In order to achieve good anti-noise effect, the ultrasonic degaussing current has the following requirements: firstly, the magnitude of the degaussing current is large enough that the magnetic tape with any residual magnetization can be magnetized to saturation; secondly, the frequency of the signal should be sufficiently high. During the period of passing the degaussing head gap, the frequency of the demagnetizing field generated by the ultrasonic frequency signal is sufficient; in addition, the waveform of the ultrasonic frequency current is strictly symmetrical, so that the hysteresis loop is approximately symmetrical to the origin, and finally returns the origin makes the remanence become zero.
Since the analog tape recorder has various losses during recording and playback, and these losses are mainly generated at high frequencies, the playback curve is no longer a rising straight line, but gradually decreases as the frequency increases. The main causes of the high frequency drop of the playback curve are the following:
The loss during recording is: recording demagnetization loss, belt thickness loss, tape self-demagnetization loss, magnetic head hysteresis loss and eddy current loss; loss during playback: gap loss of playback head, spacing loss, head core loss and head azimuth loss. In addition, during the playback process, for low frequency signals, the recording wavelength (the length of the tape corresponding to the change of the residual magnetic force left on the magnetic tape by one week) is equivalent to the contact length of the magnetic tape and the reproducing magnetic head, which causes the low frequency band of the sounding curve to fluctuate, which is generally called low frequency contour effects.
In order to ensure that the audio recording signals of the respective frequencies can be reproduced with the same sensitivity, it is necessary to perform signal processing, that is, equalization, by using a reverse curve which is mirror-symmetrical with the actual playback curve. In practical applications, equalization is divided into two phases, namely recording equalization and playback equalization. In AC bias recording, the magnitude of the AC bias current will be directly related to the quality of the recorded signal.
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