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BACKGROUND OF THE INVENTION
The present invention generally relates to facsimile machines, and more
particularly to a facsimile machine which has a function of retransmitting
an image data when a transmission error is detected.
Conventionally, there are facsimile machines which transmit an image data
in conformance with an error correction mode (ECM) of a Group III standard
recommended by the CCITT. It will be assumed for convenience' sake that
one page of a document shown in FIG. 1A is read on a source facsimile
machine. When coded data dl through dn of first through nth lines are
obtained, an end of line (EOL) code indicating an end of a line is added
to each of the coded data dl through dn. The EOL code is used for
synchronizing each line. According to the ECM, the 256 bytes or 64 bytes
of the image data are set at a time into a high level data link control
(HDLC) frame having a predetermined format, and a maximum of 256 frames
are transmitted as one block of the image data.
A destination facsimile machine checks a data error in the received image
data for each frame, and sends back a retransmission request to the source
facsimile machine when a data error is detected so as to request a
retransmission of a frame in which the data error exists. Hence, the
source facsimile machine retransmits the image data of the requested frame
in response to the retransmission request.
Recently, a facsimile communication is made through a mobile telephone set
and a multi channel access (MCA) radio telephone set. In such a case, a
radio line is used for the facsimile communication and thus a transmission
error easily occurs when the state of the radio line deteriorates.
Accordingly, when the facsimile communication is made in the ECM under
such a condition, it becomes necessary to repeatedly retransmit the image
data many times.
Generally, a data transmission rate is shifted down in the ECM every time
the retransmission is repeated three times. When the retransmission is
repeated three times at a minimum data transmission rate of 24000 bps, the
facsimile transmission is ended. On the other hand, when the transmission
is unsuccessful even when the same frame is retransmitted three times, the
retransmission of this frame is discontinued and a next predetermined
transmission is made such as transmitting an end signal in conformance
with a protocol of the facsimile machine.
It will be assumed that the destination facsimile machine receives one page
of the document in one block and the transmission of a specific frame
f.sub.s is discontinued, for example. In this case, the image amounting to
this specific frame f.sub.s is not recorded, but instead a blank or space,
for example, is formed at a position of the erroneous frame which contains
the data error as shown in FIG. 1B. Line information of a large number of
lines are transmitted within one frame, but conventionally, the image
corresponding to the entire frame drops out even when the data error
occurs in only one line of the frame. Thus, there is a problem in this
case that a length L.sub.1 of the recorded image shown in FIG. 1B recorded
on the destination facsimile machine is greatly reduced compared to a
length L.sub.0 of the original image shown in FIG. 1A transmitted on the
source facsimile machine.
On the other hand, according to the ECM, one block of the image data has a
data quantity of 64 kbytes when one frame has 256 bytes because one block
has a maximum of 256 frames. In a case where the image data of one page of
the document exceeds the data quantity of one block, this one page of the
document is transmitted in divisions in a plurality of blocks as partial
pages. For example, when one page of the document is transmitted in
divisions in two partial pages, the retransmission may be ended during the
transmission of a first partial page in a state where the error correction
is not completed. In such a case, the image not only drops out in a first
half of one page as shown in FIG. 1C but a latter half of this page will
not be received, and there is a problem in that one page of the document
cannot be recorded.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a
novel and useful facsimile machine in which the problems described above
are eliminated.
