1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 G0“TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.34!.$!2$):!4)/.G0G0/ amended at Geneva, 1972, 1976, 1980 and Malaga-Torremolinos, 1984)1 Frequency-modulated voice-frequency telegraph (FMVFT) systems, with a spacing of 36
2、0 Hz between themean frequencies, can accommodate seven channels. In the case of telephone bearer channels with 4-kHz spacing,channel position 8 can be used.2 The nominal modulation rate is fixed at 200 bauds.3 The nominal mean frequencies are 540 + (n 1) 360 Hz, n being the channel position number.
3、 The meanfrequency is defined as half the sum of the characteristic frequencies corresponding to the conditions A and Z. For thenumbering of channels that has been adopted in the international service see Recommendation R.70 bis.4 The mean frequencies at the sending end must not deviate by more than
4、 3 Hz from their nominal value.5 The difference between the two characteristic frequencies in the same channel is fixed at 180 Hz.6 The maximum tolerance on this difference should be 4 Hz.7 The unbalance due to the modulation process = 2 FFFFlAZ0should not exceed 2%,whereFAand FZare the two characte
5、ristic frequencies measured over a period of 10 s;F0is the mean static frequency measured FFAZ + 2;F1is the mean dynamic frequency measured with 1:1 rectangular signals during 10 s.Measurements should be made applying to the input of the transmitter 1:1 rectangular signals with the build-upand hango
6、ver time below 1 s and with the unbalance below 0.1%. 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:l signal generator and the input to the trans
7、mitter. Both forms of measurementsneed not necessarily 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 ca
8、lculate the mean frequency F0and the unbalance = 2 FFFFlAZ0.2 Fascicle VII.1 - Rec. R.38 BA 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
9、 during 10 s; = 2 = 4 FFFFFFFFlAZmAZ00or to subtract:()()FF F F F Flm AZ AZ= 1414 1.3 HzThe absolute value of the difference between the two frequencies measured, Fland Fm, must be less than1.3 Hz.8 The mean power per channel at relative zero level should not be more than 19.2 microwatts.9 In servic
10、e, the levels of the signals 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 8 above.10 The condition A frequency is the higher of the two character
11、istic frequencies, and the condition Z is the lowerone (see Recommendation V.1 1).11 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 tra
12、nsmission totake place immediately after the control current has been cut.12 The receiving equipment should operate satisfactorily when the receiving level falls to 17.4 dB below thenominal level. The receiving equipment should have been restored to condition A when the receiving level has fallen to
13、23.5 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/360-Hz FMVFT equipment, the following values must not beexceeded for the degree of distortion on a telegraph channel. These values correspond to closed
14、 circuit measurements,made with the audio-frequency line terminals of the sending and receiving equipments connected together through anartificial line. Before the series of measurements taken in accordance with Recommendation R.51, the levels areadjusted to their normal values, the mean frequencies
15、 are checked to see whether they are within 3 Hz of theirnominal value (see 4 above) and the difference between the two characteristic frequencies is within the permittedtolerance of less than 4 Hz (see 6 above). Bias distortion is eliminated by adjustment in the channel receivers. Theother channels
16、 of the system are modulated with unrelated signals when the effect of interchannel interference is to beincluded in the measurement. These “unrelated signals” can conveniently be 1:1 signals from different generators atapproximately 200 bauds but not synchronous to each other or to the signal on th
17、e channel under test.a) The transmission levels being normal, the artificial line introducing no frequency drift, but the measuredchannel being subject to fortuitous distortion due to interchannel interference: 6% for the degree ofinherent isochronous distortion.b) The level being maintained constan
18、t, 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 normal reception level, the otherconditions being the original measurement conditions: 8% for the degree of inherent isochronousdistortion.c) In the presenc
19、e 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 signal level, the other conditions for the start ofmeasurements being maintained: 15% for the degree of inherent isochronous distortion (i.e. total dist
20、ortionincluding the increase due to the interfering frequency, not distortion due to the interfering frequencyalone).Fascicle VII.1 - Rec. R.38 B 3d) By introducing a frequency drift (f Hz) of the signals during transmission through the artificial line,f being not more than 10; and the initial condi
21、tions of the test otherwise being preserved: (6 + 1.2 fHz)% for the degree of inherent isochronous distortion; the measurements shall be made after the transienteffects of changing frequency have ceased.14 Frequency drifts on modern telephone-type circuits are generally less than 2 Hz. Hence, it is
22、not necessary torecommend frequency drift control. For circuits on 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: compensation for ea
23、ch channel up to about 15 Hz; 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 ofsending the pilot and the choice of frequency. The frequency 300
24、Hz is recommended, with a tolerance of 1 Hz. The mean power emitted at the relative zero point on this frequency should not exceed thatrecommended for a telegraph channel in the case of a 24-channel group, i.e. 22.5 dBm0.15 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, Vol. VIII, Fascicle Vlll.1, Rec. V.1.
