Magazine article Science News

Putting a Notch into Digital Sound

Magazine article Science News

Putting a Notch into Digital Sound

Article excerpt

Putting a notch into digital sound

Digital audio tape offers the possibility of making crystal-clear copies of recordings, with none of the background hiss typically heard on tape. However, that possibility alarms the recording industry, which has been championing an electronic system designed to prevent people from freely copying recorded music. After five months of tests done at the request of three congressional subcommittees trying to resolve conflicting claims, the National Bureau of Standards (NBS) in Gaithersburg, MD., last week concluded that the proposed copy-prevention scheme "does not achieve its stated purpose." Moreover, some listeners can hear a difference in sound quality.

The copy-prevention system, developed by CBS Records in Milford, Conn., and known as Copy-code, involves electronically filtering out a narrow band of frequencies from any recorded music. This "notch" in the recording is centered at 3,840 hertz, a frequency that lies between the highest B-flat and B notes on an 88-key piano. Circuitry in the tape recorder would scan incoming signals and stop the machine from recording if the notch were detected.

NBS researchers found that such a copy-prevention system is not foolproof. Sometimes the system allows notched music to be recorded, and sometimes it fails to record music that is not notched. Moreover, NBS engineers designed and constructed five different circuits that could be attached to a tape recorded to defeat the copy-prevention system. …

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