Another and more specific object of the present invention is to provide a
facsimile machine comprising modem means coupled to a data transmission
path for modulating a transmitting image data which is transmitted to the
data transmission path and for demodulating received image data which is
received from the data transmission path, where the transmitting image
data and the received image data are transmitted and received in frames
respectively including image data related to a plurality of lines and
error correction codes for use in detecting an error in the image data of
each of the lines, communication control means coupled to the modem means
for controlling communications to and from the data transmission path,
reading means for reading a document image which is to be transmitted and
for outputting the transmitting image data describing the document image,
recording means for recording an image described by the received image
data onto a recording sheet, memory means for temporarily storing the
transmitting image data and the received image data, and system control
means for controlling operations of the communication control means, the
reading means and the recording means. The system control means includes
first error detecting means for detecting an error in the received image
data related to an arbitrary frame by detecting the error correction code,
discriminating means for discriminating whether or not a retransmission
request of the arbitrary frame is permitted, retransmission means for
controlling the communication control means to output a retransmission
request only when the discriminating means discriminates that the
retransmission request is permitted, second error detecting means for
detecting whether or not a data error exists in the image data related to
the lines within the arbitrary frame for each of the lines within the
arbitrary frame when the discriminating means discriminates that the
retransmission is not permitted, and enabling means for enabling the
recording means to record the image data related to each of the lines in
which no data error is detected in the second error detecting means, so
that the image data related to the arbitrary frame is recorded on the
recording means even when an error is detected therein. According to the
facsimile machine of the present invention, the error detection is made
with respect to each of the lines within the frame when the retransmission
request is not permitted and a data error is detected in the frame, and
the error-free lines are successively recorded. Hence, it is possible to
minimize a dropout of image information when the transmission error
occurs, and it is also possible to minimize a reduction in a length of the
recorded image caused by the transmission error compared to a length of
the original image.
Still another object of the present invention is to provide a facsimile
machine wherein the data transmission path is a radio line, said
transmitting image data and said received image data are transmitted and
received in frames with a data format of a high level data link control
(HDLC), and a number of bytes set in one high level data link control
(HDLC) frame is less than 64 bytes. According to the facsimile machine of
the present invention, it is possible to reduce the data error rate when
the retransmission is ended.
A further object of the present invention is to provide a facsimile machine
wherein the system control means further includes first counting means for
counting a number of retransmission requests received, second counting
means for counting a number of retransmission requests made, first
disabling means for disabling a retransmission to be made when the number
in the first counting means reaches a predetermined value, where the
discriminating means includes means for discriminating that the
retransmission request is permitted when the number in the second counting
means is less than the predetermined value, page discriminating means for
discriminating whether or not one block of the received image data relates
to a last partial page of the document image, and second disabling means
for disabling the retransmission means by prohibiting a retransmission
request to be made when the number in the second counting means is one
less than the predetermined value and the page discriminating means
discriminates that one block of the received image data does not relate to
the last partial page of the document image, so that a next block of the
image data is received. According to the facsimile machine of the present
invention, it is always possible to record one page of the document image
and the dropout of the image information is minimized.
Another object of the present invention is to provide a facsimile machine
wherein the memory means stores the received image data related to a frame
and image data related to the same frame and received in response to each
retransmission request until one of the image data related to the same
frame is read out and recorded on the recording means, and the system
control means further includes means for reading out from the memory means
portions of the image data related to the same frame in which no error is
detected by the first error detecting means for being recorded on the
recording means when the retransmission is ended before a error-free image
data is received and detected by the first error detecting means.
According to the facsimile machine of the present invention, it is
possible to obtain substantially error-free image data.
A further object of the present invention is to provide a facsimile machine
wherein the memory means stores the received image data related to a frame
and image data related to the same frame and received in response to each
retransmission request, and the system control means further includes
correcting means for correcting errors in the image data stored in the
memory means and for outputting a corrected image data related to the same
frame for being recorded on the recording means when the retransmission is
ended before an error-free image data is received and detected by the
first error detecting means. According to the facsimile machine of the
present invention, it is possible to correct the errors in the image data
by a majority operation, for example.
