1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 1 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G02%#/-%.$!4)/.3G0G0/.G0G04%,%0(/.%37)4#().G0G0!.$G0G03).!,).).4%2.!4)/.!,G0G0!54/-!4)#G0G0!.$3%-)G13!54/-!4)#G0G07/2+).#!53%3G0G0/b) the use of a common battery with separate power supply leads for s
2、peech and switching circuits. Betterresults may be obtained at an increased cost by independent batteries adequately separated;c) the arrangement of the cells of the battery in a U formation.1.3 Earth returnsIndependent earth returns should be used for signalling-frequency supply circuits.2 Noise ge
3、nerated in the speech circuit2.1 Contact noise caused by vibrationThis kind of noise is caused by contact resistance variations of the various commutator, switch and relaycontacts due to mechanical vibration.This contact noise may be reduced by:a) the use of damping devices to reduce the generation
4、of vibration caused in particular by relay sets,mechanical and electromagnetic clutches;b) the use of multiple brushes, spring or resilient mountings to reduce the transmission of vibration;c) a suitable choice of contact materials;d) the use of the best contact shape and of twin contacts;2 Fascicle
5、 VI.1 - Rec. Q.29e) maintaining atmospheric conditions at an appropriate relative humidity and the use of air filters; use ofdust covers on equipment, arranging design of columns, window sills, radiators and floor to avoidharbouring dust;f) careful maintenance cleaning and lubrication in accordance
6、with specifications.2.2 Frying noiseIn speech circuits some contact materials are liable to cause frying noise.This noise may be reduced by the use of suitable contact materials and by keeping an appropriate relativehumidity.2.3 Contact noise caused by wetting currentsSpeech circuits without d.c. cu
7、rrents are liable to fading due to contact resistance fluctuations. Fading may bereduced by wetting. However, wetting currents may introduce frying noise on the lines.2.4 Charge and discharge clicksClicks may frequently be caused by the charging or discharging of capacities (cable capacity) by switc
8、heswhen rotating over occupied and non-occupied terminals.Objectionable clicks are also likely to result from sudden battery reversals, dialling and other abrupt changesin the current flowing in the speech circuits.These effects may be reduced:a) by disconnecting the speech circuits from the brushes
9、 during the hunting period of the switch;b) by the use of twisted pairs, by limiting the length of cabling and also by locating relays as close aspossible to the selectors they control.2.5 Unsound contactsObjectionable noise may be due to unsound contacts on distribution frames, particularly when wo
10、rk is inprogress such as adding or changing jumpers, etc. Such unsound contacts may be due to “dry“ contacts inadequatelysoldered, poorly wrapped joints, or to the use of distribution frame equipment having inadequate contact pressure. It issuspected that this type of trouble is responsible for most
11、 of the “hits“ and “misses“ and usually for an increase in noise.2.6 Tapping lossesWhen lines are tapped for service interception, observation, etc., the tapping circuit should be designed to givethe minimum of unbalance to earth and the transmission loss introduced should be a minimum. Semi-permane
12、ntconnections should be used in preference to base-metal sliding connections at the tapping point.2.7 Reduction of the number of switching contactsCircuits should be designed so that at each switching stage there is a minimum number of contacts in thespeech circuit in order to reduce the risk of mic
13、rophonic noise from “dry“ contacts.3 Noise induced in the speech circuit3.1 Noise induced in the speech circuit may be due to:a) speech crosstalk;b) signalling frequency crosstalk;c) induction from tone supplies;d) direct current pulses;e) clicks caused by abrupt changes in inductive and capacitive
14、circuits.Fascicle VI.1 - Rec. Q.29 3Clicks may be reduced at the source by the use of spark quench devices or other means to reduce the steepnessof the interfering wave-front concerned. In addition, noise may be reduced by balancing, by using twisted pairs and/orby screening.3.2 Noises due to unbala
15、nced transmission bridge circuitsA well-balanced circuit is necessary for the transmission bridge to avoid noise interference. This can beachieved by:a) the use of balanced components;b) the separation of components used for speech from those used for control and switching;c) the separation of indiv
16、idual transmission bridges by screening or spacing;d) the addition of balancing components, e.g. balancing transformers of retardation coils;e) taking the precautions listed at the end of 3.1 above.3.3 Low-level speech circuitsLow-level electronic speech circuits are particularly susceptible to nois
17、e induction and should therefore bescreened.3.4 Longitudinal interferenceSuch noise may be induced into the speech circuit from the line by power distribution systems and tractioncircuits or by earth potential differences.These may be reduced by balancing the line or by the addition of transformers.Note - Interference which is sufficiently severe to cause unwanted operation of relays, etc., may be overcomeby the use of loop circuits which should also reduce noise.
