ITU-T R 31-1988 STANDARDIZATION OF AMVFT SYSTEMS FOR A MODULATION RATE OF 50 BAUDS《调制速率为50Bd的调幅音频电报(AMVFT)系统的标准化》.pdf

1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.34!.$!2$):!4)/.G0G0/amended at Malaga-Torremolinos, 1984)The CCITT,unanimously declares the following view:1 It is advisable to adopt, for amplitude-modulated voice-frequency

2、telegraph (AMVFT) systems and for amodulation rate not exceeding 50 bauds, the series of frequencies formed by odd multiples of 60 Hz, the lowestfrequency being 420 Hz as shown in Table 1/R.31.TABLE 1/R.31ChannelpositionFrequencyHzChannelpositionFrequencyHz1234567891011124205406607809001020114012601

3、3801500162017401314151617181920212223241860198021002220234024602580270028202940306031802 This numbering is valid whatever use is made of the channel (e.g. traffic channel, pilot channel, etc.) or themethod employed to obtain the line frequencies, e.g. by group modulation. For the numbering of channe

4、ls that has beenadopted in the international service see Recommendation R.70 bis.3 In the case of systems on telephone-type circuits with a spacing of 3-kHz operating in accordance with thestandardized frequency series, channel positions 23 and 24 cannot be used.4 The frequencies applied to the tele

5、phone-type circuit that is used as the voice-frequency telegraph bearer circuitshould not deviate by more than 6 Hz from the nominal value when the telegraph channels supplied are operating overa telephone-type circuit composed exclusively of audio-frequency sections, and not more than 3 Hz in other

6、 cases.5 The power levels of carrier waves transmitted on the line and measured successively in as short a period aspossible should not differ from one another by more than 1.74 dB when they are operating on a constant impedance.2 Fascicle VII.1 - Rec. R.316 The power of each of the carrier waves tr

7、ansmitted on the line should not vary in operation by more than 0.87 dB when it operates on a constant impedance.7 The amplitude of the signals transmitted by a channel modulator during a transition from condition A tocondition Z should remain within the tolerances of Figure 1/R.31 in which the valu

8、es t0and y2and ylare fixed asfollows:t0= 11 ms,yl= 95%,y2= 110%.8 Receivers with rapid-action level correction should not be so sensitive to secondary pulses following the signalpulse provided that the amplitude of the signal emitted does not exceed the reference level by more than 10% and thatthe r

9、eference level does not exceed the normal level by 10.4 dB. (This provision applies only to new systems.)9 If 1:1 reversals at frequency fpcorresponding to the modulation rate are sent over a channel with mid-frequencyF0, the voltage at frequency F0 3 fp, must not exceed 3% of the nominal voltage at

10、 frequency F0and the voltage at thefrequencies F0 5 fpmust not exceed 0.4% of the nominal voltage at frequency F0.Note These tolerances will be required only for future systems. Administrations should try as far as possibleto use systems satisfying these tolerances on international relations.10 The

11、unbalance of the emitted signal should not be greater than 4% (methods of measuring this unbalance aredescribed in 1 and 2). This tolerance takes account of the limit in 11 below for new systems.11 For new systems, the static relay should introduce a difference of not less than 45 dB between the two

12、signalling conditions. (For existing systems the limit is 30 dB.)12 In the event of failure of the control current in the sending static relay, the attenuation of the residual signalrelative to this nominal level should be at least 27 dB. This attenuation of the signal need not occur immediately on

13、thefailure of the control current.Fascicle VII.1 - Rec. R.31 313 Systems should be able to tolerate slow level variations of at least 6 dB. Administrations should equipsystems that are unable to tolerate such variations with a common amplifier to enable them to tolerate variations of atleast 6 dB.14

14、 The permissible limit for the power of the telegraph signal on each telegraph channel when a continuous tone isbeing transmitted is given in Table 2/R.31.TABLE 2/R.31Normal limits (nominal values) for the power per telegraph channel in AMVFT systemsNumber of telegraph channels in the AMVFT systemAl

15、lowable power per telegraph channelat a point of zero relative levelwhen sending a signal corresponding tocontinuous Z conditionmicrowattsdecibels12 or less35 14.51815 18.3249 20.45Note These limits are such that the maximum instantaneous voltage will exceed that of a sinusoidal voltage with a power

16、 of5 milliwatts at a point of zero relative level. This is the maximum permissible for voice-frequency circuits.15 Audio-frequency is transmitted to line when stop polarity (condition Z) is sent.16 When a signal, whose frequency is equal to the nominal frequency of the channel and whose level is 18.

17、3 dBbelow the normal signal level of the channel, is applied to the detector of a 24-channel AMVFT system, the receivingrelay should not respond.17 It must be possible to subject any channel to a test without withdrawing from service a channel other than thereturn channel of the circuit planned.18 I

18、n graded harmonic frequency telegraphy, it is desirable that the same frequencies be used separately forcircuits established on different successive sections of a 4-wire circuit.19 In graded harmonic frequency telegraphy, the attenuation of the filters that pass a group of frequencies must, inthe su

19、ppressed frequency band, be higher by at least 35 dB than that shown in the transmission band.20 In graded harmonic frequency telegraphy, in order to facilitate local tests, the frequencies used forcommunications set up between two international offices in one direction should also be used in the op

20、posite direction,if possible.4 Fascicle VII.1 - Rec. R.31References1 Measuring method to determine the asymmetry of an amplitude-modulated telegraph signal, Blue Book,Vol. VII, Supplement No. 11, ITU, Geneva, 1964.2 The measurement of the distortion produced in the sending terminal equipment of an A.M.-V.F. telegraphsystem, Blue Book, Vol. VII, Supplement No. 12, ITU, Geneva, 1964.