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Putting a notch into digital sound.

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. According to NBS, a competent electronics technician could build any of these circuits for about $100 in parts.

Because the system requires the deletion of certain frequencies from a recording, another key question was whether a listener can readily hear the difference. A series of listening tests showed that for some listeners and for some musical selections the inclusion of the notch has a discernible effect.

The effects of the notch are "extremely subtle," says Irwin Pollack of the University of Michigan in Ann Arbor, who conducted the listening tests. "I reject the extreme position that the [system's] action is so evident and pervasive that it will be immediately recognized by unsophisticated listeners. I also reject the extreme position that the [system's] action is so benevolent that it cannot be detected." However, there is enough of a difference to warrant caution about allowing such electronic tampering.

"The record industry and the electronics industry have strived for the last 20 or 30 years to get towards perfect reproduction of sound," says Gary Shapiro of the Home Recording Rights Coalition in Washington, D.C. The proposed copy-prevention system represents a step backward, he says.

The Recording Industry Association of America, also based in Washington, D.C., says it accepts the NBS results. But the association plans to continue pressing Congress for some form of copy-protection or compensation.
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Title Annotation:National Bureau of Standards report on anti-copying system
Author:Peterson, Ivars
Publication:Science News
Date:Mar 12, 1988
Words:453
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