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U.S. Geological Survey Open-File Report 75–663

Notes on Some Experiments on the Application of Subtractive Compensation to USGS Seismic Magnetic Tape Recording and Playback Systems

By Jerry P. Eaton

Thumbnail of and link to report PDF (25.5 MB)Introduction

The purpose of these experiments is to lay the groundwork for the implementation of subtractive compensation of the USGS seismic network tape playbacks utilizing the Develco model 6203 discriminators at a x1 playback speed. Although the Develco discriminators were designed for this application and a matching Develco compensation discriminator was purchased, effective use of this system for subtractive compensation has been blocked by the inadequate (frequency dependent) matching of the phase of the compensation signal to that of the data signal at the point compensation is carried out in the data discriminators.

John Van Schaack has ameliorated the phase mismatch problem by an empirical alteration of the compensation discriminator input bandpass filter. We have selected a set (of eight) Develco discriminators and adjusted their compensation signal input levels to minimize spurious signals (noise) originating from tape speed irregularities. The sensitivity of the data discriminators was adjusted so that deviations of +125 Hz and -125 Hz produced output signals of +2.00 volts and -2.00 volts, respectively. The eight data discriminators are driven by a multiplex signal on a single tape track (subcarriers 680, 1020, 1360, 1700, 2040, 2380, 2720, and 3060 Hz). The Develco-supplied compensation discriminator requires an unmodulated 3125 Hz signal on a separate tape track.

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Suggested citation:

Eaton, J.P., 1975, Notes on some experiments on the application of subtractive compensation to USGS seismic magnetic tape recording and playback systems: U.S. Geological Survey Open-File Report 75–663, 36 p. (Available at


I. Introduction

II. Test VCO Bank Design and Performance

III. Compensation Tests on the B&H VR-3700B

IV. Performance of the System Under Strong Modulation and with TimingSignals

V. Compensation Tests with the Sony TC-126 Cassette Recorder

VI. Influence of the Discriminator Output Filter and of the Subcarrier Channel Density on Frequency Response and Dynamic Range of the Data Channel

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