Device for long-distance transmission of images

A device for long-distance transmission of images includes a transmitter and a receiver, connected by a communications line. The transmitter contains a device which scans a picture in a linear pattern to form an electric signal modulated to represent the brightness of the picture in picture points along the scanned lines. The receiver has a picture reproduction device and a driver for moving the reproduction device relative to a picture-supporting surface in a linear pattern corresponding to the linear scanning pattern of the transmitter. Upon receiving this signal the picture points are reproduced line by line with a brightness which corresponds to the modulation of the electric signal. The transmitter contains a memory and a controller arranged to divide the transmission of the signal created by the scanning device into segments and, upon sensing a maneuver signal, to direct the memory to retransmit a portion of the signal corresponding to one segment to the receiver. The receiver is provided with a memory and a control unit arranged to verify each signal segment received via the communications line with respect to errors while simultaneously storing each signal segment in the memory. If a preselected level of error has not been exceeded, the signal segment is transferred from the memory to the picture reproduction device. If this error level has been exceeded the aforementioned maneuver signal is sent to the transmitter to direct it to retransmit the erroneous signal segment and the erroneous signal segment, store in the receiver's memory, is erased.

TECHNICAL FIELD 
The present invention relates to a device for the long-distance 
transmission of images such as a picture, such longdistance transmission 
being effected by means of a transmitter and a receiver connected by means 
of a communications line such as a conductor or a wireless connection, the 
transmitter containing a scanning means for scanning an image in a linear 
pattern and deviced so as to form an electric signal modulated to 
represent the picture's brightness in picture points along the scanned 
lines, the receiver having a reproduction means and a moving means for 
moving of same relative to a picture supporting surface in a linear 
pattern corresponding to the transmitter's linear pattern, means for 
receiving said signal via the communications line and tranferral of same 
to the picture reproduction means which is so arranged that, in the course 
of its motion over said surface, it reproduces the picture points line by 
line with a brightness which corresponds to the modulation of the signal. 
BACKGROUND 
The longdistance transmission of images is undertaken for different 
purposes. Two main purposes can be distinguished: the transmission of 
monochrome or color images with high transmission quality for reproduction 
in newspapers and magazines as the most important application, and the 
transmission of images and texts, often black and white in nature, for 
informative reasons, without high requirements for accurate reproduction 
and which has as its main object the transmission of documents in an 
office environment. The invention relates to apparatuses for the former 
purpose, which, however, does not prevent its possible use in the other 
main area if the quality which can be achieved by the invention is 
required even in this area. The following presentation will, however, 
mainly concentrate on apparatuses for the former purpose, i.e., 
apparatuses for the transmission of images. In the following it is 
assumed, for purposes of example only, that the image involved is a 
picture. 
Previously known apparatuses of this type transmit a picture in a 
continuous synchronized fashion. The picture is scanned line by line in 
the transmitter and its varying levels of brightness give rise to a 
transmitted signal by means of lightsensitive elements. During the 
continuous scanning of the picture the signal is transmitted to the 
receiver via a telephone line, some other line, or wirelessly. The 
surface, normally a paper surface, on which the picture is to be created 
is scanned in the receiver in synchronization with the scanning of the 
original picture and a printing means gives the surface the blackening or 
the nuance of color which corresponds to the signal which is transmitted 
at that instant. Previously, an analog signal was used for the gray scale, 
but now a digital signal is widely used which lends good fidelity to the 
reproduction. Precise equipment is used for synchronization so that 
satisfactory agreement can generally be attained between the original and 
the reproduction. 
Technical problem 
When it comes to quality of transmission, however, one difficulty remains. 
Disturbances in the transmission link can distort the received signal so 
much that the reproduction of the picture becomes inaccurate. In general, 
the inaccuracies affect the reproduction of the gray scale such that a 
line is rendered lighter, darker, longer or shorter than was intended. 