Other objects and further features of the present invention will be
apparent from the following detailed description when read in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a diagram for explaining a document image and an image data
thereof;
FIGS. 1B and 1C respectively show an image recorded on a conventional
facsimile machine for explaining the problems thereof;
FIG. 2 is a system block diagram showing a first embodiment of a facsimile
machine according to the present invention;
FIGS. 3A and 3B respectively are diagrams for explaining a frame structure
of a frame of an image data
FIGS. 4A and 4B respectively are flow charts for explaining an operation of
a system control part for carrying out a reception process in the first
embodiment;
FIG. 5 shows an image recorded on the first embodiment when a frame
contains a data error;
FIG. 6 is a diagram for explaining procedure signals transmitted at a start
of a communication for explaining a second embodiment of the facsimile
machine according to the present invention;
FIG. 7 is a flow cart for explaining an operation of the system control
part for carrying out a transmission process in the second embodiment;
FIG. 8 is a diagram showing an erroneous frame which is received during a
retransmission process;
FIG. 9 is a flow chart for explaining an operation of the system control
part for carrying out a reception process in the second embodiment;
FIG. 10 is a flow chart for explaining an operation of the system control
part for carrying out a retransmission process in the second embodiment;
FIG. 11 shows an image recorded on the second embodiment when the erroneous
frame exists;
FIG. 12 is a graph showing a communication time versus a data error rate
characteristic of the second embodiment;
FIGS. 13A through 13C are diagrams for explaining data stored in a memory
for explaining an operation of a third embodiment of the facsimile machine
according to the present invention;
FIG. 14 is a flow chart for explaining an operation of the system control
part for carrying out a reception process in the third embodiment;
FIG. 15 is a flow chart for explaining an operation of the system control
part for carrying out an acknowledge process in a fourth embodiment of the
facsimile machine according to the present invention;
FIGS. 16A through 16C respectively are diagrams for explaining a
transmission procedure of the fourth embodiment;
FIG. 17 is a system block diagram showing a fifth embodiment of the
facsimile machine according to the present invention;
FIG. 18 is a diagram for explaining a frame structure of a frame of an
image data transmitted and received in the fifth embodiment;
FIGS. 19A and 19B respectively are flow charts for explaining an operation
of the system control part for carrying out a reception process in the
fifth embodiment;
FIGS. 20A through 20C are diagrams for explaining the storage of the image
data in the fifth embodiment;
FIGS. 21A and 21B respectively are flow charts for explaining an operation
of the system control part for carrying out a decoding and recording
process in the fifth embodiment;
FIG. 22 is a diagram for explaining a switching of a memory area in the
fifth embodiment;
FIG. 23 is a diagram for explaining a read-out position of a next image
data after the switching of the memory area;
FIG. 24 is a flow chart for explaining an operation of the system control
part for carrying out a reception process in a sixth embodiment of the
facsimile machine according to the present invention; and
FIG. 25 is a diagram for explaining a majority operation carried out in the
sixth embodiment.
DETAILED DESCRIPTION
FIG. 2 shows a first embodiment of a facsimile machine according to the
present invention. This embodiment is especially suited for use with a
mobile telephone (radiophone) set. In FIG. 2, a facsimile machine 1 is
coupled to a mobile telephone set 3 through an interface circuit 2. A
handset 4 for controlling functions of the mobile telephone set 3 and for
making a conversation is coupled to the interface circuit 2.
In the facsimile machine 1, a scanner 11, a plotter 12, an encoding and
decoding part 13, a memory 14, a communication control part 16, an
operation and display part 17 and a system control part 18 are coupled to
each other through a bus 19. In addition, a modem 15 is coupled to the
communication control part 16.
The scanner 11 reads a document image and outputs an image data having a
predetermined resolution. The image data is supplied to the plotter 12
which records an image described by the image data onto a recording sheet.
The encoding and decoding part 13 encodes the image data which is to be
transmitted to a destination facsimile machine when the facsimile machine
1 is a source and decodes the image data which is received from a source
facsimile machine when the facsimile machine 1 is a destination. The
memory 14 temporarily stores the image data. The modem 15 modulates the
image data which is to be transmitted to the interface circuit 2 and
demodulates the image data which is received through the interface circuit
2. The modem 15 also transmits various procedure signals of transmission
control procedures. The communication control part 16 controls a
communication in the ECM and makes a predetermined facsimile
communication. The operation and display part 17 has an operation portion
for setting various operations of the facsimile machine 1 and a display
portion for displaying modes of the facsimile machine 1, guidances for
operating the facsimile machine 1 and the like. The system control part 18
is made up of a microcomputer, for example, and controls the scanner 11,
the plotter 12, the encoding and decoding part 13, the memory 14, the
communication control part 16 and the operation and display part 17 so as
to carry out a predetermined operation on the facsimile machine 1.
