When you hear the term “copy protection”, these days you’re probably more likely to
think along the lines of DRM or some clever way to prevent copying a digital something-or-other,
such as a music file, DVD, Blu-ray, or whatnot.
If this is your frame of reference, you might be a little surprised to learn that a form
of copy protection had been in use for pre-recorded video cassettes as far back as 1985.
♫ sweet music with tuba ♫
The Macrovision company, wh… which later became the Rovi corporation, which later bought
TiVo and decided to absorb that identity so is now TiVo, came up with a way to exploit
the automatic gain control circuitry in a videocassette recorder, and create pre-recorded
tapes which would mess around with that circuitry to prevent copying them.
Now to be clear, this wasn’t so much a copy protection scheme as it was a copy deterrent.
This wouldn’t exactly prevent you from making a copy,
but that copy would be almost unbearable to watch.
So first let’s see what it does.
I have this VCR plugged into this VCR, and the TV is currently displaying the output
of this VCR through This VCR, and I’m gonna stick a prerecorded tape into this one and
a blank tape into this one.
I want to make a copy of Rain Main.
So now, I’m going to hit record on this VCR, which will copy Rain Main onto the blank
tape inside it.
Pay attention to what happens as soon as I hit record.
That’s not exactly a good sign.
Well, aside from that, it seems to be working...OK, no apparent issues so far.
But when you play the copy back, in addition to the generational loss brought about by
being a second generation analog copy, the brightness of the image changes wildly, and
it’s not at all stable.
Why is that happening?
And why only when I’m recording?
Well, what Macrovision does is screw around with the vertical blanking interval of the
video signal in such a way that a TV won’t really be bothered, but that a VCR attempting
to record would incorrectly interpret as a change in signal strength.
I’m not going to go too much into detail here, as I’ve already made a video explaining
the basics of analog television, but at the start of every field there are a number of
lines in the signal that are blank.
They are transmitted at the highest intensity which creates a blacker than black segment
that the TV can use to synchronize itself to a broadcast image and display it properly.
To see the vertical blanking interval, you need to get yourself an old TV that allows
you to adjust the vertical hold.
This particular set to the rescue once more!
I have the vertical hold ever so slightly off so the image will roll.
This dark bar is the vertical blanking interval, and the really dark portion is the sync pulse
that the TV is looking for.
You can tell that it almost catches it each time it makes its way to the bottom, but with
the adjustment just a tad off, it won’t stay in one place.
Many videocassette recorders would use this pulse for determining the signal strength.
Not all channels come in with the same strength, which in the land of video means they could
appear brighter or darker depending on how clear the signal is coming in.
A TV doesn’t care too much about the relative signal strength, but a VCR really does.
Just like audio tape, a recording onto video tape needs to be made at the correct level.
Too much saturation and detail is lost through clipping, and too little saturation and the
picture becomes noisy.
By calibrating its signal gain to the level of vertical blanking interval, the VCR would
get a known reference point for brightness.
This meant that no matter how strong the signal was or how bright or dark the scene it was
recording was, a VCR would be able to make a recording correctly.
So then, with this knowledge in their back pocket, Macrovision Solutions Corporation
devised a way to mess with the blanking interval in order to confuse the gain control circuitry
of a VCR.
Let’s look at how Macrovision changed the blanking interval.This copy of the Magic School
Bus wasn’t encoded with Macrovision.
Notice how there’s really nothing in there.
So what does a tape with macrovision look like?
Well, with the Wrong Trousers, the BBC took care to encode their tape with Macrovision.
Notice these new blocks inside the blanking interval.
That IS Macrovision.
You’ll see that next to the black blocks, which are actually high intensity pulses because
a TV signal is inverted, are white blocks that keep fading in and out.
This is likely what causes the brightness changes in the copied tape.
A VCR that is recording this video signal keeps seeing the brightness of the vertical
blanking interval go up and down, and it will treat it as though the entire signal were
getting stronger or weaker.
This means that it keeps erroneously adjusting the strength of the recording, which causes
the recorded image to suffer.
Warning: the following section of the video contains some speculation.
OK, I ran into a wall regarding some specifics.
A lot of the information about Macrovision you can find online seems incomplete and doesn’t
really jive with what I’ve observed.
Most descriptions of Macrovision will talk about the high intensity pulses and the automatic
gain control, but this only explains the image getting brighter and darker.
This might be particular to these two VCRs, but I tried dubbing in both directions and
the resulting recordings were wildly unstable--not just too light or too dark.
Because you can see the white blocks change in intensity when you look at the blanking
interval, it would seem that this is what causes the image to go light and dark.
And that’s fine.
But in addition to these pulses, which are described as “excessive voltage pulses”,
there’s a little something else.
These handy oscilloscope captures on the Wikipedia page show that the intensity of the high voltage
spike goes up and down, and the lower portion here stays constant.
So this means that this part is the white boxes we can see appear and disappear.
Assuming these two graphs show these two states of the vertical blanking interval, then the
only component that changes is the level of these upper pulses.
This make sense because the light portion follows a dark portion.
Within the lines, you can see dark, light, grey.
Dark, light, grey.
And the graph does the same--dark light grey, dark light grey.
I think that not enough attention is given to these lower pulses, as these appear to
be at the same level as the synchronization pulse in the vertical blanking interval.
If we go back to the tape without Macrovision, you can see this very dark portion here in
the blanking interval.
