Note: Descriptions are shown in the official language in which they were submitted.
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B~CKGROUND OF T~E IN~,IENTION
The present invention relates to facsimile com-
munication and more specifically to a variable speed
facsimile transmitter which employs a storage mode photo-
detection array as a means of deriving a video line signal
and method of operatin~ the facsimile transmitter employ-
ing such photodetection arrays.
The present invention incorporates self-scanning
photodetection arrays as a means of sensing visual
information recorded on a printed document to be trans-
mitted over long distances. The photodetection arraysutilize solid state nhotosensitive devices which operate
in a storaqe mode. mese devices, when operated in the
storage mode with a junc-tion reverse biased, have the
characteristics of a capacitor. When the junction is
open circuited the junction slowly discharges as elec-trons
and holes are generated thermally and neutralize the
stored charge on each side of the junction. With the
application of light to the junction the discharge of
the junction occurs much more rapidly and hence the
junction may be used to sense light. Typically, the
junction is recharged periodically and the recharging
current is sensed; this current is a function of the
total incident liqht on the junction, i.e. the time
integral of the incident light. In many pattern recogni-tion
applications which utilize an array of photodiodes in
a storage mode of operation, an elonga-ted row of the
photosensitive device is scanned electrically. Typi-
cally, the electrical circuit utilized to scan the
photosensitive devices is incorporated on the same
semiconductor substrate or chip as the photosensitive
devices. The photodetection array incorporated in the
present invention is commercially available from Reticon
Corporation, U. S. A.
Because of the storage mode of operation with the
magnitude of the available output varying as a function
of the time integral of the quantity of incident light,
and because of the possible difference between the
scanning speed of the photosensitive device and the
speed at which an electronic circuit processes the
video signal into a format suitable for transmission,
it is necessary to drive the photosensitive device such
that a same line path of a printed document is scanned
repeatedly until the processor is ready to process the
output of the photosensitive device. Otherwise, the
photosensitive device would reach the saturation level
upon prolonged exposure to the sensed line. This re-
quires that the photosensitive device be repeatedly
discharged at intervals.
In practice, facsimile communication systems are
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designed to operate.on a number of transmission speeds
to meet the user's requirements. ~here the aforesaid
photodetection array is employed as a means of scanning
documents, the amplitude of the scanner output will vary
depending on the transmission speed.
To keep the video.output constant over the differing
transmission speeds, the prior art system, disclosed in
Japanese Patent Application 51-105221, laid open to public
inspection on September 17, 1976, discharges the photo-
detection array at a constant frequency which is a common
multiple of the different scanning frequencies of the
system. For example,assume the system have 3Hz and 5Hz
scanning frequencies, the photodetector is discharged always
at 15Hz, regardless of the selected scanning frequencv.
However, since the amplitude of the photodetector
output decreases inversely with the discharge frequency,
the video output of the prior art system is of small
amplitude, which would require amplification of the signal
or high performance optical system to reduce the loss of
incident light.
SU~RY OF THE INVENTION
In a facsimile transmitter which employs a storage
mode photodetection array and drives it at one of a
plurality of manually selectable clock rates, the present
invention contemplates to discharge the photodetection
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array at an instant of time which is delayed from the time of
occurrence of the clock pulse by an interval responsive to the
difference between the selected clock rate and the highest clock
rate of the system so that the video signal available at that
instant is discarded. Upon the discharge of the photodetection
array, the array is allowed to develop charges for a constant
time interval until the occurrence of a subsequent clock pulse,
whereupon the array is energized to generate a video line signal
having a constant amplitude regardless of the selected clock rate.
Specifically, the interval is determined by counti`ng a unit time
period which is equal to reciprocal of a common multiple of the
selectable clock rates, and the discharge is effected when the
count reaches a number determined by the difference between the
selected clock rate and the highest clock rate.
It is an object of the invention to generate a video
line signal of constant signal level from a storage mode photo-
detection array driven at variable clock rates regardless of
the selected clock rate.
To achieve this object, a variable speed facsimile
transmitter in accordance with the invention includes means for
generating synchronization pulses at one of a plurality of
manualiy selectable rates, a storage mode photodetection array
and means for driving the photodetection array in response to
each of the sync pulses, comprising means for discharging the
photodetection array at an instant delayed from the time of
occurrence of the sync pulse by an interval in dependence on
the difference between the selected synchronization rate and
the highest one of the sync rates so that the photodetection
array is allowed to develop charges during a constant interval
regardless of the sync rate so selected.
The invention provides also a method for operating
a variable speed facsimile transmitter having a plurality of
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selectable llne scanning frequenc:ies to provide dlfferent
transmisslon speeds and a storage mode photodetection array
adapted to generate a_video line signal, comprlslng the steps of:
a) generatlng sync pulses at a frequency corres-
ponding to a selected one of the scannlng frequencles;
b) determining the difference between the period
of the selected frequency and the period of the highest one of
the scanning frequencies;
c) causing the photodetection array to discharge
its stored electrical energy at an instant delayed from the
time of occurrence of the sync pulse by an interval detexmined
by the difference in period of the scanning frequencies when
the selected frequency is other than the highest freqùency;
d) allowing the photodetection array to charge
electrical energy in response to the end of the time interva~
when the selected frequency is other than the highest frequency,
or allowing the photodetection array to charge electrical
energy in response to the sync pulse when the highest frequency
is selected; and
e) driving the photodetection array to generate
the video line signal in response to each of the sync pulses.
, The invention will be further described with
reference to the accompanying drawings,in which:
Fig. 1 is a schematic block diagram of an
embodiment of the invention; and - ~~~---~-~- ~-~ ~ ~~~~- -~~~ -- -
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Fig. 2 is a timing diagram useful for describinq
the operation of Fig. 1.
