1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.6/)#%G0G13G0 amended at Geneva, 1964,Mar del Plata, 1968, Geneva, 1976 and Melbourne, 1988)It is necessary to distinguish between the case in which the radio frequency used is
2、 below approximately30 MHz, and the case in which the radio frequency used is greater than approximately 30 MHz.1 Radio circuits the frequency of which is below approximately 30 MHz1.1 In the case of radio circuits whose frequency is less than approximately 30 MHz, it appears that the use ofamplitud
3、e-modulated voice-frequency telegraph systems, as defined by Recommendation R.31, cannot berecommended. In such a case, the nature of the telephone-type circuits available for telegraph operation may varywidely according to the radio system used, and several systems of telegraph transmission are ava
4、ilable (e.g. two- orfour-tone telegraph systems, frequency modulated systems, etc.).1.2 However, frequency-shift systems are in use on many routes and the frequency-exchange method (Definition32-32, Recommendation R.140) of operation is in use on long routes suffering from severe multipath distortio
5、n.1.3 Synchronous telegraphy operating at approximately 100 bauds (see CCIR Recommendation 436-2 1)Radiotelegraph channels that operate synchronously at a modulation rate of 96 bauds and employ automaticerror correction are being increasingly used. The channel arrangement shown in Table 1/R.39 is pr
6、eferred for voice-frequency multi-channel frequency-shift systems operating at a modulation rate of approximately 100 bauds over HFradio circuits. For frequency-exchange systems (Definition 32.32, Recommendation R.140), the central frequencies ofTable 1/R.39 should be used, and should be paired in t
7、he manner found to be best suited to the propagation conditionsof the route. (A typical arrangement would take alternate pairs giving 340 Hz between tones.)TABLE 1/R.39Central frequencies of voice-frequency frequency-shift telegraph channels with a channel separationof 170 Hz and a modulation index
8、of about 0.8(Frequency shift: 42.5 Hz or 40 Hz)Channel positionCentral frequency(Hz)Channel positionCentral frequency(Hz)123456742559576593511051275144589101112131415161517851955212522952465263528052 Fascicle VII.1 - Rec. R.391.4 Start-stop telegraphy at 50 baudsFor several years, various Administra
9、tions have had in service, on certain selected circuits, equipment with achannel spacing of 120 Hz, the central frequencies and frequency deviations of which are in agreement withRecommendation R.35. The central frequencies of these systems are given in Table 2/R.39.TABLE 2/R.39Central frequencies o
10、f voice-frequency frequency-shift telegraph channels with a channel separationof 120 Hz and a modulation index of about 1.4(Frequency shift: 35 Hz or 30 Hz)Channel positionCentral frequency(Hz)Channel positionCentral frequency(Hz)1234567891042054066078090010201140126013801500111213141516171819201620
11、1740186019802100222023402460258027002 Radio circuits whose frequency is greater than approximately 30 MHzThe use of voice-frequency telegraphy on line-of-sight radio-relay links and on trans-horizon radio-relaysystems is under study.Reference1 CCIR Recommendation Arrangement of voice-frequency telegraph channels working at a modulation rate ofabout 100 bauds over HF radio circuits, Vol. III, Rec. 436-2, ITU, Geneva, 1978.
