Transmitting device, receiving device, system, and method for transmitting image at high efficiency

A transmitting device encodes an animated image by detecting a difference between one image frame and a reference image frame, then transmits the encoded image to a receiving device by a radio line at high efficiency. The transmitting device comprises an image storing unit for storing the reference image frame, an initialization unit for initializing the image storing unit, a difference encoding unit for detecting and encoding the difference, an encoded data storing unit for storing the encoded difference, a transmitting unit for detecting an error detection code in the encoded difference, and transmitting the encoded difference together with transmission information including the error detection code to the receiving device, a receiving unit for receiving reception information from the receiving device which indicates whether the receiving device finds either OK or NG as a result of error detection check which is based on the error detection code, a reproducing unit for reproducing the encoded difference and storing it as one reference frame into the image unit when the reception information indicates OK, and a controlling unit for controlling the difference encoding unit to encode the next image frame when the reception information indicates NG, while controlling the difference encoding unit to encode the next image frame only after the reproducing unit stores a new reference image frame when the received reception information indicates OK.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a transmitting device, a receiving device, 
a system including these devices, and a method of transmitting digital 
data which is processed by animated image compression methods (frame 
encoding, animated compensation prediction) to a radio circuit at high 
efficiency. 
(2) Description of the Related Art 
Recently, various methods have been realized to process digital data of 
animated images. Generally, the information volume of an image is 
considerably large, so that it is necessary to compress/reduce the 
information volume before recording or reducing the image. For example, 
"ITU advice H. 261 video encoding method for audio visual service" is a 
method of encoding difference information between frames of an animated 
image. 
In this encoding method or the like, the problem of transmission error is 
overcome by adding an error correction code to encoded image data. Also, a 
single frame of compressed data is transmitted at a certain interval so as 
to prevent the transfer of an error in difference information to 
succeeding frames. 
However, when transmitting data by a radio line, the conventional encoding 
method has the following problem. Adding an error correction code to image 
data does not eliminate a burst error derived from a fading which is 
unique to a radio line. Originally, an error correction code was invented 
to overcome the problem of random error in a wire communication line. 
Also, the addition of an error correction code increases the transmission 
amount, and this slows down the actual transmission rate. 
Further, compared to the data amount taken to send difference information, 
the data amount taken to send one frame data is considerably large. 
Therefore, data transmission will be slow down at large. Especially when 
sending data at a low bit rate, the slow down of data transmission extends 
the time taken since the transmission starts until the reproduction ends 
(reproduction delay). Accordingly, scheduled smooth movements of animated 
images are interfered very badly. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a high efficiency 
transmission method which is capable of transmitting digital data of 
animated images by a radio line whose transmission late is slow. 
The above object may be fulfilled by a transmitting device for encoding to 
compress an animated image including a plurality of image frames by 
detecting a difference between one image frame to be encoded and a 
reference image frame, then transmitting data of the encoded image to a 
receiving device by a radio line at high efficiency, the transmitting 
device comprising an image storing unit for storing the reference image 
frame which is referred to detect the difference relating to the image 
frame to be encoded, an initialization unit for, before a first 
transmission, initializing the image storing unit by initial data which is 
identical to initial data held by the receiving device, a difference 
encoding unit for detecting the difference between the image frame to be 
encoded and the reference image frame, and encoding to compress the 
difference, an encoded data storing unit for storing the encoded 
difference data received from the difference encoding unit, a transmitting 
unit for detecting an error detection code in the encoded difference data, 
and transmitting the encoded difference data together with transmission 
information including the error detection code to the receiving device, a 
receiving unit for receiving reception information from the receiving 
device, the reception information including information which indicates 
whether the receiving device finds either OK or NG as a result of error 
detection check which is based on the error detection code, a reproducing 
unit for decoding to reproduce the encoded difference data stored in the 
encoded data storing unit, then storing it as one reference frame into the 
image storing unit when the received reception information indicates OK, 
and a controlling unit for controlling the difference encoding unit to 
encode to compress the next image frame when the received reception 
information indicates NG, while controlling the difference encoding unit 
to encode to compress the next image frame only after the reproducing unit 
stores a new reference image frame when the received reception information 
indicates OK. 
In this construction, the transmitting device can transmit only encoded 
difference data as assuring each transmission to the receiving device, 
without transmitting an encoded image frame unit. 
The transmitting unit may divide the encoded difference data into a 
plurality of radio frames, and transmitting each radio frame and 
transmission information which includes the error detection code which 
corresponds to the radio frame to the receiving device; the receiving unit 
may receive from the receiving device the reception information which 
corresponds to the transmitted radio frame; and the controlling unit may 
control the reproducing unit to reproduce the encoded difference data for 
a single image frame when all the reception information for it indicates 
OK, while the controlling unit may control the difference encoding unit to 
encode the difference for a next image frame when at least one of the 
reception information for a single image frame indicates NG. 
Each radio frame may be fixed length, and an excess of encoded data over 
the data amount of a predetermined number of radio frames may be 
abandoned. 
In this construction, the above effect can be realized by a radio line at a 
lower speed. 
The transmission information may include a transmission frame number which 
indicates the number of image frames which have been transmitted safely, 
and the controlling unit may increment the transmission frame number by 
one upon each updating of the reference image frame in the image storing 
unit by the reproducing unit, whereby the receiving device can check if 
its own reference image frame is identical to the reference image frame 
stored in the transmitting device. 