When the facsimile machine 1 is used as the destination for receiving the
image data from the source, an operator confirms by a conversation through
the handset 4 that he is ready to receive a document image and then pushes
a receive button (not shown) provided on the operation portion of the
operation and display part 17. As a result, a facsimile transmission is
started in the ECM according to a predetermined transmission control
procedure.
In the ECM, the image data is transmitted in frames with the data format of
the HDLC frame. In this case, one frame of the image data has a frame
structure shown in FIG. 3A. One frame includes a flag sequence F, an
address field A, a control field C, a facsimile control field FCF, a
facsimile information field FIF, a frame check sequence FCS and a flag
sequence F. An information FCD which indicates that the frame relates to
the image data is set in the facsimile control field FCF. A frame number
NO and a coded image data CD are set in the facsimile information field
FIF. In addition, an error correction code CRC is set in the frame check
sequence FCS.
A maximum of 256 frames respectively having the frame structure shown in
FIG. 3A are arranged and information frames related to a preamble and a
transmission control are added thereto to constitute one block.
As shown in FIG. 3B, 256 bytes or 64 bytes of coded image data d.sub.1
through dn are set at a time in the image data CD of each of frames
f.sub.1 through f.sub.m within one block, where the coded image data
d.sub.1 through d.sub.n are related to each of the lines respectively
sectioned by the EOL code. A return to control (RTC) code which indicates
the end of the image data is set in the image data CD of the last frame
f.sub.m, and in addition, dummy data containing all "0"s are added to the
RTC code so as to make the number of bits a predetermined value.
FIGS. 4A and 4B are flow charts for explaining an operation of the system
control part 18 for carrying out a reception process in the first
embodiment, that is, when the facsimile machine 1 is the destination. In
FIG. 4A, a step 101 receives one block of the image data described above
and stores the image data in a data buffer (not shown). This data buffer
may be a part of the system control par 18, for example. Then, a step 102
reads out one frame of the image data and makes an error detection using
the CRC code in the frame. A step 103 discriminates whether or not the one
frame is error-free, that is, no data error exists in the one frame. When
the discrimination result in the step 103 is YES, a step 104 stores the
one frame of the image data into the memory 14 and a step 105
discriminates whether or not the process is completed for all of the
frames within the one block. The process returns to the step 102 so as to
make an error detection with respect to another frame when the
discrimination result in the step 105 is NO.
In this embodiment, it is assumed for convenience' sake that the
transmission process of the source is ended when retransmissions are made
three times at a minimum data transmission rate of 2400 bps and that the
retransmission of a frame is discontinued to make a next predetermined
transmission such as transmitting an end signal in conformance with a
protocol of the facsimile machine when the transmission is unsuccessful
even when the same frame is retransmitted three times. In this case, the
source transmits an end of retransmission (EOR) signal and informs the end
of the retransmission to the facsimile 1 after transmitting the image data
of a last block. The EOR signal is generated in the communication control
part 16.
On the other hand, when a data error exists in the one frame and the
discrimination result in the step 103 is NO, a step 106 discriminates
whether or not the frame is a last frame before the end of the
retransmission. When the discrimination result in the step 106 is NO, a
step 107 sets a predetermined bit of a frame data of a partial page
request (PPR) signal which is transmitted with the retransmission request
to "1" and stores an information indicating that the retransmission
request of this frame is necessary. The PPR signal is generated in the
communication control part 16.
