A nonmystical treatment of tape speed compensation for frequency modulated signals

After briefly reviewing frequency modulation and demodulation, tape speed variation is modeled as a distortion of the independent variable of a frequency-modulated signal. This distortion gives rise to an additive amplitude error in the demodulated message, which comprises two terms. Both terms depend on the derivative of time base error, that is, the flutter of the analog tape machine. It is pointed out that the first term depends on the channel's center frequency and frequency deviation constant, as well as on the flutter, and that the second depends solely on the message and flutter. A description is given of the relationship between the additive amplitude error and manufacturer's flutter specification. For the case of a constant message, relative errors and signal-to-noise ratios are discussed to provide insight into when the variation in tape speed will cause significant errors. An algorithm is then developed which theoretically achieves full compensation of tape speed variation. After being confirmed via spectral computations on laboratory data, the algorithm is applied to field data.