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From a technical standpoint, it would seem impossible to completely prevent all users from making copies of such media as CDs, DVDs, videotapes, computer software discs, or video game discs. The basic technical fact is that all these types of media require a "player"—a CD player, DVD player, videotape player, computer, or video game console, in these five examples. The player has to be able to read the media in order to display it to a human. In turn, then, logically, a player could be built that first reads the media, and then writes out an exact copy of what was read, whether to the same type of media that was read, or to some other format, such as a file on a hard disk.
Copying of information goods which are downloaded (rather than embedded in physical media) can be restricted more effectively. They can be encrypted in a fashion which is unique for each user's computer, and the decryption system can be made tamper-resistant (see also traitor tracing ).
At a minimum, digital copy prevention is subject to the analog hole: regardless of the digital restrictions, if music can be played on speakers, it can also be recorded. Copying text in this way is more tedious, but if it can be printed or displayed, it can also be scanned and OCRed.
Since this basic technical fact exists, copy prevention is not intended to stop professional operations involved in the unauthorized mass duplication of media, but rather to stop casual copying in which one friend makes a copy of a disc for another friend and thus (arguably) decreases the possible market for that disc by one copy.
Publishers who implement copy prevention have historically referred to the technology as copy protection as it carries the implication of preventing destruction or suffering. However, as the act of copying information does not necessarily result in destruction, suffering, or even loss of business profits, many people believe this term is misleading. Advocates of copyright reform and copyright abolishment in particular believe the term wrongly encourages people to identify with publishers who benefit from copy prevention rather than the users who are restricted by it. Copy prevention is a neutral term which simply describes the purpose of the technology without passing judgment on whether the act of copying is inherently damaging.
Copy prevention has been attempted in many ways, long before computers and digital media entered the picture. For example, the ancient practice of watermarking is an attempt to, if not prevent a copy, at least prove the authenticity of the original. The music industry in particular has long sought a reliable copy prevention method—early attempts included adding a high frequency spoiler signal to an analog recording so that tape recorders would generate an unpleasant whistle when the spoiler heterodyneIn telecommunication, to heterodyne is to generate new frequencies by mixing two or more signals in a nonlinear device such as a vacuum tube, transistor, or diode mixer. The mixing of each two frequencies results in the creation of two new frequencies, ond with the biasBias has several different meanings, most relating to an offset or prejudice of some sort. Viewpoint A bias is a prejudice in a general or specific sense, usually in the sense for having a predilection to one particular point of view or ideology. One is s oscillator. These attempts were largely unsuccessful since the spoiler was either audible to the listener, or else so high that it would not be reproduced reliably when played back. Videotape manufacturers had more success, with companies like MacrovisionMacrovision is a company that creates electronic copy prevention schemes. Macrovision is notable for its video copy prevention scheme of the same name. A VHS videotape or DVD (no laserdisc or video CD players implement it) encoded with Macrovision will ca inventing clever schemes that would make copies unusable if they were created with a normal VCR, and licensing this technology to videotape manufacturers.
Some modern forms of copy prevention are invisible to the end-user, such as CD subchannel data or other mechanisms such as SafeDisc which only become apparent once an attempt to copy is made.
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