The transmitting device may further comprise an error detection/correction 
code generating unit for generating an error detection/correction code 
relating to the reference image frame in the image storing unit, wherein 
the reception information received by the receiving unit includes the 
error/correction code relating to the reference image frame stored in the 
receiving device, and the controlling unit controls the initialization 
unit to compare the error detection/correction code included in the 
reception information received by the receiving unit with the error 
detection/correction code generated by the error detection/correction code 
generating unit, and controls the initialization unit to initialize the 
reference image frame stored in the image storing unit when they do not 
coincide with each other, as well as controls the transmitting unit to 
transmit the transmission information which directs to initialize the 
reference image frame stored in the receiving device. 
In this construction, there may a difference between the reference image 
frame in the transmitting device and a reproduction image in the receiving 
device, and this difference can be overcome by operating the 
initialization. 
The error detection/correction code generating unit may divide the 
reference image frame in the image storing unit into a plurality of areas, 
and generates the error detection/correction code relating to each area; 
the transmitting unit may divide the encoded difference data into a 
plurality of radio frames, and transmit the transmission information which 
includes each radio frame, the error detection code corresponding to the 
radio frame, and the error detection/correction code corresponding to the 
area to the receiving device; the receiving unit may receive the reception 
information from the receiving device which includes a check result 
according to the error detection code, and information which indicates if 
the area corresponding to the error detection/correction code is identical 
to one of areas into which the reference image frame in the receiving 
device is divided; and the controlling unit may control the transmitting 
unit to transmit the radio frame which was transmitted at the last 
transmission together with the error detection/correction code for the 
next area when the check result included in the reception information 
indicates an error, also control the initialization unit to initialize all 
the areas which are not identical to those in the receiving device 
according to the reception information after the error 
detection/correction codes for all the areas are transmitted and the 
reference image frame in the image storing unit is updated. 
In this construction, a radio frame unit can be re-transmitted. Also, the 
above difference can be overcome effectively. 
The above object may be fulfilled by a receiving device for receiving 
encoded difference data at high efficiency from a transmitting device via 
a radio line, the receiving device comprising an image storing unit 
including an area where a reproduced image basing upon the encoded 
difference data is stored, an initialization unit for initializing the 
image storing unit by initial data which is identical to initial data held 
by the transmitting device, a receiving unit for receiving the encoded 
difference data from the transmitting device, and detects an error 
according to an error detection code included in transmission information 
which was transmitted together with the encoded difference data, an 
encoded data storing unit for storing the encoded difference data received 
by the receiving unit, a reproducing unit for decoding the encoded 
difference data in the encoded data storing unit when a result of the 
error detection is OK, reproducing the image frame by adding the decoded 
data to content of the image storing unit, and storing the reproduced 
image frame into the image storing unit, a controlling unit for generating 
reception information which includes the result of the error detection by 
the receiving unit, and a transmitting unit for transmitting the reception 
information to the transmitting device. 
The receiving unit may receive radio frames into which the encoded 
difference data is divided, and the controlling unit may control the 
reproducing unit to reproduce one image frame of the encoded difference 
data when the error detection result as for all of the radio frames 
included in it is OK. 
The receiving device may further comprise an error detection/correction 
code generating unit for generating an error detection/correction code 
relating to a reference image frame stored in the image storing unit, 
wherein the transmission information includes information which directs 
whether or not the reference image frame in the image storing unit should 
be initialized, and the controlling unit generates reception information 
which includes the generated error detection/correction code, controls the 
transmitting unit to transmit the reception information, and controls the 
initialization unit to initialize the reference image frame in the image 
storing unit when the information included in the transmission information 
received by the receiving unit directs the initialization. 
The transmitting system which fulfills the above object comprises the 
transmitting device and the receiving device. 
The above object may be fulfilled by a high efficiency transmitting method 
employed in a system which comprises a transmitting device and a receiving 
device, for encoding to compress an animated image including a plurality 
of image frames by detecting a difference between one image frame to be 
encoded and a reference image frame, then transmitting data of the encoded 
image to a receiving device by a radio line at high efficiency, wherein 
the transmitting device comprises a first initialization step for, before 
a first transmission, initializing a first image storing unit by initial 
data which is identical to initial data held by the receiving device, a 
first difference encoding step for detecting the difference between the 
image frame to be encoded and the reference image frame, and encoding to 
compress the difference, and storing the encoded difference data into a 
first encoded data storing unit, a first transmitting step for detecting 
an error detection code in the encoded difference data, and transmitting 
the encoded difference data together with transmission information 
including the error detection code to the receiving device, a first 
receiving step for receiving reception information from the receiving 
device, the reception information including information which indicates 
whether the receiving device finds either OK or NG as a result of error 
detection check which is based on the error detection code, a first 
reproducing step for encoding to reproduce the encoded difference data 
stored in the first encoded data storing unit, then storing it as one 
reference frame into the first image storing unit when the reception 
information indicates OK, and a first controlling step for controlling the 
first difference encoding step to encode to compress the next image frame 
when the reception information indicates NG, while controlling the first 
difference encoding step to encode to compress the next image frame only 
after a new reference frame is stored at the first reproducing step when 
the reception information indicates OK, and the receiving device comprises 
a second initialization step for initializing a second image storing unit 
which includes an area where a reproduced image basing upon the encoded 
difference data is stored by initial data which is identical to initial 
data held by the transmitting device, a second receiving step for 
receiving the encoded difference data from the transmitting device and 
storing it into a second encoded data storing unit, as well as detecting 
an error according to an error detection code included in transmission 
information which was transmitted together with the encoded difference 
data, a second reproducing step for decoding the encoded difference data 
in the second encoded data storing unit when a result of the error 
detection at the second receiving step is OK, reproducing the image frame 
by adding the decoded data to content of the second image storing unit, 
and storing the reproduced image frame into the second image storing unit, 
a second controlling step for generating reception information which 
includes the result of the error detection at the second reproducing step, 
and a second transmitting step for transmitting the reception information 
to the transmitting device. 
