The Recommendation considering that the performance characteristics of international sound-programme circuits used for programme exchange are based on a hypothetical reference circuit (HRC) defined in Recommendation 502; that the performance of a realistic international sound-programme circuit shorter or longer than the HRC can be derived by calculation according to rules given in Recommendation 605; that the performance of the sound-programme transmission chain from the broadcasting house to the output of the receiver should therefore be based on a reference chain; that sound-programme signals intended for international exchange are conveyed from the broadcasting house to the international sound-programme centre (ISPC) on national circuits which are either the same as, or similar to, the national circuits which form part of the sound-programme transmission chain; that the performance of the sound-programme transmission chain should be described by a number of parameters; that for reasons of comparison, clearly defined parameters measured using test signals, should be used; that these parameters and their target values should be based on the perceptibility of the impairments by the human ear.