1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.42!.3-)33)/.G0G0#(!2!#4%2)34)#G0G0 amended at Geneva, 1976)1 Standardized carrier systems with 4-kHz and 3-kHz spacing permit homogeneous voice-frequency telegraph(VFT) system
2、s providing the capacities of telegraph channels given in Table 1/R.30.TABLE 1/R.30Bearerbandwidth50-baud120-Hz spacing100-baud240-Hz spacing200-baud360-Hz spacing200-baud480-Hz spacing4 kHz 24 128(not normally used)63 kHz 22 11 7 52 Audio-frequency circuits with heavy or semi-heavy loading permit 1
3、2-channel 50-baud systems; circuits withlighter loading permit 18 channels at 50 bauds.3 Four-wire links are to be preferred for voice-frequency telegraphy.4 The composition of a 4-wire link for voice-frequency telegraphy differs from that of a telephone circuit in thatthere are no terminating sets,
4、 signalling equipment and echo suppressors.5 With 2-wire links, a duplex arrangement would not be feasible since the links could not be balanced with thenecessary precision to avoid mutual interaction. If the low frequencies are used for transmission in one direction andhigh frequencies for the othe
5、r direction, a 2-wire link can be used for voice-frequency telegraphy.6 The conditions of use of international VFT links are described in detail in Recommendation H.22 1.7 PCM (pulse code modulation) channels complying with Recommendation G.712 2 are also suitable as bearersfor FMVFT (frequency-modu
6、lated voice-frequency telegraph) links. However, the increase in telegraph distortion inrelation to the transmission level and the number of tandem-connected PCM channels is the subject of further study.References1 CCITT Recommendation Transmission requirements of international voice-frequency telegraph links (at 50,100 and 200 bauds), Rec. H.22.2 CCITT Recommendation Performance characteristics of PCM channels at audio frequencies, Rec. G.712.