The above effects may be realized by this transmitting method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
[Embodiment 1] 
(overall configuration) 
FIG. 1 shows a simple digital cordless telephone system to which a high 
efficiency image transmission device relating to a first embodiment of the 
present invention is applied. The simple digital cordless telephone system 
in FIG. 1 is a so-called PHS (Personal Handy-Phone System) where a master 
terminal and a subordinate terminal communicate digital data at 32 kbps 
transmission rate. The high efficiency image transmission device comprises 
an image transmitting unit and an image receiving unit. In this system 
example, the image transmitting unit is constructed at a master terminal 
while the image receiving unit is constructed at a subordinate terminal. 
In FIG. 1, a master terminal 100 comprising an image transmitting unit 101 
and an antenna 102 functions as one master terminal in the PHS. The image 
transmitting unit 101 encodes an image signal received by communication 
line, and transmits it to one of subordinate terminals by the antenna 102. 
Subordinate terminals 101, 120, 130 comprising image receiving units 111, 
121, 131 and antennas 112, 122, 132 respectively functions as subordinate 
terminals in the PHS. Each image receiving unit reproduces an image from 
signals received via the corresponding antenna, then displays it. 
(configuration of image transmitting unit) 
FIG. 2 shows a block diagram describing the configuration of the image 
transmitting unit 101 at a master terminal. The image transmitting unit 
101 comprises a controller 1, a communication cable 2, a line interface 3, 
an image storing unit 4, a difference compressing unit 5, a data storing 
unit 6, a transmitting unit 7, a radio unit 8, a receiving unit 9, and an 
expansion adder 10. The thus constructed image transmitting unit 101 
applies difference encoding to each image frame so as to compress image 
data, divides a difference encoded image frame into a plurality of radio 
frames, and transmits each radio frame to a subordinate terminal as 
assuring every transmission. 
The communication cable 2 is connected to a dedicated line or ISDN. An 
animated image signal or an encoded animated signal is sent to the 
communication cable 2. 
An animated image signal or an encoded animated image signal is inputted 
from the communication cable 2 to the line interface 3. The line interface 
3 then outputs image frames which make up an animated image (normally, 30 
image frames per second) one by one. When an encoded animated signal is 
inputted from the communication cable 2, the line interface 3 decodes it, 
then outputs image frames one by one. Herein an image frame outputted from 
the line interface 3 is an intermediate format CIF (Common intermediate 
Format) or a QCIF (Quater CIF) of H. 261. 
The image storing unit 4 stores initial data beforehand, then replaces it 
with an expanded image frame upon each expansion by the expansion adder 
10. Initial data means one image frame of data which is commonly held by 
the image receiving units 111, 121, 131, and an image frame where every 
bit value is "0" is held as initial data herein. 
The difference compressing unit 5 encodes and compresses an image frame 
inputted from the line interface 3. To be concrete, the difference 
compressing unit 5 encodes a difference between a received image frame and 
an image frame stored in the image storing unit 4 (hereunder called 
"reference frame"). The variable length encoding method which conforms 
with CCITT advice H. 261 is employed. The difference compressing unit 5 
operates difference encoding on every image frame. Although the above H. 
261 also includes encoding of an image frame, the difference compressing 
unit 5 does not operate this. The data storing unit 6 stores a single 
image frame of compressed data received from the difference compressing 
unit 5. A compressed image frame was generated according to the variable 
length encoding method; therefore, the data amount varies for each image 
frame. 
The transmitting unit 7 divides a compressed image frame stored in the data 
storing unit 6 into a plurality of radio frames for radio transmission; 
adds an image number, difference compression data, an error detection 
code, and a synchronous pattern to each radio frame so as to generate a 
fixed length data packet; then transmits it. FIG. S shows how a compressed 
image frame is divided by the transmitting unit 7. In the figure, the 
first 3000 bits of a compressed image frame are divided into 20 radio 
frames 0-19. Accordingly, one radio frame has 150 bit data amount. The 
remaining part of the compressed image frame, that is after the 3000-th 
bit, is abandoned. 
FIG. 4 shows the format of a fixed length data packet generated by the 
transmitting unit 7. The data packet in FIG. 4 comprises the cells of 
control information 1, packet type, control information 2, information 
field, and control information 3. The control information 1-3 are mainly 
for TDMA (Time Division Multiple Access) in the PHS, and the further 
explanation thereof is omitted. The cell of information field includes 
10-bit transmission information and 150-bit difference compression data. 
