1、 1 Scope This practice specifies the technique of measuring, by the intermodulation method, the signal distortion introduced by audio systems. 2 Test method 2.1 An arrangement of apparatus as shown in figure 1 may be used for the measurement of intermodulation distortion. 2.2 The signal supplied to
2、the audio system under test shall consist of a linear combination of a low frequency, nominally 60 Hz; and a high frequency, nominally 7000 Hz. The amplitude of the high-frequency signal shall be 12 dB 1 dB below that of the low frequency. The test signals shall be not more than 3% from the nominal
3、frequencies to be used. Neither frequency shall contain more than 0.5% harmonic distortion. The peak amplitude of the complex wave at either the input or the output of the audio system under test shall be specified along with the measurement results. 3 Test equipment A block diagram of an acceptable
4、 intermodulation analyzer is shown in figure 2. The output of the analyzer shall be calibrated to provide an indication of percentage modulation of the high-frequency component after removal of the low-frequency component. The meter should be sensitive to modulating frequencies in the range from app
5、roximately 10 Hz to 400 Hz. Figure 1 Arrangement of intermodulation test apparatus to determine distortion in audio systems Page 1 of 2 pagesRP 120-2005 Revision of RP 120-1994 Copyright 2005 by THE SOCIETY OF MOTION PICTURE AND TELEVISION ENGINEERS 595 W. Hartsdale Ave., White Plains, NY 10607 (914
6、) 761-1100 SMPTE RECOMMENDED PRACTICE Measurement of Intermodulation Distortion in Audio Systems Approved April 6, 2005 RP 120-2005 Page 2 of 2 pages Figure 2 Acceptable intermodulation analyzer Annex A (informative) Additional data The method of measuring distortion by intermodulation was originall
7、y developed as a means for controlling the quality of variable-density audio tracks. Previous documents described the use of intermodulation tests for this purpose. Variable-density audio tracks have now become obsolete. However, a variation of the original intermodulation test, with a high frequenc
8、y of 7000 Hz instead of the original 2000 Hz, is commonly used in the measurement of distortion in audio systems. It is this version of the test which is described in this practice. Those wishing to use the intermodulation test for control and evaluation of variable-density audio tracks are urged to
9、 consult the references listed in annex B. Annex B (informative) Bibliography Frayne, J.G. and Scoville, R.R. Analysis and measurement of distortion in variable-density recording. Journal of the SMPE 32: 648-673; June 1939. Frayne, J.G. and Wolfe, Halley. Sound Recording. John Wiley 1949. LeBel, C.J
10、 An experimental study of distortion. Journal of the Audio Engineering Society 2: 215-218; October 1954. MacDonald, J.R. The calibration of amplitude modulation meters with a heterodyne signal. Proceedings of the IRE 52: 1515-1518; October 1954. Read, G.W., and Scoville, R.R. An improved intermodulation measuring system. Journal of the SMPE 50: 162-173; February 1948. Warren, W.J. and Hewlett, W.R. An analysis of the intermodulation method of distortion measurement. Proceedings of the IRE 36: 457-466; April 1948. IEC 60268-3 (2000-08), Sound System Equipment Part 3: Amplifiers