That is the high intensity pulse that the TV locks onto for vertical synchronization.
But the rest of the blanking interval is just a grey color.
That’s just the black level.
Many black and white televisions including this one can’t produce a true black unless
there’s a good deal of contrast in the scene, which is why it appears grey.
It will however always produce true black during the sync pulse, because that pulse
is blacker than black--meaning the electron gun is entirely shut off.
When you go back to the tape with Macrovision, you can see that the added black bars are
also at that blacker than black intensity.
It seems to me that these probably screw around with a VCR’s ability to maintain vertical
synchronization.
These dark boxes extend the sync pulse farther into the blanking interval than it should
be, and the pulses appear multiple times within a single line.
The white bars that appear and disappear are likely what interferes with the automatic
gain control and will cause the brightness issues, but that shouldn’t mess with tracking
or image stability.
There seems to be more than just that going on.
Some descriptions of Macrovision call dubbed tapes simply annoying to watch, but this?
This is unwatchable.
Just to check that it wasn’t simply this TV that was struggling to maintain synchronization
from the copied tape, I played the tape back through the LCD TV in my kitchen, which doesn’t
use the vertical blanking interval in quite the same way a CRT television does.
It had a more stable image, but not only did the image repeatedly become darker, it lost
all color completely and the VCR was clearly struggling to maintain tracking.
You can hear the pitch of the audio waver continuously.
♫ severely warbled music ♫
The signal got so bad that the TV kept blanking the screen and giving up.
It seems possible to me that not only does Macrovision mess with the automatic gain control,
but it might also mess with automatic tracking circuits of a VCR.
Which may help to partially explain the next part:
Regardless of the exact specifics of Macrovision, there’s one interesting quirk to the system.
Really old VCRs, which don’t have the same automatic gain control circuitry of their
modern counterparts, are impervious to Macrovision.
(every time I need to move this thing, I regret it.)
This beast just really doesn’t care what’s going on in the vertical blanking interval.
It will dub that tape just fine for ya.
See, it’s recording right now, and the TV doesn’t show anything weird.
Let’s play the copy back to be sure.
Yep, just fine.
Hey, it’s me from the future.
While shooting the B-roll I needed for this video, I noticed that the VCR which was copying
Rain Man was making really odd sounds as it recorded.
Take a listen.
[sound of VCR recording but with wild speed fluctuations]
Normally you never hear a VCR make noises like this, except for maybe when it’s adjusting
its tracking.
This really seems like the Macrovision is confusing the VCR on where the start of the
video field actually is, and it keeps trying to compensate for that.
The video head drum needs to spin in perfect alignment with the video signal, and it seems
like this VCR is being fooled into thinking the start of each frame moves as the macrovision
blocks fade in and out.
The old VCR probably isn’t affected because it doesn’t try to chase a wavering signal--
I would imagine it has much less of an ability to adjust the speed of its video head and
tape transport to signal fluctuations, and as a result Macrovision just doesn’t affect it.
Anyway,
Even into the days of Macrovision, there were some VCRs that weren’t affected by it.
It was estimated that 85% were, but that leaves a significant number of VCRs floating around
which just don’t care about it.
Also, for those that were so inclined, getting a Macrovision defeat device from the grey
market wasn’t all that hard.
In fact, you could build one yourself.
Because Macrovision can’t entirely eliminate the pulse for vertical synchronization, otherwise
the TV couldn’t display an image at all, a defeat device just needs to remove the altered
parts that were messed up with the boxes and replace it with standard black lines.
Now, when DVD came along, Macrovision was incorporated here, too.
Because DVDs are a digital format, they don’t contain the video fields like a videocassette
does, so the player actually generates the Macrovision in the vertical blanking interval
of its composite output.
It will only do it so long as the disc asks for it, which not all of them did.
For example, this copy of the Princess Bride doesn’t use Macrovision, but this copy of
Spirited Away does.
The fact that the analog Macrovision system was shoved into the DVD format caused some
problems for early DVD adopters.
Remember how the image got messed up only after I had hit record?
That happened because unless it’s recording, the automatic gain control circuitry isn’t
active and it passes the signal straight through to the TV.
But many VCRs weren’t so kind and instead applied automatic gain control to whatever
input they received, regardless of if it was recording or not.
Many TVs only had one composite input, so for anyone who hooked their new DVD player
into their TV through their VCR, and if their VCR didn’t simply pass-through the video
from the DVD player, they might get a horrible mess.
Imagine how annoying it would be to play your first DVD and all of the sudden have the image
come out like that.
Many people thought their new DVD players were somehow broken, but thankfully newer
VCRs like this one won’t cause a fuss unless you tried to record from the DVD player.
And while we’re at it, this attempted copying of Spirited Away shows a second thing added
to Macrovision, which was called Level II ACP.
This would invert the phase of the colorburst occasionally, which produced stripes of the
wrong color on a copy.
Quite honestly that’s the least annoying of the issues with this recording.
Now these days, far more complicated copy protection schemes are in use, much to the
chagrin of anyone who wants to make legal copies of their media which is, by the way,
allowed.
But Macrovision was I think quite clever.
By messing with the fundamental structure of the video signal just enough to keep a
recording VCR from working correctly, but not enough for a TV or a playing to become
confused, they actually had a pretty elegant solution.
Thanks for watching, I hoped you enjoyed the video!
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