The first 9 bits of transmission information indicate a transmission frame 
number and the last one bit of the same is reserved. Difference 
compression data represents radio frames which correspond to transmission 
information. The cell of packet type encloses information which indicates 
whether difference compression data or voice data is set at the 
information field cell. The cell of error detection code encloses an error 
detection code with which an error between the packet type cell and the 
information field cell is detected. 
The radio unit 8 modulates a data packet received from the transmitting 
unit 7, and outputs its radio wave. Also, the radio unit 8 demodulates 
radio wave received from a subordinate terminal. A data packet outputted 
by the radio unit 8 is illustrated at the bottom of FIG. 4. The cells 1-4 
in FIG. 4 indicate that four subordinate terminals are multiplexed 
according to TDMA. In FIG. 4, the radio unit 8 transmits a data packet 
according to a time slot 1 in TDMA time frame. 
The receiving unit 9 establishes synchronization according to a synchronous 
pattern included in a data packet demodulated by the radio unit 8 to 
receive a data packet; simultaneously, it detects an error by the error 
detection code included in the data packet. FIG. 5 shows the format of a 
data packet received by the receiving unit 9. The upper part of FIG. 5 is 
substantially same as that of FIG. 4 except the content of the information 
field cell. To the last one bit of the reception information cell, "0" 
(success) is set when the subordinate terminal receives information 
safely; whereas "1" (failure) is set when an error occurs so that the 
subordinate terminal fails to receive information. The first nine bits are 
reserved. A data packet transmission timing according to the time slot 1 
in TDMA time frame is illustrated at the bottom of FIG. 5. 
When receiving a expansion request from the controller 1, the expansion 
adder 10 expands difference compression data in the data storing unit 4, 
adds the expanded result to reproduction image data stored in the storing 
unit 4 so as to generate image data. Subsequently, the expansion adder 10 
stores the image data into the image storing unit 4 as a newly 
reproduction image. The thus reproduced image is the same as the one 
reproduced at the receiving end originally. 
The controller 1 controls difference encoding of an image frame, 
transmission of a data packet which includes radio frames, and reception 
of a data packet which includes reception information derived from a 
subordinate terminal. More specifically, the controller 1 comprises a 
transmission counter (not illustrated) which counts up upon each 
transmission of a data packet which includes radio frames, a first 
register (not illustrated) for storing a transmission frame number which 
indicates how many image frames have been transmitted safely, an error 
flag which indicates if an error occurs in transmission of a data packet 
which includes reception information, and a transmission flag which 
indicates if an error is included in reception information received from a 
subordinate terminal. The controller sets 20 times as utmost transmission 
frequency per an image frame, controls to transmit the radio frames in 
order as examining reception information from a sub-ordinate terminal, 
then, if 20 radio frame transmissions are carried out for one image frame, 
it controls to make another 20 transmissions for the next image frame. To 
the error flag, "0" is set when every transmission is carried out safely, 
while "1" is set when any transmission error is detected. To the 
transmission flag, "0" is set when no error is included, while "1" is set 
when an error is included. 
(detailed control by controller 1) 
FIG. 6 is a flow chart depicting detailed control processing by the 
controller 1. When the communication cable 2 starts to encode, the 
controller 1 establishes a radio link between itself and a subordinate 
terminal, sets initial data to the image storing unit 4 (Step 500), resets 
a transmission frame number in the first register to 0 (Step 501). When 
the line interface 3 writes an encoded image frame into the data storing 
unit 6 (Step 502), the controller 1 resets all of the transmission 
counter, error flag, and transmission flag to 0 (Step 503). Subsequently, 
the controller 1 generates a data packet from a radio frame in the data 
storing unit 6 which corresponds to a number at the transmission counter 
(Step 504), and controls the radio unit 8 to transmit it by radio wave 
(Step 505). 
As for a reception time slot in TDMA time frame corresponding to the radio 
wave transmission (For example, a reception time slot "Rx1" corresponds to 
a transmission time slot "Tx1") at the bottom of FIGS. 4 and 5, when the 
receiving unit 9 receives a data packet which includes reception 
information via the radio unit 8 (Step 506), and an error is detected in 
transmission of the same data packet (Step 507: failure), the controller 1 
sets "1" to the error flag. On the other hand, if no error is detected in 
transmission of the data packet (Step 507: success) and the least 
significant bit of the reception information is 1 (Step 508: error), it 
sets "1" to the transmission flag, and increments the transmission counter 
by one (Step 511). If not error is detected in transmission of the data 
packet and the least significant bit of the reception information is other 
than 0, it skips to Step 511. 
Upon each increment, the controller checks if the transmission counter does 
not reach its utmost value, 20. If the transmission counter does not reach 
20 (Step 512: no), the controller 1 returns to Step 504 and repeats the 
above operation on the next radio frame until the transmission counter 
reaches 20. 
When the transmission counter 20 reaches 20 (Step 512:yes), and the error 
flag is not 0 (Step 513: no), the controller 1 judges that an error occurs 
in the transmission of the data packet which includes the reception 
information. Therefore, the controller 1 returns to Step 503 to repeat the 
same operation so as to transmit difference image data for the same image 
frame again. 
