When in the years 50 appeared with success the components qualified as "High Fidelity", the difficulty of comparison among apparatuses of equal functions but of distinct manufacturer arose.  Each one it specified what believed convenient and of the form more favorably.  To put order in this chaos they elaborated norms upon the diverse parameters to measure in the players of sound in high fidelity.  Some norms are only norms of measurement, that is to say only they indicate forms it in which this should be performed and the form of expressing it so that they can be compared among itself measurements originating in distinct manufacturers.  Such it is the case of the American norm of measurement IHF (Institute of High Fidelity). 

Other they are norms of quality, they specify requisite minimums so that the apparatus can enter the denomination of high fidelity.  The but used of them is the one that was edited in the years 1963-63, in the republic Federal of Germany and in charge of the Institute of Normalization (DIN).  These norms carried the number 45.500, destined for the fame.  Another norm, used frequently by the Japanese, is the JIS that gives in general values better than the DIN, thus, upon comparing values of the distinct apparatuses we should assure of which have been the norms upon the that did the measuring; otherwise we would be able to commit errors of consideration.  A concept used in the measurements and that should be cleared up is that of the weighting that is specified for some measured.  Weigh, is to measure keeping in mind the weight and by extension, in the case that occupies us, is to measure a parameter keeping in mind the sonorous sensation that produces in the brain each one of the audible frequencies.  Thus, it is not the same to have a certain level of noise in a plate turntables in the frequency of 400 Hz, that in the frequency of 50 Hz.  The second creates in the brain a notably smaller sensation.  From here that the measured should be done by electric filters that respond to this peculiar sensibility of the human ear.  Some of these norms have been revised in the course of the years, but they have remained essentially unalterable.  In some aspects the technique has exceeded them extensively, in other difficulties for their fulfillment exist still.  We do not forget that is a matter of requisite minimums. 


Manual of High Fidelity and Professional Sound. Electronic world series. Marcombo editions p.p  53, 54.