Analog hole

The analog hole (also known as the analog loophole) is a fundamental and inevitable vulnerability in copy protection schemes for noninteractive works in digital formats which can be exploited to duplicate copy-protected works that are ultimately reproduced using analog means. Once digital information is converted to a human-perceptible (analog) form, it is a relatively simple matter to digitally recapture that analog reproduction in an unrestricted form, thereby fundamentally circumventing any and all restrictions placed on copyrighted digitally-distributed work. Media publishers who use digital rights management (DRM), to restrict how a work can be used, perceive the necessity to make it visible and/or audible as a "hole" in the control that DRM otherwise affords them.



Although the technology for creating digital recordings from analog sources has existed for some time, it was not necessarily viewed as a "hole" until the widespread deployment of DRM in the late 1990s. It should be pointed out that this kind of duplication is not a direct digital copy, and therefore has flaws, the magnitude of which depends on the nature of the reproduction methods used. This kind of reproduction is, in many ways, similar to the initial digitization of any analog medium or performance, with all the pitfalls and benefits of such digitization. For example, bootleg films may have poor audio, or highly washed-out video. At a minimum, copy protection can be circumvented for types of material whose value is aesthetic, and does not depend on its exact digital duplication. In general, performing a digital-to-analog conversion followed by an analog-to-digital conversion results in the addition of noise in an information-theoretic sense relative to the original digital signal. This noise can be measured and quantified. Naturally, the use of high quality conversion equipment reduces the amount of noise added, to the point where such noise is essentially imperceptible to the human senses.

Regardless of any digital or software copy control mechanisms, if sound can be heard by an ear, it can also be recorded by a microphone. And if images (static images or video/film), including text, can be seen by the eye, it can also be recorded by a camera. In the case of text the image can be converted back to text using optical character recognition.

In 2002 and 2003, the U.S. motion picture industry publicly discussed the possibility of legislation to "close the analog hole" — most likely through regulation of digital recording devices, limiting their ability to record analog video signals that appear to be commercial audiovisual works. These proposals are discussed in the Content Protection Status Report, Consumer Broadband and Digital Television Promotion Act, and Analog Reconversion Discussion Group. Inventors of digital watermark technologies were particularly interested in this possibility because of the prospect that recording devices could be required to screen inputs for the presence of a particular watermark (and hence, presumably, their manufacturers would need to pay a patent royalty to the watermark's inventor).

The motion picture industry has also pursued several private-sector approaches to eliminating the analog hole; these might be implemented without additional legislation.

  • Analog signals can be degraded in ways that interfere with or confuse some recording devices. For example, Macrovision attempts to defeat recording by VCRs by outputting a deliberately distorted signal, crippling the automatic gain control for video, causing the brightness to fluctuate wildly. While this is only supposed to happen to copies, it may, as an inadvertent side-effect, happen when viewing the original video as well. Some vendors claim to have developed equivalent techniques for preventing recording by video capture cards in personal computers. Devices exist, however, to counteract this measure.
  • Manufacturers of recording devices can be required to screen analog inputs for watermarks (or Macrovision or DCS Copy Protection or CGMS-A) and limit recording as a condition of private contracts. For example, a manufacturer who licenses patents or trade secrets associated with a particular DRM scheme might also be obliged as a purely contractual matter to add recording limitations to digital recording products.
  • Manufacturers of certain playback devices such as set-top boxes can be required, as a condition of private contracts, to allow publishers or broadcasters to disable analog outputs entirely, or to degrade the analog output quality, when particular programming is displayed. This capability is one example of selectable output control. A broadcaster could then prevent all recording of a broadcast program by indicating that compliant receiving devices should refuse to output it through analog outputs at all.

In theory, it is possible to bypass all these measures by constructing a player that creates a copy of every frame and sound it plays. Although this is not within the capability of most people, many bootleggers simply record the video being displayed with a video camera or use recording and playing devices that are not designed to use the protection measures. In fact, the Motion Picture Association of America has recommended use of a camcorder as an alternative to circumventing the Content Scrambling System on DVDs.

Engineering vs. business and political views


The notion of "plugging the analog hole" may be based on fundamental misconceptions of the meaning of analog and digital. There is a history of business and political desires combined with core misunderstandings of technology leading to legislation and industry practices that are counterproductive or fundamentally flawed on an engineering theory level.

One example was an early law passed by the European Parliament to support DRM in response to widespread buzz about unauthorized digital music downloads being held in computer memory caches. Apparently reasoning by analogy to "caches of arms," the use of computer memory caches was outlawed. The legislators, hearing a very general piece of computer jargon (caching) associated with infringement, banned it, not realizing it was a basic digital storage technique found in most modern equipment. A BBC article describing the controversy itself demonstrates the difficulty of explaining to legislators and the general public the aspect that every computer and most digital devices of any kind would have to be destroyed were the law to be evenly enforced. Far from a specialized illegality, caching is a universally used computer memory technique, leading to comparisons of this law to the Indiana Pi Bill.

A body of opinion in the engineering community puts the buzz about the "analog hole" in the same category: an impossible strategy based on fundamental misunderstandings by people who are not engineers that will not solve the stated problem but cause expense and confusion. Both "analog to digital" and "digital to analog" conversion are such basic technologies, with so many possible implementations, that the idea of being able to block conversion by these means is unrealistic. Engineers are aware of mathematical and physical principles that often begin with "It is not possible to..." which sometimes come in direct conflict with business and political goals. One does not have to be an engineer to understand that it is simply not possible to simultaneously display and conceal a signal. In particular, an audio signal must be converted to analog before it reaches the listener.

In addition to this general principle, theory says that digital watermarking and other restrictions on the "analog hole" can be simply defeated by a variety of well-known techniques, such as dithering.

Copyright law vs. particular techniques

Copyright law has been defined in terms of general definitions of infringement in any concrete medium. This classically focused such law on whether there is infringement, rather than focus on particular engineering techniques. Detecting infringement within the social and legal system avoids a legacy of outlawing generic, universal, popular, widespread, useful, and possibly uncontrollable engineering techniques in response to specific misuses.

Consumer vs. professional equipment

In every copy-restricted medium, there are two grades of equipment: consumer, which may include copy restriction, and professional, which by necessity, allows access in a way that is above copy restriction. In most countries, the sale of professional equipment is not regulated, although price alone prevents most users from getting access to it. The price may not discourage piracy organizations that understand such a purchase as an investment with a good return, especially as the cost of such equipment continues to drop and as direct digital piracy becomes more difficult.