NBSIR 88-3725 Evaluation of A Copy Prevention Method for Digital Audio Tape Systems NATIONAL ENGINEERING LABORATORY NATIONAL BUREAU OF STANDARDS B.A. Bell and G.N. Stenbakken Electronic Instrumentation arid Metrology Group D.R. Flynn, D.J. Evans, E.D. Burnett, and V. Nedzelnitsky Acoustic Measurements Group K.R. Eberhardt Statistical Engineering Division February 1988 U.S. Department of Commerce C. William Verity , Secretary National Bureau of Standards, Ernest Ambler, Director . EXECUTIVE SUMMARY Digital audio tape (DAT) systems have all the attractive audio qualities now associated with compact disc technology plus the ability to make very high quality copies of copies. Concern over loss of intellectual property rights has produced proposals for legislation requiring that DAT systems sold in the United States be fitted with a copy prevention decoder such that suitably coded material could not be copied by the DAT recorder. The CBS Records system, the basis for the proposals, removes a narrow band of frequencies from the audio signal - termed putting a "notch” in the frequency spectrum of the signal In considering this proposal, several committees of the Congress asked the National Bureau of Standards (NBS) three questions about the copy prevention system: 1. Does the copy prevention system achieve its purpose to prevent digital audio taping machines from recording? 2. Does the system diminish the quality of the prerecorded material into which the notch is inserted? 3. Can the system be bypassed, and, if so, how easily? The National Bureau of Standards received from CBS Records descriptive material, specifications, circuit diagrams, and operating encoding and recording/decoding devices for the tests. The results and conclusions of the NBS tests apply to these devices. Suggestions for recorded material to be used in listening tests were received from several sources. NBS conducted a series of laboratory studies to understand the copy prevention system and to exercise it in several ways to obtain answers to question 1 above. The NBS also constructed and evaluated several circuits designed to bypass the system (question 3) . With the aid of an expert in acoustical evaluations on contract from a university, listening studies were conducted using subjects drawn from the local section of the Audio Engineering Society, NBS staff, and a few local audiophiles and musicians. These listening studies were carried out to answer question 2 above. The NBS findings are as follows: 1. Answer to question 1 - "Does the system achieve its purpose?": NBS Conclusion : The system does not achieve its stated purpose. The system does prevent the copying of notched material much of the time. However, for about half of the recorded tracks studied, the system exhibited false negatives : i.e., notched material was nonetheless recorded. In addition, the system also exhibited false positive behavior, i.e., the system failed to record unnotched material. NBS studied 502 tracks on 54 compact discs and found false positives for 16 tracks on 10 discs. iii . .: - 2 . Answer to question 2 "Does the system diminish the quality of the prerecorded material?": NBS Conclusion : The system's encoder alters the original electrical signal. For some listeners for some selections, this results in a discernible difference between prerecorded notched and unnotched material NBS interpreted this question both in terms of objective electrical measurements and in terms of whether or not listeners can detect a difference between notched and unnotched material in carefully controlled listening tests. Electrical measurements showed that the encoded signal is degraded relative to the signal as it was prior to copy prevention coding, degraded in terms of adding a deep notch in signal amplitudes near 3840 Hz, and degraded in terms of changes in signal phase relationships near 3840 Hz. In a double-blind series of listening tests, 87 subjects listened to a prerecorded tape of short selections from 24 different sources - 20 compact discs and four selections produced on a keyboard synthesizer. The results show that, although the effects of the encoder are fairly subtle for some musical selections, there are some selections for which the subjects detected differences between notched and unnotched material In a second series, 15 experienced listeners worked with 10 selections presented on parallel tape tracks such that the subject could switch back and forth from notched to unnotched material. For 2 of the 10 selections the encoded version was correctly identified 12 out of 15 times; these results are statistically significant. 