1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.34!.$!2$):!4)/.G0G0/ amended at New Delhi, 1960,Geneva, 1964, Mar del Plata, 1968, Geneva, 1972, 1976, 1980,Malaga-Torremolinos, 1984 and at Melbourne, 1988)Note In this Recom
2、mendation, frequency-modulated voice-frequency telegraph (FMVFT) equipment withand without crystal control are distinguished. In order to improve the quality of transmission and to minimizemaintenance costs, the application of equipment with crystal control is recommended.1 The nominal modulation ra
3、te should be standardized at 50 bauds.2 For the nominal mean frequencies, the series formed by the odd multiples of 60 Hz should be adopted, thelowest frequency being 420 Hz in accordance with Recommendation R.31, 1, the mean frequency F0being definedas half the sum of the two characteristic frequen
4、cies corresponding to the permanent start polarity FAand stop polarityFZ.For the numbering of channels that has been adopted in the international service, see Recommendation R.70 bis.3 The mean frequencies at the sending end should not deviate from their nominal value by more than:a) for equipment w
5、ithout crystal control 2 Hz;b) for equipment with crystal control 0.5 Hz.1)4 The unbalance due to the modulation process = 2 FFFFlAZ0should not exceed 2%,whereFAand FZare the two characteristic frequencies measured over a period of 10 s;F0is the mean static frequency measured = 2 + FFAZ2;Flis the me
6、an dynamic frequency measured with 1:1 rectangular signals during 10 s.Measurement should be made applying to the input of the transmitter 1:1 rectangular signals with the build-upand hangover time below 1 s and with the unbalance below 0.1%. In the event that in service the transmitter iscontrolled
7、 by an electromechanical relay (with a certain transit time), the measurement should also be made with thattype of relay inserted between the 1:1 signal generator and the input to the transmitter. Both forms of measurementsneed not necessarily be included in the maintenance procedure but should be i
8、ncluded in laboratory type tests.Note To determine the unbalance due to the modulation process by the method indicated above, it isnecessary to measure the frequencies FA, FZand Fl and to calculate the mean frequency F0and the unbalance = 2 FFFFlAZ0_Error! Bookmark not defined.1)The tightening of th
9、is tolerance is for further study.2 Fascicle VII.1 - Rec. R.35A more rapid method for checking whether or not the unbalance is less than the limit fixed is to measure: the mean dynamic frequency Fl with 1:1 signals during 10 seconds; the mean dynamic frequency Fm with 2:2 signals during 10 seconds;
10、= 2 = 4 FFFFFFFFlAZmAZ00or to subtract:FF FF FFlm AZ AZ= 14( ) 14( 0.4 Hz. )The absolute value of the difference between the two frequencies measured, Fl , and Fm , must be less than0.4 Hz.5 The difference between the two characteristic frequencies (corresponding to the start and the stop conditions
11、)should be 60 Hz.6 The maximum tolerance on this difference should be 3 Hz.7 The total average power transmitted to the telephone-type circuit is normally dependent on the transmissioncharacteristics of the circuit as follows:a) For circuits with characteristics not exceeding the limits given in Ann
12、ex A, the total average powertransmitted by all channels of a system should preferably be limited to 50 W at a point of zero relativelevel. This sets, for the average power of a telegraph channel (at a point of zero relative level), the limitsgiven in Table 1/R.35.b) For other circuits, the total av
13、erage power transmitted by all channels of a system is limited to 135 W at apoint of zero relative level. This sets, for the average power of a telegraph channel (at a point of zerorelative level), the limits given in Table 2/R.35.TABLE 1/R.35Normal limits (nominal values) for the power per telegrap
14、h channel in FMVFT systems for bearer circuitswith charactistics not exceeding the limits given in Annex AAllowable power per telegraph channelat a point of zero relative levelNumber of telegraph channels in the FMVFT systemin microwattsin absolute power leveldecibels12 or less 4.0 24.018 2.7 25.724
15、 2.0 27.0Fascicle VII.1 - Rec. R.35 3TABLE 2/R.35Normal limits (nominal values) for the power per telegraph channelin FMVFT systems for others bearer circuitsAllowable power per telegraph channelat a point of zero relative levelNumber of telegraph channel in the FMVFT systemin microwattsin absolute
16、power leveldecibels12 or less 10.8 19.718 7.2 21.424 5.4 22.7Note The figures in Tables 1/R.35 and 2/R.35 assume the provision of a pilot channel on the telegraph bearer at a level of 27 dBm0 and 22,7 dBm0 respectively.8 In service, the levels of the signals corresponding to continuous condition Z a
17、nd continuous condition A shouldnot differ by more than 1.7 dB in the same channel. Both of these levels must lie between 1.7 dB with reference tothe level given in Table 1/R.35 or Table 2/R.35 as applicable.9 The frequency for the transmitted condition corresponding to the condition A is the higher
18、 of the twocharacteristic frequencies and that corresponding to the condition Z is the lower.10 In the absence of a channel-modulator control telegraph current, a frequency should be transmitted within 5Hz of the frequency normally transmitted for the start polarity. This frequency need not be sent
19、immediately afterinterruption of the control current.11 The frequency spectrum of the emitted signal, when transmitting 1:1 reversals (Definition 31.401,Recommendation R. 140) at the modulation rate of 2 fp(fp= frequency of modulation), should be in accordance withthe limits specified in Figure 1/R.