When the error flag is 0 (Step 513: yes), as well as the transmission flag 
is not 0 (Step 514: no), the controller 1 judges that the reception 
information includes an error (the subordinate terminal failed to 
receive). Then, the controller 1 returns to Step 502 to transmit 
difference image data for the next image frame and repeats the above 
operation on it. In this case, the content of the image storing unit 4 is 
not updated, so that the same reference frame is referred, but only an 
image frame to be encoded is updated to next one. 
When the transmission flag is 0 (Step 514: yes), the controller 1 
increments the transmission frame number by one (Step 515), controls the 
expansion adder 10 to expand the difference image data in the data storing 
unit 6 (Step 516), then moves to Step 502. Consequently, both the content 
(reference frame) of the image storing unit 4 and an image frame to be 
encoded are updated. 
(configuration of image receiving unit ) 
FIG. 7 is a block diagram showing the configuration of the image receiving 
unit 111 constructed at the subordinate terminal 110. The image receiving 
units 121 and 131 have the substantially same configuration as the image 
receiving unit 111, and the description of those will not be repeated. 
The image receiving unit 111 comprises a controller 20, a radio unit 21, a 
receiving unit 22, a data storing unit 23, an image storing unit 24, an 
expansion adder 25, a transmitting unit 26, a video interface 27, and a 
monitor 28. The thus constructed image receiving unit 111 receives a data 
packet transmitted from its master terminal as monitoring a radio frame 
number included in it, and reproduces an image from difference image data 
included in the received data packet. 
The radio unit 21 receives radio wave from the transmitting unit, and 
demodulates it. Also, the radio unit 21 modulates a data packet generated 
by the transmitting unit 26 to output its radio wave. 
The receiving unit 22 establishes synchronization according to a 
synchronous patter included in a demodulated data packet to receive a data 
packet; simultaneously, it detects an error with reference to the error 
detection code included in the demodulated data packet as well as obtains 
a transmission frame number and difference compression data. 
Controlled by the controller 20, the data storing unit 23 stores difference 
compression data obtained by the receiving unit 22. 
The image storing unit 24 stores image data reproduced by the expansion 
adder 25 as new reproduction image data. 
When receiving a expansion request from the controller 1, the expansion 
adder 25 extracts one radio frame of compression data from the data 
storing unit 23 and expands it, adds the expanded difference data to the 
reproduction image data in the image storing unit 24 so as to reproduce 
image data, then stores it into the image storing unit 24. 
When receiving reception information from the controller 1, the 
transmitting unit 26 adds the error detection code and synchronous pattern 
to the same so as to generate one data packet. 
The video interface 27 D/A converts reproduction image data to output 
analog image signals. 
The monitor 28 receives input of analog image signals, and displays an 
image of the analog image signals. 
The controller 20 controls reception of a data packet which includes radio 
frames, transmission of a data packet which includes reception 
information, and decoding of each image frame. More specifically, the 
controller 20 comprises a reception counter (not illustrated) which counts 
up upon each reception of a data packet which includes radio frames, and a 
second register (not illustrated) for storing a radio frame number which 
indicates how many image frames have been received safely so far, and a 
reception flag which indicates that a transmission error is included in a 
data packet received from the master terminal. The controller sets 20 
times as utmost reception frequency (including re-transmission) per an 
image frame; upon each reception of one data packet which includes radio 
frames, controls to transmit reception information to its master terminal; 
then, if 20 radio frame receptions are carried out relevant for one image 
frame, controls to make another 20 receptions for the next image frame. 
(detailed control by controller 20) 
FIG. 8 is a flow chart depicting detailed control processing by the 
controller 20. When the controller 20 establishes a radio link between 
itself and a master terminal, it sets the same initial data as the master 
terminal to the image storing unit 24, and resets the reception frame 
number to 0 (Steps 600, 601). Also, the controller 20 resets the reception 
counter and the second register to 0 (Step 602). When the receiving unit 
22 receives a data packet from the master terminal via the radio unit 21 
(Step 603), the controller 20 examines if a transmission error is included 
in the data packet. When an error is included in the data packet (Step 
604: error), the controller 20 sets 1 to the reception flag. However, if 
no error is included in the data packet (Step 605: no error), the 
controller 20 stores difference image data included in the data packet 
into the data storing unit 23 (Step 606). 
Then, the controller 20 sets the examination result into reception 
information (sets 1 when an error is detected at Step 605, and sets 0 when 
no error is detected), converts the reception information into a packet, 
and controls the radio unit 8 to transmit it (Step 607). Further, the 
controller 20 increments the reception counter by 1 (Step 608). Next, if 
the reception counter does not reach its utmost value, 20 (Step 609: no), 
the controller 20 returns to Step 603 to repeat the above operation until 
the reception counter reaches 20. 
When the reception counter 20 reaches 20 (Step 609:yes), and the reception 
flag is not 0 (Step 610: error), the controller 20 returns to Step 602 to 
repeat the same operation so as to receive the next image frame. In this 
case, neither the reception frame number nor the content of the image 
storing unit 24 is not updated. 
When the reception flag is 0 (Step 610: no error), and the transmission 
frame number included in the data packet does not coincide with the 
reception frame number held by itself (Step 611: no), the controller 20 
returns to Step 602. 