When the above described process is carried out with respect to all of the
frames within one block, the discrimination result in the step 105 becomes
YES and a retransmission request is made if needed. In other words, a step
108 discriminates whether or not the retransmission request is to be made,
and the discrimination result in the step 108 becomes YES when the
retransmission is not to be ended and one of the bits of the frame data of
the PPR signal is set to "1". In this case, a step 109 transmits a
predetermined PPR signal to the source and makes the transmission request.
The source retransmits the image data of the requested frame in one block,
and the facsimile machine 1 which is the destination receives and checks
each frame of the received block. The correct image data free of data
errors are stored in the memory 14. But when a data error again exists in
the retransmitted frame, the frame data of the PPR signal is set with
respect to this frame containing the data error. Then, after the process
is carried out for all of the frames within the block, the retransmission
request is made again and the source carries out the retransmission
process in response to this retransmission request.
When the retransmission process is carried out and the retransmission of
the same frame is made three times, for example, the EOR signal indicating
the end of the retransmission is transmitted from the source. In this
case, the discrimination result in the step 106 becomes YES and the
process advances to a step 110 shown in FIG. 4B. The step 110 obtains one
line of the image data from the frame by detecting the EOL code, and a
step 111 discriminates whether or not the line is a last line. When the
discrimination result in the step 111 is NO, a process is carried out to
discriminate whether or not a data error exists in the image data of the
line.
In this embodiment, a modified Huffman (MH) coding scheme is employed as
the data compression (encoding) system. According to the MH coding scheme,
one line of the image data is made up of a termination code, a make up
code, the EOL code, a return to control (RTC) code and a dummy data
containing all "0"s. When decoding the image data, the termination code
and the make up code are subjected to a code conversion by referring to a
code table so as to obtain the original image data having a predetermined
number of bits.
When a data not included in the code table exists within one line of the
image data, when the make up code is continuous, or when the EOL code
exists next to the make up code, it may be judged that a data error
exists. It may also be judged that a data error exists when the original
image data obtained by the decoding does not have the predetermined number
of bits.
Next, in this embodiment, the one line of the image data is decoded under
the following four conditions so as to detect the data error. Firstly, a
step 112 discriminates whether or not there exists an image data included
in the code table. When the discrimination result in the step 112 is YES,
a step 113 discriminates whether or not the make up code is discontinuous.
When the discrimination result in the step 113 is YES, a step 114
discriminates whether or not the EOL code exists not next to the make up
code. When the discrimination result in the step 114 is YES, a step 115
discriminates whether or not the original image data obtained by the
decoding has the predetermined number of bits. A step 116 stores the image
data of the one line into the memory 14 when the discrimination result in
the step 115 is YES.
On the other hand, when the discrimination result in any of the steps 112
through 115 is NO, a step 117 stores a white image data amounting to one
line in the memory 14 in place of the image data amounting to one line and
containing the data error.
Thereafter, the process returns to the step 110 and a similar process is
repeated until the image data of each of the lines within the one frame
and free of data errors are stored in the memory 14.
In many cases, the image data of the last line is divided between two
successive frames and may not amount to one line in one frame. For this
reason, measures are taken so that the above described process is not
carried out with respect to the last line. In other words, when the
discrimination result in the step 111 is YES in FIG. 4B, the process
advances to the step 105 shown in FIG. 4A so as to discriminate whether or
not the process is completed for all of the frames within the one block.
When the discrimination result in the step 105 is NO, the process returns
to the step 102 and the process described before is repeated.
When the process with respect to the one block is completed and the
discrimination result in the step 105 becomes YES and the discrimination
result in the step 108 becomes NO, a step 118 shown in FIG. 4A
successively reads out the image data stored in the memory 14, decodes the
read out image data and records the image on the plotter 12. A step 119
discriminates whether or not a next block is received after the recording
of the image on the plotter 12. The process advances to the step 101 to
receive this next block when the discrimination result in the step 119 is
YES. But the process is ended when there is no next block and the
discrimination result in the step 119 is NO. In other words, the reception
process is ended when the last block is received.
