Based on a practical implementation within an archival workflow and prior studies of bias signal retrieval from analogue magnetic tape, the authors focus on a comparison of reproduction and pre-processing methods of the bias signal. Previous approaches are compared to the authors' proposed method. The signal pre-processing in analogue as well as digital domain are outlined, and basing on analyses of bias signals from professional and semi-professional recordings, the various practical problems are discussed: Level instability and unknown frequency of the recorded bias signal, frequency variations mainly with semi-professional devices of older types of recording equipment due to the instability of the bias oscillator, as well as effects of signal distortions, interferences and ultrasonic artefacts.
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