3. Answer to question 3 - "Can the system be bypassed, and, if so, how easily?" NBS Conclusion : The copy prevention system can be bypassed easily. This question was interpreted by NBS as asking whether or not a DAT recorder can be made to record notched prerecorded material. It is not possible to restore all of the actual information removed by the encoding process. NBS engineers designed and constructed several electronic circuits for implementing five different methods to circumvent or defeat the copy prevention system by the use of external signal conditioning. All five methods succeeded in bypassing the copy prevention system. The circuits are simple and easy to construct. The cost would be of the order of $100. iv Table of Contents Executive Summary i List of Figures V List of Tables X 1. Introduction 1 2. Description and Characterization of the System 3 2.1 Encoder: Operation 3 2.2 Encoder: Physical Measurements 6 2.3 Decoder: Operation 18 2.4 Decoder: Physical Measurements 22 3. Does the System Achieve Its Purpose? 44 3.1 Interpretation of the Question 44 3.2 Decoder False Positives 44 3.3 System False Negatives 46 4. Does the System Diminish the Quality of the Prerecorded Material? 49 4.1 Interpretation of the Question 49 4.2 Effect on the Performance Specifications of an Audio System 49 4.3 Subjective Response Testing Procedure 49 4.4 Selection and Characterization of Recorded Material 51 4.5 Results of Listening Tests 53 5. Can the System be Bypassed, and if so, How Easily? 65 5.1 Interpretation of the Question 65 5.2 Description of Defeat Methods 6 5 5.3 Ease of Implementation 70 5.4 Audibility of Defeat Methods 71 V 6 . Summary of Results 72 6.1 Does the System Achieve Its Purpose?.. 72 6.2 Does the System Diminish the Quality of the Prerecorded Material? .. 73 6.3 Can the System be Bypassed, and if so, How Easily? 74 Acknowledgements 76 Appendices Appendix A: Congressional Committee Letters A.1 Appendix B: Compact Discs Used in False Positive and False Negative Studies B.1 Appendix C: Contractor's Report C.1 Appendix D: Sponsors' Lists of Recorded Material D.1 Appendix E: Description of Recorded Material for Listening Tests E.1 Appendix F: Preparation of Recorded Material F.1 Appendix G: Audio Systems Used for Listening Tests G.1 Appendix H: Listener Instructions H.1 Appendix I: Schematic Diagrams of Defeat Circuits 1.1 VI List of Figures Figure 1 Conceptual block diagram of the major elements contained in the Encoder, as determined in the NBS study 4 Figure 2 Amplitude- frequency characteristic for the encoding notch filter in the left channel of Encoder 006 7 Figure 3 Phase characteristic for the encoding notch filter in the left channel of Encoder 006 7 Figure 4 Amplitude- frequency characteristic of the encoding notch filters in Encoder 006 8 Figure 5 Phase characteristic of the encoding notch filters in Encoder 006 9 Figure 6 Amplitude -frequency characteristic of the encoding notch filters in Encoder 012 9 Figure 7 Phase characteristic of the encoding notch filters in Encoder 012 10 Figure 8 Impulse response of the left- channel notch filter for Encoder 006 11 Figure 9 Amplitude -frequency characteristic of the on-notch and off-notch filters in Encoder 006 12 Figure 10 Amplitude -frequency characteristic of the on-notch and off -notch filters in Encoder 012 12 Figure 11 The Encoder state and the voltage levels at the output of the on-notch and off-notch filters for selection "Prokofiev" 15 Figure 12 The Encoder state and the voltage levels at the output of the on-notch and off-notch filters for Track 3 of "Drumming" by Steve Reich, Electra/Nonesuch 9079170-2 16 Figure 13 Amplitude -frequency characteristic for the left channel of Encoder 006 with the encoding notch filter out of the circuit 17 Figure 14 Phase characteristic for the left channel of Encoder 006 with the encoding notch filter out of the circuit 17 Figure 15 Basic diagram of the decoder and recorder electronics in the CBS-supplied DAT Recorder/Decoder 18 Figure 16 Block diagram of the major circuits contained in the Decoder section (within the dashed outline) of the DAT Recorder/Decoder, as determined in the NBS study 20 vii Figure 17 Amplitude response for the three bandpass filters in the Decoder versus the effective input signal frequency 23 Figure 18 Amplitude -frequency response of the anti-aliasing filter and automatic gain control (AGC) circuits in the Decoder 24 Figure 19 Difference level as a function of the lower sideband level (upper sideband level constant at about -43 dBV) that causes the POSSIBILITY AND RECORD INHIBIT states in the Decoder 26 Figure 20 Difference level as a function of the lower sideband level (upper sideband level constant at about -23 dBV) that causes the POSSIBILITY AND RECORD INHIBIT states in the Decoder 28 Figure 21 Difference level as a function of the upper sideband level . (lower sideband level constant at about -43 dBV) that causes the