Even the synchronization can be affected if, for example, the sync pulse 
disappears or is delayed, which can result in an almost total distortion 
of the picture. Prior methods provide a certain ability to make 
corrections if errors are detected in the received signal, but systems of 
prior art have not been suitable for high quality transmission of, for 
example, photographs. 
The solution 
A solution to the described problem is according to the invention, obtained 
by that the transmitter contains a memory and a control means arranged in 
part to divide the transmission via the communication line of the signal 
created by means of the scanning means into time intervals, each 
containing a specific picture segment, preferably consisting of a line, 
and partly, in the presence of a maneuver signal, to direct the memory to 
retransmit a portion of the signal to the receiver and that the receiver 
is designed with a memory and a control unit arranged so as to verify each 
signal section received via the communications line with respect to errors 
while simultaneously storing same in the memory and arranged so as to, 
while verifying that a preselected level of error has not been exceeded, 
transfer the signal section from the memory to the reproduction means for 
reproduction of a picture segment corresponding to said signal section 
and, when detecting that said error level has been exceeded, to send via 
the communications line, which is two-way, said maneuver signal to the 
receiver causing the destruction of the erroneous signal section stored in 
the receiver's memory as well as thereafter verifying the retransmitted 
signal section until such time as a signal is received which has a level 
of error less than the predetermined value, whereupon the control means 
transfers same to the reproduction means in order to reproduce a picture 
segment which corresponds to the signal section. 
Advantages 
The purpose of the present invention is to provide a device for 
longdistance transmission which makes possible a continuous check of the 
transmitted signal in such a manner that corrections can be made in 
individual portions of the signal before they are used to direct the 
printing means. In this way a faultless picture is produced even when 
disturbances arise at the moment of transmission. 
Another purpose of the invention is to provide a device which is 
independent of the particular rate of synchronization used for 
transmission between a transmitter and a receiver. In this manner a 
transmission rate can be chosen to correspond to the proper transmission 
capacity for all types of connections and may even be chosen during the 
course of a transmission.

BEST MODE OF CARRYING OUT THE INVENTION 
An apparatus for sending picture information of the type for which the 
invention can be used is shown in FIG. 1. It comprises a casing 1, 
illumination means 2, a container 3 for the film 4 which is maneuverable 
in the beam path of the illumination means, as well as a first mirror 5 on 
a supporting beam 11, a lens system 6 providing variable magnification and 
a device 7 for scanning pictures. 
The scanning device 7 contains a row scanner 9, which can, for example, be 
of the type CCD and which is connected to a movable slide 8. Using a motor 
10, the slide is movable in the longitudinal direction of the row scanner 
9 by known means, so that a row by row scanning can be accomplished of the 
picture projected onto the device 7 from the film 4 via the mirror 5 and 
the lens system 6. 
A second mirror 12, which is connected to an axel 13, and which is 
rotatable between two positions, is positioned in the beam path in the 
direction of the scanning device 7. When the mirror 12 assumes the 
position shown in the figure, the projection is reflected from the lens 
system upwards to a focusing screen 14 on which it is possible to observe 
the projected picture. By observing the picture which appears on the 
focusing screen 14 it is possible to focus and the section of the picture 
which is most pertinent may be selected. When the picture section is 
chosen the mirror 12 is dropped so that the scanning device may read the 
picture whereby keyboard 15 can be used for maneuvering. The sender is 
equipped with a monitor 17 in order to allow checking of the scanned 
picture which is stored electronically in a memory 16. 
FIG. 2 shows the picture scanning device 7 which is contained in the 
transmitter, shown in FIG. 1. It comprises a frame 20 with two guides 22 
and 23 along which a scanning element, the aforementioned row scanner 9, 
can be moved. The guides 22 and 23 can, for example, consist of worm gears 
which are driven by electric motor 10 in the frame 20 so that the row 
scanner is moved back and forth along the guides in a direction determined 
by the direction of rotation of the worm gears. The row scanner is 
equipped with a row of photo sensors 25, sufficient in number to ensure 
high resolution. The photo sensors are shielded in the scanner's 
transverse direction in such a manner that they are only affected by a 
narrow band which extends perpendicular to the element's direction of 
movement. The plane of the photo sensor lies in the focal plane of the 
said projection of the picture which is to be sent. 
