IEEE Standard Method of Measuring Recorded Flux of Magnetic Sound Records at Medium Wavelengths

Abstract

The knowledge of the recorded signal amplitude on a magnetic record (which may be in the form of a tape, a disc, oτ a perforated film) has several practical uses: (1) for determining and standardizing references for the levels of audio programs on these magnetic records; (2) for specifying the recording-performance properties of magnetic media (for example, the distortion versus flux, etc); and (3) for measuring and specifying the sensitivity of reproducing heads (and also, by reciprocity, recording heads). The quantity for specifying the recorded signal is the short-circuit magnetic flux, sometimes simply shortened to flux. Several possible methods exist for measuring tape flux, but the measurement given here involves only the use of a calibrated ring-core head which is described in the reference of the standard. Note that the present standard measurement is only for "medium wavelengths". A further standard method for measuring short-circuit flux at long wavelengths and at short wavelengths - that is, the "frequency response" of the flux on a magnetic record - is under consideration. The technique utilizing a calibrated ring-core head to measure directly the flux at medium wavelengths is superior to magnetometer methods and to "single turn head" methods which have been used in the past because the head is easily manufactured using highly perfected commercial methods. The transducer (head) is commercially available to any laboratory at a reasonable cost; it is easily used, and requires no specialized auxiliary equipment. A further advantage of this method over magnetometer methods is the elimination of the errors inherent in making the "transfer" recording which is necessary for a magnetometer measurement. This standard represents the joint efforts of the Recording and Reproducing Standards Subcommittee of the Standards Committee of IEEE Group on Audio and Electroacoustics and the Sound Engineering Committee of the Society of Motion Picture and Television Engineers. It was originally prepared by an SMPTE Ad Hoc Committee as a method of measurement of flux on magnetic sound tracks used in motion-picture films, and was later modified for measurement of recorded flux in any form of magnetic sound record at medium wavelengths. This standard specifies a method of measuring the recorded flux per unit width of a magnetically recorded test signal at medium wavelengths for all magnetic sound record formats. It also specifies the equipment needed to implement this method.