Suppose that the image shown in FIG. 1A described before is transmitted
from the source and the correct image data is not received by the
facsimile machine 1 even when the retransmission of the specific frame
f.sub.s is made three times. In this case, the image data of the lines
which are within the frame f.sub.s and contain no data error are recorded
as shown in FIG. 5 in this embodiment. As a result, it is possible to
record to a certain extent the image of the frame f.sub.s containing the
data error.
According to this embodiment, when a data error is detected within one
frame which is received during the last retransmission process, the
existence of the data error is checked for each of the lines within the
frame. Those lines having no data error are recorded. Therefore, a dropout
of the image due to the data error is considerably reduced compared to the
image recorded on the conventional facsimile machine as may be seen by
comparing FIGS. 5 and 1B. In addition, it may be seen from these figures
that a reduction in the length of the recorded image due to the data error
is considerably reduced compared to the image recorded on the conventional
facsimile machine.
Next, a description will be given of a second embodiment of the facsimile
machine according to the present invention. In the first embodiment, 256
bytes or 64 bytes of the image data to be transmitted are set in one HDLC
frame as recommended by the CCITT. However, in this second embodiment, 32
bytes of the image data to be transmitted are set in one HDLC frame. The
block system shown in FIG. 2 can also be employed in this second
embodiment.
As shown in FIG. 6, the facsimile machine 1 which is the destination
transmits a called station identification (CED) signal, a non-standard
facilities (NSF) signal and a digital identification signal (DIS) at a
start of a communication. The CED signal informs the source that the
destination is ready to receive the transmission, and the NSF and DIS
signals inform the functions of the destination to the source. The CED,
NSF and DIS signals are generated in the communication control part 16.
When transmitting these signals CED, NSF and DIS, the destination informs
the functions thereof to the source by the NSF signal such as a function
of receiving the image data 32 bytes at a time. On the other hand, the
source discriminates the functions of the destination based on the signal
NSF, and transmits a non-standard facilities set-up signal (NSS) which
informs the destination the conditions under which the transmission will
be made. In this embodiment, the NSS informs the destination that a data
quantity of the image data is 32 bytes per frame. The NSS signal is
generated in the communication control part 16.
FIG. 7 is a flow chart for explaining an operation of the system control
part 18 shown in FIG. 2 for carrying out a transmission process in the
second embodiment, that is, when the facsimile machine 1 is the source.
After a predetermined transmission control procedure, a step 201 shown in
FIG. 7 reads each line of the document image which is to be transmitted,
and a step 202 successively encodes the image data of the lines. Then, a
step 203 adds the EOL code and the RTC code to the (coded) image data of
each line, and a step 204 temporarily stores the (coded) image data in the
memory 14.
A step 205 reads out from the memory 14 the image data amounting to one
frame, that is, 32 bytes as shown in FIG. 8 and a step 206 constitutes by
the read out image data predetermined frames with the data format of the
HDLC frame. A step 207 transmits from the modem 15 the image data in
frames with the data format of the HDLC frame. Next, a step 208
discriminates whether or not other image data exist. The process returns
to the step 205 when the discrimination result in the step 208 is YES. On
the other hand, when only one block of image data is to be transmitted and
the discrimination result in the step 208 is NO, a step 209 transmits a
partial page signal (PPS) and an end of procedure (EOP) signal to inform
the destination of the end of the image data, that is, the transmission of
the last page is completed. The EOP signal is generated in the
communication control part 16.
FIG. 9 is a flow chart for explaining an operation of the system control
part 18 shown in FIG. 2 for carrying out a reception process in the second
embodiment, that is, when the facsimile machine 1 is the destination. In
FIG. 9, those steps which are substantially the same as those
corresponding steps in FIGS. 4A and 4B ar designated by the same reference
numerals, and the illustration of steps identical to those shown in FIG.
4A will be omitted.
In FIG. 9, the step 106 is carried out when the discrimination result in
the step 106 shown in FIG. 4A is NO. When the discrimination result in the
step 106 is YES, a step 120 stores a white image data amounting to one
line in the memory 14 in place of the image data amounting to one line and
containing the data error and the process advances to the step 105 shown
in FIG. 4A. The rest of the process is identical to that described before
in conjunction with FIG. 4A.
