Velocity and Amplitude Systems

Author: N.H. Crowhurst

Velocity and amplitude pickup

As well as different kinds of transducers for the cutter or pickup head, there are different principles of operation; some work on a velocity principle in which the output voltage is proportional to the rate at which the stylus moves. Others work on an amplitude principle in which the output voltage is proportional to the amount that the stylus moves. The velocity-type transducers include the magnetic, moving coil, and ribbon. The amplitude-type transducers include the crystal or ceramic and the capacitor.

The important difference between these types is in the kind of frequency response that they produce. If the disc has grooves of equal magnitude from side to side at different frequencies, the amplitude-type pickup will give a flat response. The rapidity with which the stylus moves is, however, proportional to frequency; hence, this type of recording would produce an output proportional to frequency, if the pickup were of the velocity kind. (This would require special equalization on playback.)

To produce a recording of the velocity kind that gives uniform output with frequency, the amplitude must vary inversely with frequency. (Doubling the frequency must halve the amplitude, so that the maximum rate at which the stylus moves at the different frequencies will be constant.)

Standards of disc recording have been based on the velocity principle, using magnetic, moving coil, or ribbon type transducers for cutters or pickups. Assuming a recording from 20 cycles to 20,000 cycles, this would mean the magnitude of movement to give the same output at 20 cycles would be 1000 times what it is at 20,000 cycles. Obviously, this would be impractical. If the movement is 1/10,000 inch at 20,000 cycles, 1/10 inch movement will be needed at 20 cycles. 1/10,000 of an inch is so small that the roughness of the record will make more noise than the signal recorded; on the other hand, 1/10 of an inch is so wide that it would be impossible to get more than 10 grooves to the inch.

Standard recording curve of Recording Industry Association of America (RIAA)

For this reason, there must be a change at some point to constant-amplitude recording. The Recording Industry Association of America (RIAA) puts this turnover at 500 cycles. To overcome the fact that the extremely small movement, above, say 10,000 cycles, would mean that the surface noise would be louder than the audio, constant amplitude is also applied above a frequency of 2,120 cycles. These two changes from the velocity characteristic avoid excessive stylus movement at low frequencies and prevent noise from overriding the audio at high frequencies. This varying frequency response is provided by an equalizer in the recording amplifier and must be compensated for by opposite equalization in the playback amplifier.