1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 G0!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 1 This is the standardized system for op
2、eration at 200 bauds.Note 2 In this Recommendation 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
3、 recommended.1 The nominal modulation rate is fixed at 200 bauds.2 The nominal mean-frequencies are 600 + (n 1) 480 Hz, n being the channel position number. The meanfrequency is defined as half the sum of the characteristic frequencies corresponding to conditions A and Z. For thenumbering of channel
4、s 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 without crystal control 4 Hz;b) for equipment with crystal control 0.8 Hz1).4 The difference between th
5、e two characteristic frequencies in the same channel is fixed at 240 Hz.5 The maximum tolerance on this difference should be 6 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 t
6、he mean static frequency measured + FFAZ2;Flis the mean dynamic frequency measured with 1:1 rectangular signals during 10 signals.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%
7、. In the event that in service the transmitter iscontrolled 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 necessari
8、ly be included in the maintenance procedure but should 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
9、 FFFFlAZ_1)The tightening of this tolerance is for further study.2 Fascicle VII.1 - Rec. R.38 AA 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 si
10、gnals during 10 s: = 2 0 FFFFlAZ= 40 FFFFmAZor to subtract:()()FF F F F Flm AZ AZ= 1414 1.8 HzThe absolute value of the difference between the two frequencies measured, Fland Fmmust be less than1.8 Hz.7 The total average power transmitted to the telephone-type circuit is normally dependent on the tr
11、ansmissioncharacteristics of the circuit as follows:a) For circuits with characteristics not exceeding the limits given in Annex A to Recommendation R.35, themean power per channel at a point of relative zero level should not be more than 8.0 W (21.0 dBm0).The pilot channel, where employed, should h
12、ave a level of not more than 2.0 W (27.0 dBm0).b) For other circuits, the mean power per channel at a point of relative zero level should not be more than21.6 W (16.7 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 the signa
13、ls corresponding to continuous condition Z and 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 in 7 above.9 The condition A frequency is the higher of the two characteristic frequencies, and the
14、condition Z frequency isthe lower one (see Recommendation V.1 1).10 In the absence of a channel-modulator control telegraph current, a frequency shall be transmitted that shall bewithin 20 Hz of the frequency normally transmitted for the condition A. It is not necessary for this transmission totake
15、place immediately 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 2fp(fp= frequency of modulation) should be in accordance with thelimits specified in Figure 1/R
16、.37, 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 re
17、ceiving 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 200-baud/480-Hz frequency-modulated voice-frequency telegraph(F
18、MVFT) equipment, the following values must not be exceeded for the degree of distortion on a telegraph channel.These values correspond to closed circuit measurements, made with the audio-frequency line terminals of the sendingand receiving equipments connected together through an artificial line. Be
19、fore the series of measurements taken inaccordance with Recommendation R.51, the levels are adjusted to their normal values, the mean frequencies arechecked to see whether they are within:1) for equipment without crystal control 4 Hz;2) for equipment with crystal control 0.8 Hz,Fascicle VII.1 - Rec.
20、 R.38 A 3of their nominal value (see 3 above) and the difference between the two characteristic frequencies is within thepermitted tolerance of less than 6 Hz (see 5 above). Bias distortion is eliminated by adjustment in the channelreceivers. The other channels of the system are modulated with unrel
21、ated signals when the effect of interchannelinterference is to be included in the measurement. These “unrelated signals” can conveniently be 1:1 signals fromdifferent generators at approximately 200 bauds but not synchronous to each other or to the signal on the channel undertest.a) The transmission
22、 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 value different from the nor
23、mal 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 interference by a single sine-wave
24、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: 10% for the degree of inherent isochronous distortion (i.e. total distortionincluding the increase due to the
25、interfering frequency, not distortion due to the interfering frequencyalone).d) By introducing a frequency drift (f Hz) of the signals during transmission through the artificial line,f in Hz being not more than 10, and the initial conditions of the test otherwise being preserved:(5 + 0.7 f Hz)% for
26、the degree of inherent isochronous distortion; the measurements shall be made afterthe transient effects of changing frequency have ceased.e) Equipment with crystal control, with any climatic conditions specified for the tested equipment, the initialcondition of the test otherwise being preserved: 8
27、% for the degree of inherent isochronous distortion. Thebias distortion caused by changes of climatic conditions should not be eliminated.14 Frequency drifts on modern telephone-type circuits are generally less than 2 Hz. Hence it is not necessary torecommend frequency drift control. For circuits on
28、 which a maximum frequency drift of not greater than 2 Hz cannotbe guaranteed, and on which the distortion resulting from the frequency drift is not acceptable, compensation seemsnecessary. Two methods can be used:a) compensation for each channel up to about 15 Hz;b) compensation for all the channel
29、s 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 ofsending the pilot and the choice of frequency. The frequencies 3300 Hz or, preferably, 300 Hz arerecommended for this pilot, wi
30、th a tolerance of:1) for equipment without crystal control 1 Hz2) for equipment with crystal control 0.2 Hz.The mean power emitted at the relative zero point on this frequency should not exceed 27.0 dBm0 or 22.7 dBm0 as appropriate (see 7 and Tables 1/R.35 and 2/R.35 in Recommendation R.35, which ar
31、e alsoapplicable to equipment to this Recommendation).15 The number of significant conditions of the modulation is fixed at two; this number may be increased, ifnecessary, by agreement between the Administrations concerned.Reference1 CCITT Recommendation Equivalence between binary notation symbols and the significant conditions of a two-condition code, Rec. V.1.
