101 Dalmatians (NTSC).

NB: From here, it gets very technical. A basic knowledge of what video signal looks like is required.

First, some basics. A television picture consists of 25/30 (PAL/NTSC) FRAMES. Every frame contains a total of 625/525 LINES (not all are visible, though). Frames are INTERLACED, i.e. they are divided into two FIELDS that are drawn on top of another in such a way that the first field scans every alternate line and the second field 'fills in' the missing lines. The scanning is from top to bottom and from left to right.

The fields are separated by VERTICAL SYNCHRONIZATION pulses. The lines are separated by HORIZONTAL SYNCHRONIZATION pulses. Both PAL and NTSC standards specify that those approximately 20 lines that follow a vertical sync are not to be used in forming the picture. These lines are not visible in properly adjusted TVs, so they can e.g. carry TeleText data, or, in our case, Macrovision copy protection signals (see pictures 1 and 2). The vertical sync and invisible lines form the VERTICAL BLANKING REGION, or VBR for short.


  5.1) Macrovision

So, what is the trick with Macrovision? In order to understand that, some knowledge of VHS AGC circuits is needed (AGC==Automatic Gain Control). The AGC circuits adjust the amplitude of the video (luminance) signal by measuring the voltage difference between the bottom of the horizontal sync and the back (rear) porch (see picture 3).

What Macrovision does is to introduce false synchronization pulses in the VBR followed by false back porches at a very high voltage level (~15% over white level). The VHS VCR looks at the signal and thinks that it is fed with an extremely high-amplitude signal and adjusts the gain control to minimum --> the real picture gets very dim (see pictures 4 and 5).

The AGC response must be like this:

Chrominance AGC is measured from the color burst, so it is not affected by the protection, but it surprisingly enhances the annoying effect, because the colors stay strong and bright even when the luminance is low. This results in a swarm of random color blothces over the picture.

In the following pictures '|' represents a vertical synchronization pulse. Because of interlacing, the sync pulses of corresponding lines in different fields are spaced 1/2 lines apart. The fields are drawn on top of another because 1) my 'scope shows them like this, and 2) the fields are nearly identical anyway.

The scale of the video signal pictures:


    5.1.1) Picture 1: PAL vertical blanking region with Macrovision


   Scope             Interlace           Macrovision regions
   trigger           starts    ...........................................
   :                 :         1                   2 1                   2
   :                 :         1  Region 1         2 1  Region 2         2
   :  Vertical sync  :         1  (between '1's)   2 1  (between '2's)   2
   :                 :         1                   2 1                   2
_           _ _ _ _ ________________________________________________________
 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
 |_|_|_|_|_| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
 
   3   3   3   3   3   3   3   3   3   3   3   3   3   3   3   3   3   3   3
   1   1   1   1   1   1   2   2   2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 3 1 3 1 3
 1 4 2 5 3 6 4 7 5 8 6 9 7 0 8 1 9 2 0 3 1 4 2 5 3 6 4 7 5 8 6 9 7 0 8 1 9 2
 The number of the scanline starting after this sync pulse

                :
                : Picture
                : start
                :
______________________...
| | | | | | | | | | |
| | | | | | | | | | |
 
3   3   3   3   3   3
3 2 3 2 3 2 3 2 3 2 3
2 0 3 1 4 2 5 3 6 4 7

I'll explain those regions later. It is very easy to calculate the required timing for a Macrovision eliminator from this picture (more later, also). There are other line numbering systems, but this is "time sequential".


    5.1.2) Picture 2: NTSC vertical blanking region with Macrovision


   Scope                 Interlace         Macrovision region         Picture
   trigger               starts    ................................... start
   :                     :         :                                 :   :
   :                     :         :                                 :   :
   :  Vertical sync      :         :                                 :   :
   :                     :         :                                 :   :
_             _ _ _ _ _ ____________________________________________________
 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
 |_|_|_|_|_|_| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
 
   2   2   2   2   2   2   2   2   2   2   2   2   2   2   2   2   2   2   2
   6   6   6   6   6   6   7   7   7 1 7 1 7 1 7 1 7 1 7 1 7 1 7 1 8 1 8 1 8
 1 4 2 5 3 6 4 7 5 8 6 9 7 0 8 1 9 2 0 3 1 4 2 5 3 6 4 7 5 8 6 9 7 0 8 1 9 2

As can be observed, NTSC timing is a little bit tighter. I'm not sure about the line numbering, because I have no NTSC-references, so please correct me.

Now, what do those lines inside Macrovision regions contain? Back to basics, once again.


Please check attribution section for Author of this document! This article was written by filipg@repairfaq.org [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/filipg/ [Copyright] [Disclaimer]