1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 4 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%2-).!,G0G0%15)0-%.4G0G0!.$G0G002/4/#/,3G0G0 amended atNew Delhi, 1960; Geneva, 1964; Mar del Plata, 1968;Malaga-Torremolinos, 1984 and Melbourne, 1988)The CCITT,consideringthat the transmission of pictures
2、is possible only if certain characteristics of the transmitting and receivingequipments are identical,unanimously declares the viewthat phototelegraph apparatus and the associated modulating and demodulating equipment should beconstructed and employed according to the following standards:1 Scanning
3、trackAt the transmitting apparatus the message area should be scanned in a “negative” direction. The orientation ofthe document in relation to the scanning plane will depend upon its dimensions and is of no consequence.At the receiving apparatus scanning takes place in a “negative” direction for “po
4、sitive” reception and in a“positive” direction for “negative” reception.2 Index of cooperationThe normal index is 352 (corresponds to a factor of cooperation of 1105).The preferred alternative index, for use when less dense scanning is required, or when the characteristics ofcircuits (and particular
5、ly combined radio and metallic circuits) so demand, is 264 (a factor of cooperation of 829).The admissible tolerances on the above-mentioned values are 1%.3 Dimensions of apparatus3.1 Apparatus with drum scanningThe most currently used drum diameters are 66, 70 and 88 mm.The drum factor of the sendi
6、ng apparatus shall not be more than 2.4.The drum factor of the receiving apparatus shall not be less than 2.4.The width of the picture-retaining device (dead sector) may not exceed 15 mm. An allowance of 3% of the totallength of a scanning line is also made for phasing. Thus, since the total circumf
7、erence of a drum of the diameter of66 mm is about 207 mm, the usable circumference will be at least 186 mm.2 Fascicle VII.3 - Rec. T.13.2 Apparatus with flat-bed scanningThe total lengths of the most current scanning lines are 207, 220 and 276 mm of which 15 mm are not used foreffective transmission
8、, because of the possibility that the receiving station may use a drum apparatus.Before transmitting a picture to a receiving station using a drum apparatus, it is necessary to ensure that thevalue of ratio:length of document to be transmitted1) piError! Bookmark not defined.total length of a scanni
9、ng lineis less or at most equal to the drum factor of the receiver used.3.3 Table 1/T.1 gives corresponding values of index of cooperation M, factor of cooperation C, drum diameter D,total length of scanning line L, scanning pitch P and scanning density F for apparatus in most common use.TABLE 1/T.1
10、MCD (mm)L (mm) P (mm) F (lines/mm)2642642643503523528298298291099110511056670887066882072202762202072761/41/3.771/31/53/161/443.773516/34Note The maximum dimensions of the pictures to be transmitted result from the parameters given in the table.4 Reproduction ratioIn the case where apparatus working
11、 with different lengths of scanning line (but with the same index ofcooperation) are interconnected, there will be a slight change in size and the reproduction will bear the same proportionas the original, the ratio being that of the total lengths of the scanning lines.5 Drum rotation speed scanning
12、 line frequency5.1 Table 2/T.1 gives the normal and approved alternative combinations of drum rotation speeds or of scanning linefrequencies and indices of cooperation._1)Measured in the direction perpendicular to the scanning line.Fascicle VII.3 - Rec. T.1 3TABLE 2/T.1Drum rotation speedin rpm or s
13、canningIndex of cooperationline frequencyMetalliccircuitsCombined metallicand radio circuitsNormal conditions 6090352 352264Alternatives for use when thephototelegraph apparatus and metalliccircuits are suitable90120150264 and 352264 and 352264Note 1 In the case of transmitters operating on metallic
14、 circuits, the index 264 is not intended to be used with an 88-mm drum. Inthe case of transmitters operating on combined metallic and radio circuits, the index 264 associated with a drum diameter of 88-mmis intended to be used only exceptionally.Note 2 The provisions given in the table are not inten
15、ded to require the imposition of such standards on users who use their ownequipment for the transmission of pictures over leased circuits. However, the characteristics of the apparatus used should becompatible with the characteristics of the circuits used.5.2 The speed of transmitters must be mainta
16、ined as nearly as possible to the nominal speed and in any case within 10 parts in 106of the nominal speed. The speed of receivers must be adjustable and the range of adjustment shouldbe at least 30 parts in 106from the nominal speed. After regulation, the speeds of the transmitting and receiving se
17、tsshould not differ by more than 10 parts in 106.6 JudderThe stability of the speed during one rotation should be such that the maximum shift of the drum surface fromthe average position should not exceed one quarter of the scanning pitch P at normal index 352, which means that themaximum angle of t
18、he oscillations should not exceed 0.08 degree measured from the average position.7 SynchronizationWhen phototelegraph stations have available a standard of frequency which is better than 5 parts in 106,verification of the synchronism between the two stations may be dispensed with. In view of the sav
19、ing of time, thismethod should be adopted wherever possible.To compare the speeds of a transmitter and a receiver, an alternating current whose frequency bears anunvarying relationship to the transmitter speed and has a nominal value of 1020 Hz is used.Where there is the possibility that the transmi