20、35, which shows the levels of the spectra of different components with respect to theamplitude of the non-modulated carrier as ordinates and the frequencies as abscissae.12 The receiving equipment should operate satisfactorily when the receiving level falls to 17.4 dB below thenominal level. The rec
21、eiving equipment should have been restored to condition A when the receiving level has fallen to23.5 dB below the nominal level. The nominal level is the level resulting from the choice of power per channel (seeTables 1/R.35 or 2/R.35 as applicable) depending upon the number of channels (12, 18 or 2
22、4) on the circuit. The alarm-control level is left to the choice of each Administration.13 On delivery by the manufacturer of 50-baud FMVFT equipment, the following values must not be exceeded forthe degree of distortion on a telegraph channel. These values correspond to closed circuit measurements,
23、 made with theaudio-frequency line terminals of the sending and receiving equipments connected together through an artificial line.Before the series of measurements taken in accordance with Recommendation R.51, the levels are adjusted to theirnormal values, the mean frequencies are checked to see wh
24、ether they are within:1) for equipment without crystal control 2 Hz;2) for equipment with crystal control 0.5 Hz2),_2)The tightening of this tolerance is for further study.4 Fascicle VII.1 - Rec. R.35of their nominal value (see 3 above) and the difference between the two characteristic frequencies i
25、s within thepermitted tolerance of less than 3 Hz (see 6 above). Bias distortion is eliminated by adjustment in the channelreceivers. The other channels of the system are modulated with unrelated signals when the effect of inter-channelinterference is to be included in the measurement. These “unrela
26、ted signals” can conveniently be 1:1 signals fromdifferent generators at approximately 50 bauds but not synchronous to each other or to the signal on the channel undertest.a) The transmission levels being normal, the artificial line introducing no frequency drift, but the measuredchannel being subje
27、ct to fortuitous distortion due to interchannel interference: 5% for the degree ofinherent isochronous distortion.b) The level being maintained constant, but at a value different from the normal level, for all constant levelsbetween 8.7 dB above the normal reception level and 17.4 dB below the norma
28、l reception level, the otherconditions being the original measurement condition: 7% for the degree of inherent isochronous distortion.c) In the presence of interference by a single sine-wave frequency equal first to one and then to the othercharacteristic frequency, with a level of 20 dB below the s
29、ignal level, the other conditions for the start ofmeasurements being maintained: 12% for the degree of inherent isochronous distortion (i.e. total distortionincluding the increase due to the interfering frequency, not distortion due to the interfering frequencyalone).d) By introducing a frequency dr
30、ift (f Hz) of the signals during transmission through the artificial line, fbeing not more than 5 Hz and the initial condition of the test otherwise being preserved: for equipment without crystal control for equipment with crystal control but without compensationfor frequency drift+ (5 2.5 Hz) %f fo
31、r equipment with crystal control and compensationfor frequency drift 7%for the degree of inherent isochronous distortion.By introducing a frequency drift (f Hz) of the signals during transmission through the artificial line, fbeing not more than 10 Hz, and the initial conditions of the test otherwis
32、e being preserved: for equipment with crystal control and compensationfor frequency drift 13%for the degree of inherent isochronous distortion. The measurements shall be made after the transienteffects of changing frequency have ceased.e) Equipment with crystal control, with any climatic conditions
33、specified for the tested equipment, the initialcondition of the test otherwise being preserved: 8% for the degree of inherent isochronous distortion. Thebias distortion caused by changes of climatic conditions should not be eliminated.Fascicle VII.1 - Rec. R.35 514 Frequency drifts on modern telepho
34、ne-type circuits are generally less than 2 Hz. Hence it is not necessary torecommend frequency drift control. For circuits on which a maximum frequency drift of not greater than 2 Hz cannotbe guaranted, and on which the distortion resulting from the frequency drift is not acceptable, compensation se
35、emsnecessary. Two methods can be used:a) compensation for each channel up to about 15 Hz;b) compensation for all the channels by using a pilot. In this case, the receiving end must be able to requestand obtain a pilot frequency. Administrations should agree among themselves on the advisability ofsen
36、ding the pilot and the choice of frequency. The frequencies 3300 Hz or, preferably, 300 Hz arerecommended for this pilot, with a tolerance of:1) for equipment without crystal control 1 Hz2) for equipment with crystal control 0.2 Hz.The mean power permitted at the relative zero point on this frequenc
37、y should not exceed 27.0 dBm0 or 22.7 dBm0 as appropriate (see Table 2/R.35).6 Fascicle VII.1 - Rec. R.3515 The number of significant modulation conditions is fixed at two; this number may he increased, if necessary, byagreement between the Administrations concerned.ANNEX A(to Recommendation R.35)Li
38、mits required by a bearer circuit for FMVFT application if thetotal power transmitted by all channels is set at 50 microwattsA.1 Loss/frequency distortionThe variation with frequency of the overall loss of the link with respect to the loss at 800 Hz should not exceedthe limits shown in Figure A-1/R.
39、35.A.2 Random noiseThe mean psophometric noise power referred to a point of zero relative level should not exceed 32 000 pW0p(45 dBm0p), using a psophometer in accordance with Recommendation P.53 1.A.3 Impulsive noiseThe number of counts of impulsive noise that exceeds 28 dBm0 should not exceed 18 i
40、n 15 minutes whenmeasured with an impulsive noise counter in accordance with Recommendation O.71 2.Fascicle VII.1 - Rec. R.35 7A.4 Error ratesThe telegraph character error rate that may be caused by interruptions and noise in the bearer circuit should notexceed the limits stated in Recommendations R
41、.54 and F.10 3.References1 CCITT Recommendation Psophometers (apparatus for the objective measurement of circuit noise), Rec. P.53.2 CCITT Recommendation Specification for an impulsive noise measuring instrument for telephone-type circuits,Rec. O.71.3 CCITT Recommendation Character error rate objective for telegraph communication using 5-unit start-stopequipment, Rec. F.10.