If they coincide (Step 611: yes), on the other hand, the controller 20 
increments the reception frame number by 1 (Step 612), controls the 
expansion adder 25 to expand difference image data (one image frame) in 
the data storing unit stores it into the image storing unit 24 (Step 613), 
then returns to Step 602. In this case, the reception frame number is 
updated. Therefore, the transmission frame number held by the master 
terminal at Step 611 and the reception frame number held by the 
subordinate terminal remain identical. 
(operation example) 
The operation of the thus constructed high efficiency image transmission 
device (the image transmitting unit and the image receiving unit) is 
described hereunder. 
FIG. 9 shows a transmission and reception sequence of a data packet between 
a master terminal and a subordinate terminal. The left side and right side 
of FIG. 9 represent a master terminal and a subordinate terminal 
respectively. The left side (master terminal) has an image frame received 
by the line interface 3, the content of the image storing unit 4, the 
content of the data storing unit 6, and a transmission frame number. The 
right side (subordinate terminal) has the content of the image storing 
unit 24, the content of the data storing unit 23, and a reception frame 
number. A right pointing arrow (.fwdarw.) represents 20 transmissions of 
data packets which include radio frames, and a left pointing arrow 
(.rarw.) represents 20 transmissions of data packets which include 
reception information. An arrow marked with a cross (x) indicates at least 
one of the 20 transmissions failed because of some trouble such as fading 
or the like; an arrow marked with 0K indicates that all of the 20 
transmissions were completed safely; and an arrow marked with NG indicates 
that an error was included in at least one of 20 reception information. 
Difference image data is enclosed with a parenthesis (). For example (1:2) 
represents a difference between an image frame 1 and an image frame 2. "0" 
in the image storing unit 4 represents initial data. 
As shown in FIG. 9, different operations are carried out in the following 
four situations: (1) no transmission error occurs; (2) an error occurs in 
transmission of a data packet to a subordinate terminal; (3) an error 
occurs in transmission of a data packet to a master terminal; and (4) an 
error occurs both in transmission of a data packet to a subordinate 
terminal and in transmission of a data packet to a master terminal. 
(1) If a transmission frame number coincides with a reception frame number 
(for example, 1st, 5th, and 8th image frame transmissions), the 
subordinate terminal updates the image storing unit 24 and the reception 
frame number; at the same time, the master terminal updates the image 
storing unit 4 and the transmission frame number. Consequently, the same 
reference frame is stored in the master terminal and the subordinate 
terminal (image storing unit 4/image storing unit 24), as well as the same 
number is stored as a transmission frame number and as a reception frame 
number. However, if a transmission frame number does not coincide with a 
reception frame number (for example, for the 4th image frame 
transmission), the master terminal and the subordinate terminal do not 
hold the same reference terminal any longer. Accordingly, the master 
terminal updates only the image storing unit 4, while the subordinate 
terminal updates only the reception frame number. As a result, the same 
reference frame is held by the master terminal and the subordinate 
terminal, and the transmission frame number and the reception frame number 
remain identical. 
(2) When an error occurs in transmission of a data packet to a subordinate 
terminal (for example, 2nd image frame transmission), it seems apparent to 
a master terminal that its subordinate terminal does not update the image 
storing unit 24. Therefore, the master terminal does not update the image 
storing unit 4, and transmits difference image data relating to the next 
image frame. 
(3) When an error occurs in transmission of a data packet to a master 
terminal (for example, 3rd image frame transmission), the master terminal 
is not sure if its subordinate terminal updates the image storing unit 24 
and the reception frame number. Therefore, the master terminal repeats the 
same transmission without updating the image storing unit 4 and the 
transmission frame number. As a result, similarly to (1), the same 
reference frame is held by the master terminal and the subordinate 
terminal, and the transmission and reception numbers remain identical. 
(4) When an error occurs in both transmissions (for example, 6th image 
frame transmission), the same operations as (3) are carried out. 
[Embodiment 2] 
A high efficiency image transmission device (image transmitting unit, image 
receiving unit) in FIG. 1 is applied to a simple digital codeless 
telephone system in a second embodiment. 
(configuration of image transmitting unit) 
FIG. 10 is a block diagram depicting the configuration of an image 
transmitting unit constructed at a master terminal. Like components are 
labeled with like reference numerals with respect to the first embodiment, 
and the description of these components is not repeated. The second 
embodiment is different from the first embodiment in that an image check 
unit 11 is constructed, and the controller 1 is replaced with a controller 
12. 
Controlled by the controller 12, the image check unit 11 divides 
reproduction image data in the image storing unit 4 into a plurality of 
areas, and generates CRC (Cyclic Redundancy Check) code for reproduction 
image data within each area. FIG. 11 shows how reproduction image data in 
the image storing unit 4 is divided by the image check unit 11. The 
reproduction image data in FIG. 11 is QCIF (176pixels.times.144pixels) in 
the first embodiment, and one reproduction image data is divided into 18 
division areas, as well as two division areas for storing dummy data are 
added to them. Thus by constructing two dummy data areas, the total number 
of division areas coincides with the utmost transmission counting number, 
20. Each division area stores 176.times.8 pixels, and it corresponds to a 
counting number at the transmission counter. 
