📄 7_3_01.txt
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Note - The maximum dimensions of the pictures to be transmitted
result from the parameters given in the table.
Tableau 1/T.1 [T1.1], p.
4 Reproduction ratio
In the case where apparatus working with different lengths of
scanning line (but with the same index of cooperation) are inter-
connected, there will be a slight change in size and the reproduc-
tion will bear the same proportion as the original, the ratio being
that of the total lengths of the scanning lines.
5 Drum rotation speed - scanning line frequency
5.1 Table 2/T.1 gives the normal and approved alternative com-
binations of drum rotation speeds or of scanning line frequencies
and indices of cooperation.
H.T. [T2.1]
TABLE 2/T.1
______________________________________________________________________________________________
Metallic circuits {
{
______________________________________________________________________________________________
Normal conditions 60 90 352 352 264
______________________________________________________________________________________________
{
Alternatives for use when the phototelegraph
apparatus and metallic circuits are suitable
} 90 120 150 {
264 and 352
264 and 352
264
}
______________________________________________________________________________________________
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Note 1 - In the case of transmitters operating on metallic cir-
cuits, the index 264 is not intended to be used with an 88-mm drum.
In the case of transmitters operating on combined metallic and
radio circuits, the index 264 associated with a drum diameter
of 88-mm is intended to be used only exceptionally.
Note 2 - The provisions given in the table are not intended to
require the imposition of such standards on users who use their own
equipment for the transmission of pictures over leased circuits.
However, the characteristics of the apparatus used should be compa-
tible with the characteristics of the circuits used.
Tableau 2/T.1 [T2.1], p.
5.2 The speed of transmitters must be maintained as nearly as
possible to the nominal speed and in any case within _ | 0 parts
in 106 of the nominal speed. The speed of receivers must be adju-
stable and the range of adjustment should be at least _ | 0 parts
in 106 from the nominal speed. After regulation, the speeds of the
transmitting and receiving sets should not differ by more than
10 parts in 106.
6 Judder
The stability of the speed during one rotation should be such
that the maximum shift of the drum surface from the average posi-
tion should not exceed one quarter of the scanning pitch P at nor-
mal index 352, which means that the maximum angle of the oscilla-
tions should not exceed 0.08 degree measured from the average posi-
tion.
7 Synchronization
When phototelegraph stations have available a standard of fre-
quency which is better than _ | parts in 106, verification of the
synchronism between the two stations may be dispensed with. In view
of the saving of time, this method should be adopted wherever pos-
sible.
To compare the speeds of a transmitter and a receiver, an
alternating current whose frequency bears an unvarying relationship
to the transmitter speed and has a nominal value of 1020 Hz is
used.
Where there is the possibility that the transmitter and
receiver may be connected by a circuit liable to introduce fre-
quency drifts, for example, by a carrier telephone circuit, the use
of the simple 1020-Hz synchronizing tone is unsatisfactory. The
preferred method of overcoming this difficulty is to transmit the
phototelegraph carrier (of about 1900 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 the normal manner.
8 Phasing
Phasing is performed after the speeds of the transmitter and
receiver drums have been equalized.
For phasing purposes, the transmitter sends a series of alter-
nating white and black signals in such a way that the black
lasts 95% and the white 5% of the total scanning line period
(admissible tolerance: _ | .5% of the total duration of a scanning
line). The apparatus must be so adjusted that the pulses
corresponding to the signal for white are transmitted:
- 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 ).
(Tolerance 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 the middle of the
"lost time" (tolerance admitted: __R = _ 2% of a "total scanning
line length").
Note - These tolerances allow for the fact that the restitu-
tion of the original may deviate from its nominal position by 3% of
a "total scanning line length", when the sending and receiving sta-
tions are operating with the maximum authorized drift in the same
direction.
9 Contrast
The transmitter must transmit the original document without
changing the contrast of the tone scales of the picture to be
transmitted.
10 Modulation and demodulation equipment
10.1 Amplitude modulation
Phototelegraph equipment shall normally provide for transmis-
sion and reception of an amplitude modulated audio-frequency car-
rier, which 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 that the
ratio of nominal white signal to nominal black signal should be
approximately 30 decibels.
To simplify multi-destination operation and AM/FM conversion
for radio operation it is desirable that the amplitude of the
transmitted signal should vary linearly with the photocell voltage
and that no corrections for tone scale should be made at the photo-
telegraph transmitting station.
For audio-frequency telephone 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-current frequency
of about 1900 Hz is recommended.
10.2 Frequency modulation
Preferably 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 characteristics of the frequency-modulated
output should be: mean frequency 1900 Hz
white frequency 1500 Hz
black frequency 2300 Hz
The deviation of frequency should vary linearly with photocell
voltage or, in the case of conversion from amplitude modulation to
frequency modulation, 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 vary by 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 operating correctly
when the drift of black and white frequencies received does not
exceed their nominal value by more than _ | 2 Hz.
Note - It is recognized that there are difficulties operating
with these frequency limits on the public switched telephone net-
work (PSTN) where certain types of signalling equipment are used.
By prior agreement between users on the PSTN, alternative frequen-
cies of 1300 Hz for white and 2100 Hz for black may be used.
11 Positive or negative reception
Selection of positive or negative reception should be made by
adjustment at the receiver. The adaptation of the transmitted sig-
nals to the characteristics of the photographic materials must also
be effected at the receiving end according to the type of reproduc-
tion, negative or positive.
12 Colour transmission (optional)
12.1 Phototelegraphy apparatus constructed in accordance with
this Recommendation can be used in colour phototelegraphy by split-
ting the spectrum of light reflected from the picture elements into
three basic colours and transmitting the three resulting signals
sequentially. Then each signal can be treated and transmitted as a
phototelegraphy signal as specified in this Recommendation above.
12.2 The splitting of light reflected from picture element
into three spectral components should be performed simultaneously.
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 in range of 575-700 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 fourth components
(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 the order 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.
Recommendation T.2
STANDARDIZATION OF GROUP 1 FACSIMILE APPARATUS
FOR DOCUMENT TRANSMISSION
(Mar del Plata, 1968; amended at Geneva, 1972 and 1976)
The CCITT,
considering
(a) that there is a requirement for Group 1 facsimile
apparatus which enables an ISO A4 document to be transmitted over a
telephone-type circuit in approximately six minutes;
(b) that document facsimile transmission may be requested
alternately with telephone conversation or when either or both sta-
tions are unattended; in both cases the facsimile operation should
conform to Recommendation T.30,
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