1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.#!53%3G0G0/amended at Arnhem, 1953 and Geneva, 1956 and 1964)The CCITT,considering(a) that the great majority of international telegraph circuits are routed on voice-frequency
2、 telegraph (VFT)channels;(b) that VFT channels are liable to disturbance from the following causes:i) variations in the voltage and frequency of the source of telegraph carrier frequency due to variations inthe power supply, and variations in the signalling load in the case where the carrier source
3、suppliesseveral channels;ii) abrupt or gradual changes in the transmission equivalent of the telephone-type circuit;iii) intelligible crosstalk from other telephone-type circuits, particularly near-end crosstalk;iv) unintelligible crosstalk resulting from the cross-modulation of telephone-type circu
4、its when operated bycarrier currents;v) noise induced from electrical power and traction systems;vi) telegraph crosstalk from other telegraph channels, e.g. production of odd harmonics of the telegraphcarrier frequencies in certain channels falling within the passband of other channels, intermodulat
5、ion infilter coils, etc.;vii) variations of power supplies affecting the amplifier and detector of the VFT channel and sometimes thereceiving relay;viii) the effects of mechanical vibration upon valves (microphonics) and relays;ix) bad contacts (e.g. test point and valve bases) and badly soldered jo
6、ints;x) deterioration of component parts, e.g. ageing valves;xi) failure of power supplies, e.g. on changeover from main to reserve supply;xii) accidental disconnections made during the course of maintenance and construction work;xiii) on overhead lines, effects of atmospheric electricity, frost, et
7、c.;(c) that the disturbances account for practically all the distortion in telegraph channels, except forcharacteristic distortion (which is chiefly a function of filter and amplifier-detector design), some bias (due tomisadjustment of controls and relays, etc.) and, in the case of the lower frequen
8、cy channels, the distortion that arisesfrom the low ratio of carrier frequency to signalling frequency;(d) that many of the causes of disturbance are individually negligible and the more important of the othershave been found, in the experience of several Administrations, to be capable of eliminatio
9、n by careful maintenanceboth on the VFT equipment and at all points on the bearer circuit;(e) that the CCITT is also studying the causes of disturbance in telephone circuits and the precautions to betaken to minimize their occurrence;(f) that the results of the CCITT study will be of great importanc
10、e to telegraphy;(g) that, as a result of the considerable investigations already made by certain Administrations on the causesof disturbances in telephone and telegraph circuits, the relative order of importance of these causes appears to beapproximately as follows:2 Fascicle VII.1 - Rec. R.80i) in
11、the case of telephone circuits: high resistance and unsoldered connections; noisy and microphonic valves, and poor contact between valve pins and valve holders; working parties engaged on cable operations; noisy and high-resistance U-links; changes in line level not compensated at the detector input
12、; crosstalk; errors in setting up, for example incorrect equalization, line transformers incorrectly connected, faultycomponents;ii) in the case of VFT equipment high resistance and unsoldered connections; valves deteriorated beyond permissible limits; bad contacts; faults on power changeover equipm
13、ent; frequency error of the carrier supply;unanimously declares the view(1) that it is desirable for Administrations to undertake investigations of the causes, and frequency ofoccurrence of disturbances of VFT channels routed on the various types of bearer circuit likely to be employed forinternatio
14、nal telegraph circuits;(2) that in doing these tests and in order that the results may be of the greatest use to telegraphy and telephony,the incidence of disturbances should be measured according to their duration as follows: less than 1 ms, 1 to 5 ms, 5 to10 ms, 10 to 20 ms, 20 to 100 ms, 100 to 300 ms and those more than 300 ms;(3) that the results should be classified according to the type of bearer circuit, viz. audio or carrier, cable oroverhead line.(4) Measurements of disturbances should be made at the direct current output of the VFT channel that is underobservation.
