GB833929A - Improvements in or relating to arrangements for measuring noise on television circuits - Google Patents

Abstract

833,929. Television. STANDARD TELEPHONES & CABLES Ltd. Nov. 28,1958 [Nov. 29, 1957], No. 37286/57. Class 40 (3). [Also in Group XXXVI] In an arrangement for measuring the amount of noise present in a television link, means are provided for transmitting over the link a test signal, curve A, Fig. 1, consisting of synchronizing pulses 1 alternating with rectangular video pulses 2 of constant amplitude, whereby the crests of received video pulses, curve B, carry noise variations, means being provided at the receiving end of the link for separating the noise variations from the crests of the pulses and applying them to a power measuring device. The separation may be effected by an arrangement, as shown in Fig. 4, in which the received signals are first D.C. restored at 4 and then applied to an amplifier 5 which separates the pulses above level 6, curve B, Fig. 1. The output is applied via a phasesplitting amplifier 7 to further amplifiers 8 and 9 to produce oppositely-phased waves, curves C and D, Fig. 1, from the second of which the noise is eliminated by limitation in the amplifier 9. The waves are combined in equal resistors 12 and 13 to cause cancellation of the pulse portions, leaving the noise alone which is passed through an amplifier 14 and weighting circuit 15 to a noise power measuring device 16. Weighting circuit 15 may alternatively precede D.C. restorer 14. Pulses 2, curve A, Fig. 1, are chosen to be less than peak white and of an amplitude determined by anticipated noise such as to prevent noise peaks being removed in amplifier 5. Where the received signal is accompanied by transients 17 and 18, Fig. 3, which may interfere with the measurement, auxiliary pulses 19, which are shorter than pulses 2, may be added to the received signal by a pulse generator 21 and shaper 25 so as to displace the noise away from the transients. Amplifier 5 is modified so as to limit above level 20. Furthermore, where the received pulses arrive with a sloping crest, the auxiliary pulses may be shaped inversely to offset the slope. According to a modification, pulses 2 may be omitted from the transmission, so that noise is effectively measured at black level, and pulses are added at the receiving end by generator 21.