Appendix 3: Serial copy management system (SCMS)
The introduction of new digital recording systems like DAT, MiniDisc and DCC made it perfectly possible to produce high-quality copies of existing materials. When using digital interconnection, there is virtually no sound quality degradation. This raises the question for protection of copyrighted material. The SCMS limits the ability of consumer digital audio recording equipment to make digital transfers. SCMS allows a single generation copy of a copyrighted digital source. For example, a copyrighted CD can be digitally copied to a DAT recorder once (and only once). The SCMS system prevents further digital copies of that tape. Figure A3.1 shows some examples of possible situations. Note that, regardless of the source, it is always possible to make copies by using analog connections.
Three possible cases exist when digitally copying a digital source.
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The source is not copyrighted: digital copy permitted. |
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The source is copyrighted: a first generation copy is permitted. |
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The source is a digital copy of copyrighted material: digital copy prohibited. |
Each of the digital recording systems contains some auxiliary data, needed for correct recovery of the digital audio signal. The SCMS copy bits can be found in the auxiliary data, e.g., in case of DAT they are contained in ID6 bit 6 and bit 7 (see Table 15.4).
Figure A3.1 Different situations when copying via the SCMS system.
In the Digital Audio Interface Format (S/PDIF), the copy bits are included in the channel status bits.
The SCMS copy management routine is shown in Figure A3.2. The SCMS software checks the channel status bit of the digital audio signal. Several requirements must be fulfilled on the recording side before copying can start. The recording machine will alter the copy bits according to the new situation.
Note the following.
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A digital copy is never possible when the digital audio signal is not for consumer use. |
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When the category code is unidentified, the source is considered copyrighted. A single digital copy is allowed, but new copy bits will be set to prevent further copies. |
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When the category code is ‘general’ or ‘A/D converter’, the recording is considered to be a (copyrighted) master recording. The copy bits will be set to allow one further digital copy. |
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If the channel status bits do not carry a copy prohibited code, the source is considered not copyrighted. Digital copies will always be possible. |
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Even when the copy prohibited code is set, certain categories (e.g., CD) allow for a single digital copy with the new copy bits set to copy prohibited. |
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In all other cases a digital copy is prohibited. |
Figure A3.2 SCMS management routine.