This section deals with the measurement of interference on simulated radio-relay systems for television and f.d.m. telephony transmission. The interference may originate either between signals within the same composite baseband of a single r.f. channel, or from other r.f. channels. Interference noise from other signals needs to be taken into account when making measurements on simulated systems in accordance with Part 3, Section Three: Measurements for Monochrome and Colour Television Transmission, Part 3, Section Four: Measurements for F.D.M. Transmission of this publication, in order to make the best estimate of the noise which may appear in a radio-relay system under operational conditions. Measurements are normally carried out both with and without the interfering source present in order to establish the noise increment resulting from the interference.