The Harman Kardon CD 491 cassette deck is one of the high end cassette decks featuring an internal test tone generator for both bias and rec level calibration. Bias calibration is achieved with matching a 400hz and 12.5KHz signal recorded onto each channel, and rec level is achieved with a 400Hz signal recorded onto each channel. Tape used for this calibration is Sony HF 60. As you can see it calibrates nicely despite it's old age and is now ready for recording.

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