Modulation noise in magnetic tape recordings

Modulation noise remains as a major limitation on the dynamic signal-to-noise ratio obtainable in a magnetic recording system. The sources of modulation noise in magnetic tape recordings have been investigated by reducing to the smallest possible value the inherent noise of the recording and playback equipment used in the tests. Possible causes of modulation noise are then independently introduced and their effects measured. The results of these measurements indicate that when recording and reproducing equipment is properly adjusted, the major source of modulation noise is spurious amplitude modulation of the recorded signal by variations in the physical and magnetic properties of the recording tape. The amount of this modulation noise is found to increase above the value obtained with a dc signal, in proportion to the recorded signal frequency. Recommendations are made for measurement of amplitude modulation noise as a check on tape quality and on those adjustments which affect amplitude modulation noise. This measurement is in addition to the measurement of frequency modulation noise commonly known as "flutter."