20、tter and receiver may be connected by a circuit liable to introducefrequency drifts, for example, by a carrier telephone circuit, the use of the simple 1020-Hz synchronizing tone isunsatisfactory. The preferred method of overcoming this difficulty is to transmit the phototelegraph carrier (of about1
21、900 Hz) modulated by the 1020-Hz synchronizing tone.At the receiving end, the 1020-Hz synchronizing frequency is restored by detection and can then be used in thenormal manner.4 Fascicle VII.3 - Rec. T.18 PhasingPhasing is performed after the speeds of the transmitter and receiver drums have been eq
22、ualized.For phasing purposes, the transmitter sends a series of alternating white and black signals in such a way that theblack lasts 95% and the white 5% of the total scanning line period (admissible tolerance: 0.5% of the total duration ofa scanning line). The apparatus must be so adjusted that th
23、e pulses corresponding to the signal for white aretransmitted: during scanning of the “dead sector”, when drum apparatus is used, during “lost time”, when flat-bed apparatus is used,and that they are placed at the middle of the dead sector (or of the interval corresponding to the lost time).(Toleran
24、ce admitted in the position of the “white“ pulses: E= 1% of a “total scanning line length”.)At the receiving station, phasing signals are used to start the apparatus so that short white pulses occur in themiddle of the “lost time“ (tolerance admitted: R= 2% of a “total scanning line length“).Note Th
25、ese tolerances allow for the fact that the restitution of the original may deviate from its nominalposition by 3% of a “total scanning line length”, when the sending and receiving stations are operating with themaximum authorized drift in the same direction.9 ContrastThe transmitter must transmit th
26、e original document without changing the contrast of the tone scales of thepicture to be transmitted.10 Modulation and demodulation equipment10.1 Amplitude modulationPhototelegraph equipment shall normally provide for transmission and reception of an amplitude-modulatedaudio-frequency carrier, which
27、 is the normal mode of transmission for international metallic circuits.The level of the output signal of the transmitter shall be greatest for white and least for black. It is desirable thatthe ratio of nominal white signal to nominal black signal should be approximately 30 decibels.To simplify mul
28、ti-destination operation and AM/FM conversion for radio operation it is desirable that theamplitude of the transmitted signal should vary linearly with the photocell voltage and that no corrections for tone scaleshould be made at the phototelegraph transmitting station.For audio-frequency telephone
29、circuits, the frequency of the picture carrier-current is fixed at about 1300 Hz.This frequency gives the least delay distortion on lightly loaded underground cables.In the case of carrier telephone circuits providing a transmission band from 300 to 3400 Hz, a carrier-currentfrequency of about 1900
30、Hz is recommended.10.2 Frequency modulationPreferably phototelegraph apparatus should also provide for transmission and reception of a frequency-modulated audio-frequency carrier for use when necessary:a) on combined metallic and radio circuits;b) on wholly metallic circuits.In such a case, the char
31、acteristics of the frequency-modulated output should be:mean frequency 1900 Hzwhite frequency 1500 Hzblack frequency. 2300 HzFascicle VII.3 - Rec. T.1 5The deviation of frequency should vary linearly with photocell voltage or, in the case of conversion fromamplitude modulation to frequency modulatio
32、n, with the amplitude of the amplitude-modulated carrier.The stability of the transmission must be such that the frequency corresponding to a given tone does not varyby more than 8 Hz in a period of 1 second and by more than 16 Hz in a period of 15 minutes.The receiving apparatus must be capable of
33、operating correctly when the drift of black and white frequenciesreceived does not exceed their nominal value by more than 32 Hz.Note - It is recognized that there are difficulties operating with these frequency limits on the public switchedtelephone network (PSTN) where certain types of signalling
34、equipment are used. By prior agreement between users onthe PSTN, alternative frequencies of 1300 Hz for white and 2100 Hz for black may be used.11 Positive or negative receptionSelection of positive or negative reception should be made by adjustment at the receiver. The adaptation of thetransmitted
35、signals to the characteristics of the photographic materials must also be effected at the receiving endaccording to the type of reproduction, negative or positive.12 Colour transmission (optional)12.1 Phototelegraphy apparatus constructed in accordance with this Recommendation can be used in colourp
36、hototelegraphy by splitting the spectrum of light reflected from the picture elements into three basic colours andtransmitting the three resulting signals sequentially. Then each signal can be treated and transmitted as aphototelegraphy signal as specified in this Recommendation above.12.2 The split
37、ting of light reflected from picture element into three spectral components should be performedsimultaneously. Thus synchro and phase coincidence and electronic colour correction can be achieved.12.3 The triad RGB (red, green, blue) shall be used as a basis of main colours. The red colour shall be i
38、n range of575-700 nm, green 485-575 nm, blue 400-485 nm.Note For the high quality reproduction of art images by means of graphic facilities, transmission of fourthcomponents (i.e. black overtone) is desirable.12.4 The order of signal transmission shall be as follows: red, green, blue. In the case of negative reproduction theorder of colour separated signals transmission is reversed.12.5 The speeds of the transmitting and receiving sets should not differ by more than 1 part in 107.
