The RF signal-to-noise ratio at the receiver input depends, apart from other factors, on audio frequency S/N ratio for a defined grade of performance. Since the majority of the physiological and psychological factors ultimately influence only the AF signal-to-noise ratio, a series of subjective listening tests were carried out in India and the USSR to assess the minimum acceptable value of this ratio, from which the equivalent RF signal-to-noise ratio at the input of the receiver may also be derived.
ITU-R REPORT BS.1058-1986 history
1986ITU-R REPORT BS.1058-1986 Minimum AF and RF signal-to-noise ratio required for broadcasting in band 7 (HF)