1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.34!.$!2$):!4)/.G0G0/ amended at Mar del Plata, 1968, Geneva, 1972, 1976, 1980,Malaga-Torremolinos, 1984 and at Melbourne, 1988)Note In this Recommendation frequency-modulated
2、voice-frequency telegraph (FMVFT) equipment with andwithout crystal control are distinguished. In order to improve the quality of transmission and to minimize maintenancecosts. the application of equipment with crystal control is recommended.1 The nominal modulation rate is standardized at 100 bauds
3、2 The nominal mean frequencies are 480 + (n 1) 240 Hz, n being the channel position number. The meanfrequency is defined as half the sum of the characteristic frequencies corresponding to the start polarity and stoppolarity. For the numbering of channels that has been adopted in the international s
4、ervice see Recommendation R.70bis.3 The mean frequencies at the sending end should not deviate from their nominal value by more than:a) for equipment without crystal control 3 Hz;b) for equipment with crystal control 0.5 Hz1).4 The difference between the two characteristic frequencies in the same ch
5、annel is fixed at 120 Hz.5 The maximum tolerance on this difference should be 4 Hz.6 The unbalance due to the modulation process = 2 0 FFFFlAZshould not exceed 2%,whereFFAZ and are the two characteristic frequencies measured over a period of 10 s;F0is the mean static frequency measured + FFAZ2;Flis
6、the mean 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 iscont
7、rolled 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 measurementneed not necessarily be included in the maintenance procedure but should
8、 be included 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 Fland to calculate the mean frequency F0and the unbalance = 2 0 FFFFlAZ_1)The tightening of this tolerance is for fur
9、ther study.2 Fascicle VII.1 - Rec. R.37A more rapid method for checking whether or not the unbalance is less than the limit fixed is to measure: the mean dynamic frequency Flwith 1:1 signals during 10 s; the mean dynamic frequency Fmwith 2:2 signals during 10 s; = 2 0 FFFFlAZ= 40 FFFFmAZor to subtra
10、ct:()()FF FF FFlm AZ AZ= 141409 Hz.The absolute value of the difference between the two frequencies measured, Fl, and Fm, must be less than0.9 Hz.7 The total average power transmitted to the telephone-type circuit is normally dependent on the transmissioncharacteristics of the circuit as follows:a)
11、For circuits with characteristics not exceeding the limits (nominal values) given in Annex A toRecommendation R.35, the mean power per channel at a point of relative zero level should not be morethan 4.0 W (24.0 dBm0). The pilot channel, where employed, should have a level of not more than 2.0 W (27
12、0 dBm0);b) for other circuits, the mean power per channel at a point of relative zero level should not be more than10.8 W ( 19.7 dBm0). The pilot channel, where employed, should have a level of not more than 5.4 W(22.7 dBm0).8 In service, the levels of signals corresponding to continuous condition
13、Z and continuous condition A should notdiffer by more than 1.7 dB in the same channel. Both of these levels must lie between 1.7 dB with reference to thelevel in 7 above.9 The frequency for the transmitted condition corresponding to the condition A is the higher of the twocharacteristic frequencies
14、and that corresponding to the condition Z is the lower.10 In the absence of a channel-modulator control telegraph current, a frequency shall be transmitted that shall bewithin 10 Hz of the frequency normally transmitted for the condition A. It is not necessary for this transmission totake place imme
15、diately after the control current has been cut.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.37, w
16、hich 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 receivin
17、g equipment should have been restored to condition A when the receiving level has fallen to23.5 dB below the nominal level. The alarm-control level is left to the choice of each Administration.13 On delivery by the manufacturer of 100-baud FMVFT equipment, the following values must not be exceededfo
18、r the degree of distortion on a telegraph channel. These values correspond to closed circuit measurements, made withthe audio-frequency line terminals of the sending and receiving equipments connected together through an artificialline. Before the series of measurements taken in accordance with Reco
19、mmendation R.51, the levels are adjusted to theirnormal values, the mean frequencies are checked to see whether they are within:Fascicle VII.1 - Rec. R.37 31) for equipment without crystal control 3 Hz;2) for equipment with crystal control 0.1 Hz2),of their nominal value (see 3 above) and the differ
20、ence between the two characteristic frequencies is within thepermitted tolerance of less than 4 Hz (see 5 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 interchannelinterference is
21、to be included in the measurement. These “unrelated signals” can conveniently be 1:1 signals fromdifferent generators at approximately 100 bauds but not synchronous to each other or to the signal on the channel undertest._2)The tightening of this tolerance is for further study.4 Fascicle VII.1 - Rec
22、 R.37a) The transmission levels being normal, the artificial line introducing no frequency drift, but the measuredchannel being subject to fortuitous distortion due to interchannel interference: 5% for the degree ofinherent isochronous distortion.b) The level being maintained constant, but at a val
23、ue different from the normal level, for all constant levelsbetween 8.7 dB above the normal reception level and 17.4 dB below the normal reception level, the otherconditions being the original measurement conditions: 7% for the degree of inherent isochronousdistortion.c) In the presence of interferen
24、ce by a single sine-wave frequency equal first to one and then to the othercharacteristic frequency, with a level of 20 dB below the signal 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).
