Several practical problems make the use of the usual reproducer calibration test tape unsatisfactory below 1000 Hz at a tape speed of 380 mm/s. First, the low frequency equalization standards below 50 Hz are often not followed in practice. Second, there are many different multitrack formats, and this makes a true multitrack reproducer calibration tape commercially infeasible, but although one can calculate a fringing compensation, it is not accurate at the lowest frequencies. Third, the reproducer response inherently undulates, but there are not enough test frequencies to characterize that response. At long wavelengths (low frequencies) the recorded flux is directly proportional to the recording current. Therefore system calibration is best performed by the following method: standardize the recording response (recording head current versus frequency); use recording and reproducing heads of the same core width throughout the system; record a slowly swept-frequency test signal; and adjust the reproducer low-frequency equalizer for flattest overall response.
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