DRM: March 2009 Archives


March 7, 2009

I understand that in-theater videotaping of movies is a major source of piracy, but it's hard to understand the threat model under which this is a useful technique:
In recent years, the problem of camcorder piracy in theaters has become more serious due to technical advances in camcorders. In this paper, as a new deterrent to camcorder piracy, we propose a system for estimating the recording position from which a camcorder recording is made. The system is based on spread-spectrum audio watermarking for the multichannel movie soundtrack. It utilizes a stochastic model of the detection strength, which is calculated in the watermark detection process. Our experimental results show that the system estimates recording positions in an actual theater with a mean estimation error of 0.44 m. The results of our MUSHRA subjective listening tests show the method does not significantly spoil the subjective acoustic quality of the soundtrack. These results indicate that the proposed system is applicable for practical uses.

OK, so let's say that this works as advertised: why does it help. The full article is behind a paywall, but I'm assuming the way this is supposed to work is that you wait for a pirated movie to show up on the file sharing network, then what? Let's assume that each print is separately marked, so you can tell what theater it was taken in and what position the camera was in. I still see several problems.

First, you need to figure out which showing the video was taken at. The easiest way to do this is probably to inject a signal into either the audio or video. As I understand the situation, modern projection equipment generally uses digital audio, so I suppose it's possible that you can reprogram the projection system to add a time signal to the audio track somehow; if you're using digital projection you could probably add it to the video as well. Even so, it seems to me that this technique requires new equipment or at least new software on every theater. That's a pretty significant investment.

Second, you need to be able to go from the position of the camera in the theater to the person doing the taping. Even if we assume that the camera position and the perpetrator's position are the same, people typically sit within a half meter or so of each other, so in a packed theater, there are probably about 4-8 people who potentially did the taping. Or, rather, you now know what seat they were sitting in. But theaters don't typically know where people are sitting, so now we need some way to keep records of where people are sitting, which either means IDing customers and having assigned seating, photographic records of where people are sitting, or both. That's a major change in the way theaters do business.

Of course, even if the theaters (or rather the movie distributors or MPAA) did all this stuff, if they actually started going after pirates this way, it should be pretty easy to circumvent. The low tech countermeasure is just to put the microphone somewhere else in the theater. The high tech countermeasure is to use signal processing techniques to tamper with the time signal, remove the theater-specific watermarks, or just fuzz things enough to remove the information used for positioning. For that matter, when you go into the theater to pirate the film you could presumably—and this is pretty advanced stuff—wear some sort of disguise.

UPDATE: I should probably mention that there's a /. thread on this, which is where I originally saw it. The remote mike idea was suggested there, but it's pretty immediately obvious as soon as you hear about this technique.