DETAILED DESCRIPTION
The facsimile transmitter of the present invention
represented in Fig. 1 of the drawings includes a 2,048-
bit storage mode photodiode array or detector 10 which
is shown connected to a starting circuit 12 which ener-
gizes the detector to receive drive pulses from a
source (not shown) of drive clock (Fig. 2~A) through
a gate 14. The rate of this clock is so selected that
the photodetector 10 completes its delivery of a line
video output within the period of a synchronization pulse
(Fig. 2C) which appears at the start of each line scan.
The sync pulse is applied to an AND gate 16 to enable
it to pass a discharge clock pulse (Fig. 2D) used to
discharge the photodiode array to reset it to the initial
charge level. A 2,048-bit counter 20 is shown connected
to the-output of the gate 14 to count the number of clock
pulses supplied to the photodetector 10. Therefore,
the flip-flop 18 will be reset in response to a pulse
count of 2,048 bits by the output of the counter 20 to
terminate the supply of clock pulses to the photodetector
10 (Figs. 2E, 2F and 2G).
A video output is thus delivered from the photo-
detector 10 and supplied to a first input of a comparator
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22 for comparison with a reference voltage to eliminate
any ambiguous level signal components so that the com-
parator output is a clear-cut, black-and-white signal.
This signal is clocked into a shift register 24 by the
clock pulses passed through gate 14 and an OR gate 26.
A flip-flop 28 is shown connected to the output of
counter 20 which triggers flip-flop 28 into a set con-
ditlon upon the count of 2,048 pulses of drive clock. A
gate 30 is shown connected to the Q output of the flip-
clop 28 to pass discharge clock pulses on lead 32 to the
photodetector 10 in the presence of the high ~ output
state of flip-flop 28. The output circuit of the gate
30 is connected to an input of a programmable counter
34 which counts a predetermined number of input pulses
to reset the flip-flop 28. The programmable counter 34
iS 90 conditioned by means of a circuit designated by
numeral 36 as to count as many discharge pulses to be
applied to the photodetector 10 as necessary depending
on the scanning frequency selected.
Assume that the system is capable of transmitting
signals at one of a plurality of selectable transmisslon
speeds which correspond to line scanning frequencies Fl,
F2, ...... Fn where Fn is the maximum frequency.
~ach line path of document is transmitted at one of
intervals Tl, T2 ...... Tn which correspond to the cycle
period of the frequencies Fl, F2 ..... Fn, respectively.
Therefore, Tn is the minimum transmission interval of
the system.
Consider now the minimum time interval necessary
for the photodetector 10 to produce a video output of
acceptable amplitude level, and let us designate this
minimum interval Tmin. This minimum interval Tmin may
be equal to or smaller than the transmission interval
Tn of the system and the following relation can hold:
min M + N ............................. (1)
where, M is an integer including zero and N is an integer
at least unity.
Let Fcm to denote a frequency which is a common
multiple of the frequencies Fl, F2 .... Fn, and let Fi
to denote any one of the frequencies Fl through Fn.
In the present invention, Fcm is equal to the frequency
at which the photodetector 10 is discharged, i.e. dis-
charge clock rate is equal to Fcm.
Therefore, the number of discharging operations
that is required of the photodetector 10 before it starts
to generate video signals is determined by the following
Equation:
K = Fi ~ M + N x Fn ...................... (2)
For purposes of illustration, the system is assumed
to have 3Hz and 5Hz scanning frequenciy modes so that
Fcm is at 15Hz, i.e. the discharge clock rate is at 15Hz.
Assume, for the sake of simplicity, that ~1 = 0, and that
the system is operating in the 3Hz mode, it will be
appreciated that the photodetector 10 is discharged "twice"
by the output from the gate 30.
The circuit 36 determines the K value that is used
to program the counter 34 so that "two" discharge clock
pulses may be passed through the gate 30 to the photo-
detector 10. The circuit 36 is shown as comprising a
division circuit 44 which performs division of an electrial
value representing the frequency Fcm by another electrical
value representing a selected frequency Fi. A second
divisi~n circuit 46 provides division of an electrical
quantity representing Fcm by another quantity represent-
ing Fn. The output of the division circuit 46 is multi-
plied in a multiplier 48 by a quantity representing
N/(r~ + N) provided from a source 50. me output from
the multiplier 48 is used to subtract from the output
of the division circuit 44 by means of a subtractor 52
to generate a subtraction output which corresponds to
the K value of Equation (2).
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The programmable counter 34 is thus instructed to
eount two discharge pulses supplied from the output of
gate 30 and reset the flip-flop 28 to terminate the
supply of discharge clock to the photodetector 10(see
Figs. 2I, 2J and 2K).
In response to the absence of synchronization
pulses, gates 38 and 40 are activated to pass transmission
eloek pulses to the shift register 24 via OR gate 26
and clock out the video information now stored in the
shift register via gate 40 to a utilization circuit
(not shown) where the video signal will be proeessed
into a form suitable for transmission.
If the system is switehed to the SHz scanning mode,
the programming circuit 36 generates a signal whieh
instruets the counter 34 to immediately reset the fLip-
flop 28 and inhibit it from being set in response to
the output from the 2,048-bit eounter 20. Therefore,
no diseharging o~erations oeeur. With the system being
switehed to the 5Hz mode, the timinq siqnal will be so
automatieall~ adjusted that the synehronization pulses
oeeur at intervals smaller than for the 3Hz mode. The
photodeteetor 10 thus operates to aeeumulate eharge in
response to each synehronization pulse.
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