Accordingly, when a transmission error occurs and a data error is detected
in the received image data, the PPR signal is transmitted to request a
retransmission of the image data of the frame in which the data error is
detected.
FIG. 10 is a flow chart for explaining an operation of the system control
part 18 shown in FIG. 2 for carrying out a retransmission process in the
second embodiment, that is, when the facsimile machine 1 is the source. In
FIG. 10, a step 301 receives the PPR signal from the destination, and a
step 302 reads out from the memory 14 the corresponding image data of the
frame which is requested from the destination to be retransmitted.
Similarly to the process described before, a step 303 constitutes by the
read out image data predetermined frames with the data format of the HDLC
frame, and a step 304 transmits from the modem 15 the image data in frames
with the data format of the HDLC frame.
In this embodiment, the retransmission process is ended after the
retransmission is made a predetermined number of times under a
predetermined condition. Hence, a step 305 discriminates the condition,
that is, whether or not it is a last retransmission process. When the
discrimination result in the step 305 is NO, a process 306 transmits the
PPS and the EOP signal to indicate the end of the image data.
On the other hand, when a plurality of transmission errors occur and the
retransmission process is repeated, the discrimination result in the step
305 becomes YES. In this case, a step 307 transmits the EOR signal and the
EOP signal to inform the end of the retransmission to the destination.
The destination receives the last image data and a data error exists
therein, the discrimination result in the step 103 shown in FIG. 4A is NO
and the discrimination result in the step 106 shown in FIG. 9 is YES. In
this case, the step 120 shown in FIG. 9 stores the white image data
amounting to one line in the memory 14 in place of the image data
amounting to one line and containing the data error and the process
advances to the step 105 shown in FIG. 4A.
When the processing is carried out for all of the frames within the one
block and the discrimination result in the step 105 shown in FIG. 4A
becomes YES, the discrimination result in the step 108 is NO because no
retransmission request can be made. Hence, the step 118 successively reads
out the image data stored in the memory 14 and records the received image
on the recording sheet.
For example, when a data error is detected in a specific frame f.sub.s
during the last retransmission process, the image of this frame f.sub.s
drops out as shown in FIG. 11.
When a transmission error exists in even one bit of the data within one
frame, the entire frame is conventionally not recorded because it is
regarded as an erroneous frame. However, in this embodiment, the image
data transmitted in one frame is 32 bytes which is small compared to the
64 bytes or 256 bytes employed conventionally. Accordingly, when only a
slight bit error exists, an image portion corresponding to the erroneous
frame f.sub.s is extremely small compared to the entire document image.
For this reason, the image portion which drops out due to the bit error is
extremely small and a length L.sub.2 of the recorded image is only
slightly shorter than the length L.sub.0 of the original document image.
The present inventor has conducted experiments by making an image
transmission in the ECM in a state where a transmission error is likely to
occur on the line. By such experiments, a communication time and a
reduction in a data error rate observed when the retransmission process is
carried out were measured. FIG. 12 shows results of the experiments, where
the abscissa indicates the communication time and the ordinate indicates
the data error rate. It may be seen from FIG. 12 that the data error rate
gradually decreases with time after the start of the communication due to
the retransmission process as indicated by a characteristic I when the
image data quantity per frame is a large value such as 256 bytes. On the
other hand, when the image data quantity is a small value such as 32
bytes, the data error rate rapidly decreases after the start of the
communication but it takes time until the data error rate becomes zero as
indicated by a characteristic II.
Normally, the retransmission is ended under a predetermined condition.
Thus, when the retransmission process is ended at a time t in FIG. 12, for
example, the data error rate is e1 when the image data quantity per frame
is 256 bytes and is e2 when the image data quantity per frame is 32 bytes,
for example. Therefore, when the image data quantity per frame is set to a | | |