The controller 12 checks if the same reproduction image is held by the 
transmitting unit and the receiving unit according to CRC codes besides 
possessing the same capabilities of the controller 1 in the first 
embodiment. To be concrete, upon each reception of reception information, 
the controller 12 compares CRC codes included in the reception information 
and CRC codes generated by the image check unit 1. If they do not coincide 
with each other, the controller 1 initializes reproduction image data for 
the respective radio frame, and includes an initialization flag into 
transmission information to notify the image receiving unit to initialize 
reproduction image data for the corresponding radio frame. 
FIG. 12 is an exemplary format of transmission information which includes 
an initialization flag; and FIG. 13 is an exemplary format of reception 
information which includes CRC code. The transmission information and the 
reception information in FIGS. 4 and 5 are replaced with the transmission 
information and the reception information in FIGS. 12 and 13 respectively. 
In FIG. 12, an initialization flag is 1-bit data where "0" indicates not 
to initialize reproduction image data while "1" indicates to initialize 
reproduction image in the division area corresponding to a counting 
number-1 at the reception counter. In FIG. 13, CRC code is 8-bit data, and 
an effective flag indicates if 8-bit CRC code is effective or not. 
(detailed control by controller 12) 
FIG. 14 is a flow chart depicting detailed control by the controller 12. 
FIG. 14 is different from FIG. 6 in that the initial flag is set to 0 at 
Step 503, also Steps 1401-1406 are added, and only these three steps are 
described. 
When the receiving unit 9 does not detect any transmission error (Step 507: 
success), and reception information does not include any error (Step 509: 
no error), the following operations are carried out. If the effective flag 
included in the reception information is 1 (Step 1401), the controller 12 
controls the image check unit 11 to generate CRC code for a division area 
in the image storing unit 4 which corresponds to a value at the 
transmission counter (Step 1402), and checks if CRC code included in the 
reception information coincides with the generated CRC code. If they 
coincide with each other (Step 1403: yes), the controller 12 judges that 
the same image data for the division area is stored in the master terminal 
and the subordinate terminal. Accordingly, it resets the initial flag to 0 
(Step 1406). When they coincide with each other (Step 1403: no), on the 
other hand, the controller 12 judges that different image data is stored 
in the master terminal and the subordinate terminal for the same division 
area, so that it initializes reproduction image within the division area 
(which corresponds to a counting number at the transmission counter) in 
the image storing unit 4 (Step 1404), and sets 1 to the initialization 
flag so that the subordinate terminal also initializes reproduction image 
the division area (Step 1405). 
Hereinafter, the same operations as the first embodiment are carried out 
except that the initialization flag is included in transmission 
information. Only such difference will be described. 
(configuration of image receiving unit) 
FIG. 15 is a block diagram showing the configuration of an image receiving 
unit constructed at a subordinate terminal. Like components are labeled 
with like reference numerals with respect to the first embodiment, and the 
description of these components is not repeated. The image receiving unit 
in FIG. 15 is different from the image receiving unit in the first 
embodiment only in that an image check unit 29 is added, and the 
controller 20 is replaced with a controller 30. 
Controlled by the controller 30, the image check unit 29 divides 
reproduction image data in the image storing unit 24 into a plurality of 
areas, and generates CRC (Cyclic Redundancy Check) code for reproduction 
image data within each area. Division of reproduction image data in the 
image storing unit 24 by the image check unit 29 is substantially same as 
that by the image check unit 11 in FIG. 11. 
The controller 30 generates CRC code for a division area corresponding to a 
counting number at the reception counter and sets the CRC code and an 
effective flag into reception information, besides having the same 
capabilities of the controller 12 in the first embodiment. 
(detailed control by controller 30) 
FIG. 16 is a flow chart depicting detailed control by the controller 30. 
FIG. 16 is different from FIG. 8, which describes the operation of the 
controller 12, in that Steps 1601-1606 are added, and only these steps are 
described. 
After storing difference image data for a radio frame included in the 
reception frame into the data storing unit 23 (Step 606), the controller 
30 judges if the initial flag in transmission information is set. When the 
initial flag is set (Step 1601: yes), the controller 30 initializes 
reproduction image data in a division area which corresponds to a counting 
number-1 at the reception counter. When the initial flag is not set (Step 
601: no), the controller 30 does not operate initialization. 
Subsequently, the controller 30 compares a transmission frame number in 
transmission information with a reception frame number in the second 
register. If they coincide with each other (Step 1603: yes), the 
controller 30 sets 1 to the effective flag (Step 1604), controls the image 
check unit 11 to generate CRC code for a division area in the image 
storing unit 24 which corresponds to a counting number at the reception 
counter, and sets the CRC code and the effective flag into reception 
information (Step 1605). If a transmission frame number in transmission 
information does not coincide with a reception frame number in the second 
register (Step 1603: no), on the other hand, the controller 30 sets 0 to 
the effective flag since the content of the image storing unit 4 at the 
master terminal and content of the image storing unit 24 may not be 
identical to each other (Step 1606), and it does not generate CRC code. 
The controller 30 moves to Step 607 to operate the same as the first 
embodiment except that the CRC code and the effective code are set into 
reception information. 
(operation example) 
The operation of the thus constructed high efficiency image transmission 
device (image transmitting unit and image receiving unit) in the second 
embodiment is described. 