FIG. 3 shows an example of a picture reproduction device found in the 
receiver. Said device includes a supporting frame 31 in which a cylinder 
drum 32 is mounted in bearings and is arranged for mounting a picture 
medium such a photographic paper or photographic film 33. Along the drum 
32 is provided an illumination device 35 which is movable along the guides 
36 and 37. It contains an optical device 38, directed towards the drum 32, 
which focuses the light from the light source to a fine point on the 
surface of the paper/film 33. Motors 39 for moving the illumination device 
along the guides 36 and for turning the drum 32 are arranged on the frame 
31 and are connected to controlling equipment. For the sake of clarity 
only a section of the frame has been shown. The light source in the 
illumination device 35 is arranged so as to receive signals from the 
transmitter and to modulate its light strength in accordance therewith. 
The photo sensors of the row scanner 9 are connected to the electronic 
equipment of the transmitter in such a manner that they produce a pulse, 
the level of which is dependent upon the strength of the light which at 
the instant of registration affects each respective photo sensor. 
FIG. 4 shows a schematic diagram of the transmitter and the receiver. In 
the figure, the transmitter is indicated as 40 and the receiver as 41. The 
scanning device 7 with its row scanner 9 and with the photo sensors 25 
(Only four such are shown.) are recognized in the transmitter. By means of 
said motor 10 the scanner may be moved along the guide 22. 
As has been mentioned, the photo sensors can be actuated one at a time so 
as to produce a sequence of pulses such that a pulse train is created 
which represents the varying degree of illumination along a line of the 
picture which is to be transmitted. This arrangement is symbolized by a 
number of conductors 42 and a sequential switch 43 for creating said pulse 
train. This pulse train then passes to a digitizer 44 which converts the 
analog signals, which correspond to the illumination, to numerical values. 
The digital signal then passes to a memory 45. The transmitter also 
contains a control means 46 which is arranged so as to control the 
scanning of the picture by sequentially activating the photo sensors 25 in 
order to scan a line of the picture and to move the row scanner 9 by means 
of the motor in the housing 21. 
The receiver 41 contains the drum 32 described above with the picture 
medium 33 as well as the illumination device 35 and the motor unit 39 for 
turning the drum 32 and moving the illumination device 35 along same. A 
control means 50 is provided for controlling the motor unit 39. A memory 
51 is provided for receiving the signal supplied by the transmitter. The 
signal retrieved from the memory can be converted back into analog form in 
a dedigitizer 52 and is passed on to the illumination device 35 for 
modulating its light strength. 
The line of communication between the transmitter and the receiver is 
designated by 55. The digitized signal from the transmitter's memory 45 is 
transferred via a modem 56 over said line of communication and via a modem 
57 to the receiver's memory 51. The received signal is also transformed as 
is shown in the figure and is passed to the control unit 50. Additionally, 
the line 55 is arranged to allow transferral of answer signals from the 
receiver's control unit 50 to the transmitter's control unit 46. The 
answer signals' path is shown as a dotted line in the figure whereas other 
signals are shown as solid lines. 
The arrangement of the control units 46 and 50, their mutual cooperation 
and their cooperation with the device for picture scanning and the device 
for picture reproduction is essential to the invention. The control means 
46 is arranged so as to bring about a continuous scanning of the picture 
which is to be transmitted by means of sequential transferral of signals 
from the row of photo sensors 25 and movement of the scanning unit 24 so 
that the picture is scanned successively, line by line. 
The control means 46, however, is also arranged so as to be able to 
interrupt this continuous scanning of the picture and await a new 
transmission to the receiver of the most recently sent picture segment. 
The picture is thereby divided into a number of picture segments, which 
can consist of a single line corresponding to the scanning of one row of 
the row scanner's 9 photo sensors 25, or preferably a number of such 
lines, limited, however, in number, so that the picture segment in 
question uses only a small portion of the transmission time which is 
required for transmitting an entire picture. The arrangement shown in FIG. 
