An investigation of high-frequency bias-induced tape noise

Bias-induced tape noise remains a major limitation of the SNR in audio magnetic tape recording systems. Defined as the increment in system noise incurred when the bias oscillator is turned on, the noise can originate from a number of different causes; namely, bias oscillator harmonic distortion, magnetized heads, the earth's magnetic field, and an intrinsic noise source. The latter noise source is our primary concern here. Such record system parameters as head-to-tape spacing, gap length, bias current, and bias frequency were investigated with regard to their influence on this intrinsic bias noise source. Two models of the mechanism of intrinsic bias noise are examined. The first, the "amplitude modulation model." proposes that bias noise is generated by amplitude modulation of the recorded bias signal by the physical and magnetic variations of the head-tape system. In this model, bias noise is merely the lower AM sidebands of the recorded bias signal. The second model relates bias noise to the interaction fields in erased tape. This model proposes that these fields behave similarly to normal recording fields and can be "re-recorded" on the tape at an enhanced level. The two proposed mechanisms are examined in the light of the experimental data. The amplitude modulation model is shown to agree with all the observed data with the exception of the existence of bias noise at bias wavelengths smaller than the particle size. The second model, which does not incorporate a wavelength dependency of bias noise, is in qualitative agreement with the observed data. Methods are discussed for reducing the bias noise without materially affecting the system performance.