A transmission and reception sequence in this embodiment is substantially 
same as the transmission and reception sequence in FIG. 9 except that it 
can be examined if the image storing unit 4 and the image storing unit 24 
are identical in their contents as referring to transmission information 
and reception information. FIG. 17 is a transmission and reception 
sequence which shows transmissions and receptions of 20 data packets at 
the fifth transmission and reception in FIG. 9 (both the image storing 
units 4 and 24 store a reproduction image 3, and both transmission and 
reception frame numbers are 2). 
The left side and the right side of FIG. 9 represent transmission side and 
reception side respectively. The left side has the content of the image 
storing unit 4, and a counting number at the transmission counter. 
Similarly, the right has the content of the image storing unit 24, and a 
counting number at the reception counter. Both the image storing units 4 
and 24 stores the reproduction image 3. CRC (n) on an data packet arrow 
indicates that CRC code for an n-th division area in the image storing 
unit 24 is set into reception information; and I(n) indicates that an 
initial flag which directs initialization of an n-th division area is set 
into transmission information. 
The transmission counter counts 0 immediately after resetting at Step 503. 
In this case, 0 is always set for I(n), that is I(0); and the 
initialization flag does not make any change in practical terms. 
When the transmission counter counts 1, however, 0 is set for I(n) 
depending on the comparing result of CRC code. Therefore, the 
initialization flag implements its purpose. 
In FIG. 17, it is assumed that CRC (3), CRC (4), CRC (17) do not coincide 
with CRC included in reception information. Accordingly, division areas 3, 
4, 17 in the image storing unit 4, which are hatched in FIG. 17, are 
initialized. At reception side, 1 is set to I(3), I(4), and I(17) so that 
the division areas 3, 4, 17 in the image storing unit 24 are also 
initialized. 
Although PHS system links between a master terminal and a subordinate 
terminal in the above embodiments, another radio system may link them. For 
example, when a radio LAN, such as a wave LAN, links a master terminal and 
a subordinate terminal to realize high-speed transmission, more than 3000 
bit difference image data can be transmitted relating to one image frame. 
Also, a radio fame can be divided into less than 20 frames. For example, 
to transmit 10000-bit data, and to generate one division area, the 
transmitting unit 7 generates a data packet which includes 10000-bit 
difference image data; also the controllers 1, 12, 20, and 30 examine if 
the transmission counter and the reception counter count 1 (Step 512 in 
FIGS. 6 and 14, Step 609 in FIGS. 8 and 15). 
Although the transmission counter and the reception counter determine 20 as 
their utmost counting numbers in the above, their utmost counting numbers 
may be determined according to the data transmission amount for one image 
frame, and the transmission rate. 
Step 1404 and Step 1602 in FIG. 16 may be placed after Step 516 and Step 
613 respectively. In this case, a reference result and a counting number 
at the transmission counter are stored at Step 1403, while the 
initialization flag and a counting number at the reception counter are 
stored at Step 1601. 
[Embodiment 3] 
A third embodiment is different from the second embodiment in the following 
operations by controllers 12 and 20. When a transmission error occurs, a 
data packet is re-transmitted basing upon a radio frame unit. Also, a 
match between image storing units 14 and 24 is examined by a subordinate 
terminal. 
The configurations of a master terminal and a subordinate terminal are 
substantially same as the second embodiment in FIGS. 11 and 15, and only 
differences will be described hereinafter. 
FIGS. 18 and 19 show transmission information and reception information 
respectively. In FIGS. 18 and 19, the cell of transmission packet number 
indicates what number of radio frame is included in the data packet, 
counting from the beginning. The cell of image CRC indicates CRC code 
relating to a division area which corresponds to a counting number at the 
transmission counter. The cell of CRC reference bit indicates a result of 
the comparison of CRC codes by a subordinate terminal (0 for OK and 1 for 
NG). The cell of error notification bit is the same as the least 
significant bit of reception information in the second embodiment. 
FIGS. 20 and 21 are flow charts describing detailed control processing by 
the controllers 12 and 30 respectively. A master terminal encloses CRC 
code for each division area into transmission information, and transmits 
it to a subordinate terminal (Steps 2001, 504, 505). Receiving the 
transmission information, the subordinate terminal compares the CRC code 
with CRC code for the corresponding division area within the image storing 
unit 24, then encloses the comparison result into reception information 
and transmits it to the master terminal (Steps 2102, 607). The controllers 
12 and 30 repeat these operations for a single image frame in cooperation. 
When the content of the image storing unit 4 is different from that of the 
image storing unit 24, a counting number at the transmission counter and a 
reference bit at the moment are stored. Accordingly, upon each updating of 
the image storing unit 4, the controller initializes a division area which 
does not coincide with its counterpart as referring to a counting number 
and a reference bit stored (Step 2006). Similarly, the controller 30 
initializes a division area (Step 2106). 
Re-transmission of a data packet is described. When reception information 
includes a reception error, a transmission packet number is not 
incremented. Similarly, when an error is detected, a reception packet 
number is not incremented. As a result, it is possible to re-transmit a 
data packet. 
Thus, re-transmission of a data packet for each, radio frame is realized. 
Also, a match between the image storing units 4 and 24 is assured. 
Although the present invention has been fully described by way of examples 
with reference to the accompanying drawings, it is to be noted that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as being 
included therein.