4 assumes that the signal for each such picture segment is applied to the 
memory 45 in digitized form. The memory must consequently have sufficient 
capacity to contain one such picture segment. 
Additionally, the control means 46 is arranged so that the scan rate, i.e. 
the period of time required to transfer the sequence of digital signals 
from the row of photo sensors 25, can be changed from transmission to 
transmission and can even be changed during transmission. 
The control means 50 in the receiver 41 is, on the other hand, arranged for 
continuous registration of the transmitted picture onto the picture medium 
33. This is accomplished by rotating the drum 32 so that the picture 
medium 33 passes under the illumination device 35 during the time that a 
pulse train is received which corresponds to the signals from the photo 
sensors 25 in the row scanner 9, i.e. during the scanning of a line of the 
picture, so that a corresponding line is created on the picture medium. 
For each such rotational movement of the drum, during which a line is 
formed, the illumination device 35 is moved one line step so that a new 
line may be created immediately adjacent to the previous line. 
The control means 50 is, however, also arranged so as to be able to 
interrupt this continuous registration of the picture. This interruption 
is contingent upon a check of the received signal which, as is shown in 
FIG. 4, is presented to the control means 50 as well as to the memory 51. 
Said check is of whether or not the signal has been transferred correctly 
from the transmitter and thus constitutes a check for errors. One such 
error check may be accomplished by analyzing the received digital signal 
and examining it for possible deviations from the known structure of a 
correct digital signal. If an error is detected during reception of a 
picture segment the control means 50 prevents the received signal from 
being passed on from the memory 51 to the picture reproduction device 
which, simultaneously, is halted. At the same time a signal is sent via 
the communication line 55 to the transmitter's control unit 46 which is so 
arranged that it then interrupts the scanning of the picture as has been 
described above. The capacity of the memory 51 is also chosen so as to 
correspond to at least one of said picture segments. 
The transmission means of the receiver is consequently arranged so as to 
convey the signal corresponding to a picture segment from the memory 
onwards to the picture reproduction device in order to modulate its 
illumination device only after the signal for the picture segment has been 
registered as being error-free. If this is not the case, picture 
reproduction is interrupted and the picture reproduction device delays 
reproduction of the picture segment in question until the control device 
indicates that a new error-free signal has been registered in memory 
whereupon picture reproduction continues. As long as the signal is 
error-free picture reproduction can be continuous. 
Furthermore, the control unit 50 is devised so as to vary the 
circumferential velocity of the drum 32 under the illumination means 35 in 
accordance with the pulse rate of the received transmission signal. Since 
this signal is digitized it consists of a sequence of numbers each of 
which corresponds to the instantaneous illumination sensed by one of the 
photo sensor. Each such number corresponds to a particular segment of the 
line which is scanned by means of the row scanner 9 and to the 
corresponding segment of the line which is reproduced in the picture 
reproduction device. For each line, the concatenation of the line segments 
corresponding to the number sequence must form a line, the length of which 
corresponds to the length of the line produced in the picture reproduction 
device. Were this line to be too long or too short the dimension of the 
picture in the longitudinal direction of the lines would be changed in 
relation to the unchanging dimension in the direction transverse the lines 
which corresponds to the number of lines and their relative separation. If 
the transmission rate is reduced the numbers will be sent with larger time 
intervals than they would be at a higher transmission rate. This is 
registered by the control unit 46 which is arranged in such a manner that 
it regulates the circumferential velocity of the drum so that the set 
number of line segments which correspond to a scanned line always have the 
proper length in relation to the distance in the lines' transverse 
direction. 
Alternatively, the receiver can be designed so that it only receives and 
stores the correct digital picture information. At the discretion of an 
operator the picture can thereupon be output using different output means. 
One such output means could be a conventional telephoto receiver which, 
however, requires synchronous picture transmission. In this case, the 
advantage of the invention is that the synchronous transmission, using 
artificially created synchronization pulses, takes place over a much 
shorter and higher quality line which leads to a minimum of disturbances 
and consequently higher picture quality than is attained using 
conventional telephoto transmissions. 
