Optical discs, which can store high-definition digital content and distribute such digital content at low cost, are used widely. For example, a Blu-ray disc (BD) has a capacity of 25 GB per layer. This means that a dual layer BD can store high-definition video with the digital broadcasting quality of up to about 4.5 hours. Such BDs have recently been used to distribute high-definition video. When compared with the DVD, a single BD can store content of up to ten DVDs. When compared with the CD, a single BD can store content of up to as many as 75 CDs. The content of a single BD can therefore be much more valuable than the content of a CD or a DVD. Thus, the sound distribution of content in the market using optical discs would be disabled by unauthorized copies of content stored in a BD or by pirated discs manufactured and shipped in the market by illegal manufacturers. As optical discs have higher capacity, expectations for copyright protection techniques for such discs become increasingly higher.
After the emergence of DVDs, the copyright protection techniques for optical discs have mainly used encryption of content. Content is encrypted before being recorded onto an optical disc to prevent unauthorized copying by malicious users. However, the encryption is effective only when the encryption key is secret. Once the encryption key is leaked out, the copyright protection using the encryption would be disabled.
The encrypted content is recorded on the optical disc medium in the form of concave or convex marks. In this case, however, the content is easily copied onto a different disc by forming the recording marks on the different disc using readout signals for the content.
Instead of encrypting the content, another conventional copyright protection technique uses sub information recorded onto a disc. The sub information is recorded onto the disc in a manner that it cannot be copied using readout signals.
With one such conventional technique, for example, sub information is recorded by slightly shifting the edge positions of recording marks in a regular manner in the tangential direction (see, for example, Patent Citations 1 to 5). With this technique of shifting the edges of recording marks in the tangential direction, the sub information is recorded as jitter of readout signals. The jitter elements are eliminated from the readout signals for the content that are extracted in synchronization with clock signals. This prevents unauthorized copying of the sub information using the readout signals.
With another such conventional technique, sub information is recorded by slightly shifting recording marks in the radial direction (see, for example, Patent Citation 6). With this technique, the readout signals for the content do not contain information about the shifts of the recording marks in the radial direction. This technique also prevents unauthorized copying of the content using the readout signals.
With another such conventional technique, synchronization code areas that are inserted in fixed cycles of recording marks are replaced by predetermined patterns (see, for example, Patent Citation 7). With this technique, the readout signals for the content do not contain the synchronization code signals. This technique also prevents unauthorized copying of the content using the readout signals.
With the above conventional techniques, the sub information is recorded by shifting the recording marks in the tangential or radial direction, or by altering the synchronization codes, which are not the information representing the content. When the sub information is recorded with any of these techniques, the readout signals for the content do not contain the sub information. These techniques therefore prevent unauthorized copying of the sub information. For discs manufactured by duplicating a master, such as ROM discs, the sub information, which is recorded by shifting the recording marks or altering the pattern of the recording marks, needs to be recorded onto the master. In this case, the sub information is unique to the master.
With another conventional technique, an optical disc substrate is first formed by duplicating a master, a reflective film is then formed on the optical disc substrate by vapor deposition and then a protective layer is formed on the reflective film to complete the disc, and then the sub information is recorded onto the completed disc by illuminating the disc at positions at predetermined distances from the edges of the recording marks with laser light to locally change the reflectivity of the information recording surface (see, for example, Patent Citation 8). When the sub information is recorded onto the disc with this technique of changing the reflectivity of the recording surface, the readout signals for the content do not contain the sub information. As a result, this technique prevents unauthorized copying of the content using the readout signals. With this conventional technique, the sub information is recorded onto the disc after the disc is completed. In this case, unlike the previously mentioned technique, the sub information is not recorded onto the master. The sub information can thus be unique to each disc.    Patent Citation 1: Japanese Unexamined Patent Publication No. H11-126426    Patent Citation 2: Japanese Unexamined Patent Publication No. 2001-357533    Patent Citation 3: Japanese Unexamined Patent Publication No. 2002-203369    Patent Citation 4: International Publication No. 2004 or 036560    Patent Citation 5: Japanese Unexamined Patent Publication No. 2005-216380    Patent Citation 6: Japanese Unexamined Patent Publication No. 2000-195049    Patent Citation 7: Japanese Unexamined Patent Publication No. 2000-113589    Patent Citation 8: Japanese Unexamined Patent Publication No. H11-191218