The manner in which a signal is transferred will now be described with 
reference to FIGS. 4 and 5. The way in which a signal is formed in a 
transmitter's scanning device has been described above. As has been 
mentioned, scanning takes place in such a way that the portions of the 
signal corresponding to a segment of the picture to be transmitted, are 
conveyed to the memory 45 and then onwards to the receiver 41 via the 
communication line 55. Within the receiver 41 each such signal portion is 
stored in the memory 51 while the control unit 50 carries out a check of 
the correctness of the signal as has been described above. When a signal 
portion, which corresponds to a segment of the picture has been 
established as being correct by the control unit 50 the memory 51 is 
activated so as to convey the signal further so that, having been 
converted to analog form, it may regulate the modulation of the 
illumination device 35. Simultaneously, the control unit 50 causes the 
picture medium 33 to be moved using the motor device 39. 
If, however, the signal is not established as being correct by the control 
unit 50, storage in the memory 51 is interrupted and a signal is sent via 
the communication line 55 from the control unit 50 to the control unit 46 
(see the signal path shown as a dotted line in FIG. 4). The control unit 
46 then causes the scanning of the row scanner 9 to be halted. The signal 
transmission from the memory 45 of the signal corresponding to the picture 
segment in question is halted and is restarted from beginning. If the 
signal for the entire section is established as being correct by the 
receiver's control unit 50 the segment in question can be registered in 
the receiver. 
This procedure is clarified in FIG. 5. The signal is shown schematically as 
a horizontal line. A first signal section designated by I has been 
received correctly and, consequently, causes no repeat transmission. This 
is also the case for signal section II. In regard to signal section III, a 
disturbance has arisen during reception of this signal section which is 
indicated by a curve. The vertical line indicates that the transmission is 
halted at this point under the direction of the receiver's control unit 
50. The signal section in question is thereupon retransmitted from the 
memory 45 to the memory 51. The region of the memory 51 which contained 
the previous erroneous information has been erased. As the figure shows, 
the retransmitted signal is then received correctly. The same is the case 
for signal section IV. Signal section V, however, is affected by 
disturbances and is interrupted whereupon the signal is retransmitted, 
resulting in yet another interruption. Finally, on the third attempt, the 
signal is received correctly. This is an example of how a signal sequence 
might occur. 
The device may be designed in different ways without departing from the 
spirit of the invention and all such designs are considered to lie within 
the scope of the following patent claims. Certain alternatives have 
already been mentioned in the preceding text. As regards the memories 45 
and 51, these can be limited to a memory capacity corresponding to one 
picture segment. In this case each picture segment must be erased after 
establishing error-free reception so that the next picture segment may be 
stored. Alternatively, the memory can have greater capacity and, as has 
been mentioned previously, the receiver can utilize a memory with 
sufficient capacity to store the entire picture, or several pictures, for 
reproduction of the picture at a later opportunity. Within the scope of 
the invention it is also envisaged that the device may function even if 
the signal is not digitized; digitization, however, facilitates detection 
of errors in transmission. 
The design of the scanning device as well as of the picture reproduction 
device may vary greatly. For example, the transmitter could be constructed 
using a rotating drum instead of a row scanner. Conversely, the receiver 
can be designed for picture reproduction using a reciprocating procedure 
such as is used to generate a television picture. The reproduction means 
do not, of course, have to rely on a light source which exposes a 
photographic medium. Other means, such as, for example, an ink jet, could 
also be utilized. As has been pointed out, utilization of the device for 
transmission of color pictures also lies within the scope of the 
invention. In this case the transmission is generally divided into three 
parts in accordance with a color analysis and decomposition of the picture 
into the three basic colors of the color circle. The manner in which this 
is done is, however, well known, requiring no further description and, a 
person skilled in the art could also use the invention for transmission of 
color pictures based solely on the information which has been supplied 
above.