System and method for transmitting image data on a telephone network or equivalent

What is claimed is:

1. A method for transmitting image data on a telephone network, comprising the steps of:

(a) establishing a telephone circuit between apparatuses of a sending party and a receiving party through said telephone network;

(b) sending the image data from the apparatus of the sending party to the apparatus of the receiving party regardless of a presence and regardless of an absence of a response from the apparatus of the receiving party to the apparatus of the sending party after sending a tone signal having a specified frequency for a predetermined time period in response to an image data sending command; and

(c) receiving in the apparatus of the receiving party the image data being sent from the apparatus of the sending party only when the specified tone signal is continuously received for the predetermined time period.

2. A method in accordance with claim 1, wherein said step (b) includes a step of generating a DTMF signal as said specified tone signal.

3. A method in accordance with claim 1, further comprising the steps of

(d) sending at least one command signal for designating a transmission mode from the apparatus of the sending party to the apparatus of the receiving party; and

(e) returning an acknowledge signal or negative acknowledge signal from the apparatus of the receiving party to the apparatus of the sending party when said command signal is received, said steps (d) and (e) being executed before said step (b).

4. A method in accordance with claim 3, further comprising the steps of (f) sending said image data in accordance with a transmission mode which is designated by said command signal from the apparatus of the sending party when the acknowledge signal is returned in said step (e).

5. A method in accordance with claim 4, further comprising the steps of (g) sending said image data through a predetermined transmission mode from the apparatus of the sending party when the negative acknowledge signal is returned in said step (e) or when no responsive signal is returned in said step (e).

6. A method in accordance with claim 3, wherein in the case where confirmation of the transmission mode has been made between the apparatus of the both parties in said steps (d) and (e), after said image data sending command, said steps (b) and (c) are executed without executing said steps (d) and (e).

7. A method in accordance with claim 5, wherein said predetermined transmission mode is a monochrome image data transmission mode.

8. A method in accordance with claim 3, wherein the step of sending includes sending a plurality of command signals for designating respective transmission modes, at least two tone signals which are different from each other being utilized respectively as two of said command in signals.

9. A method in accordance with claim 8, wherein the tone signals are DTMF signals.

10. A method in accordance with claim 5, wherein command data is sent as said command signal in said step (d), and further comprising decoding the command data.

11. A method in accordance with claim 10, further comprising the steps of (h) sending a tone signal having a specified frequency from the apparatus of the sending party prior to said step (d).

12. A method in accordance with claim 11, wherein the tone signal having said specified frequency is a DTMF signal.

13. A method in accordance with claim 1, further comprising the steps of

(i) sending a request for sending the image data from the apparatus of the receiving party to the apparatus of the sending party; and

(j) returning the image data in response to the request for sending the image data from said apparatus of the receiving party to said apparatus of the sending party when a polling mode is allowed in said apparatus of the receiving party.

14. A method for transmitting image data on a telephone network, comprising the steps of:

(a) establishing a telephone circuit between apparatuses of both parties through said telephone network;

(b) sending a tone signal having a specified frequency from any one of the apparatuses of the both parties; and

(c) sending the image data regardless of a presence and regardless of an absence of a response from the apparatus of the receiving party to the apparatus of the sending party after the sending of the tone signal.

15. A method for transmitting image data in a video or visual telephone system, comprising the steps of:

(a) establishing a telephone circuit between video or visual telephone of both parties through a telephone network;

(b) sending the image data from the video or visual telephone of the sending party to the video or visual telephone of the receiving party regardless of a presence and regardless of an absence of a response from the apparatus of the receiving party to the apparatus of the sending party after a specified DTMF signal is sent for a predetermined time period in response to an image data sending command; and

(c) receiving in the video or visual telephone of the receiving party the image data being sent from the video or visual telephone of the sending party only when the specified DTMF signal is continuously received for the predetermined time period.

16. A method in accordance with claim 15, further comprising the steps of

(d) sending at least one command signal for designating a transmission mode from the video or visual telephone of the sending party to the video or visual telephone of the receiving party; and

(e) returning an acknowledge signal or negative acknowledge signal from the video or visual telephone of the receiving party to the video or visual telephone of the sending party when said command signal is received, said steps (d) and (e) being executed before said step (b).

17. A method in accordance with claim 16, further comprising the steps of (f) sending said image data in accordance with a transmission mode which is designated by said command signal from the video or visual telephone of the sending party when the acknowledge signal is returned in said step (e) or when no responsive signal is returned in said step (e).

18. A method in accordance with claim 17, further comprising the steps of (g) sending said image data through a predetermined transmission mode from the video or visual telephone of the sending party when the negative acknowledge signal is returned in said step (e).

19. A method in accordance with claim 17, wherein in the case where confirmation of the transmission mode has been made between the video or visual telephones of the both parties in said steps (d) and (e), after said image data sending command, said steps (b) and (c) are executed without executing said steps (d) and (e).

20. A method in accordance with claim 18, wherein said predetermined transmission mode is a monochrome image data transmission mode.

21. A method in accordance with claim 16, wherein the step of sending includes sending a plurality of command signals for designating respective transmission modes, at least two tone signals which are different from each other being utilized respectively as two of said command signals.

22. A method in accordance with claim 18, wherein command data is sent as said command signal in said step (d), and further comprising decoding the command data.

23. A method in accordance with claim 22, further comprising the steps of (h) sending a specified DTMF signal from the video or visual telephone of the sending party prior to said step (d).

24. A method in accordance with claim 15, further comprising the steps of

(i) sending a request for sending the image data from the video or visual telephone of the receiving party to the video or visual telephone of the sending party; and

(j) returning the image data in response to the request for sending from said video or visual telephone of the receiving party to said video or visual telephone of the sending party when a polling mode is allowed in said video or visual telephone of the receiving party.

25. A method for transmitting image data in video or visual telephone system, comprising the steps of:

(a) establishing a telephone circuit between video or visual telephones of both parties through a telephone network;

(b) sending a specified DTMF signal from any one of the video or visual telephones of the both parties; and

(c) sending the image data regardless of a presence and regardless of an absence of a response from the apparatus of the receiving party to the apparatus of the sending party after the sending of the specified DTMF signal.

26. A system for transmitting image data on a telephone network, comprising:

means for establishing a telephone circuit between apparatuses for both parties through said telephone network or equivalent;

means for sending a tone signal having a specified frequency from any one of the apparatuses of the both parties prior to sending the image data from any one of the apparatuses of the both parties; and

means for sending the image data regardless of a presence and regardless of an absence of a response from the apparatus of the receiving party to the apparatus of the sending party after sending the tone signal.

27. A system for transmitting image data on a telephone network, comprising:

first means for establishing a telephone circuit between the apparatuses of both parties through said telephone network;

second means for sending the image data from the apparatus of the sending party to the apparatus of the receiving party regardless of a presence and regardless of an absence of a response from the apparatus of the receiving party after sending a specified signal for a predetermined time period in response to an image data sending command; and

third means for receiving in the apparatus of the receiving party the image data being sent from the apparatus of the sending party.

28. A system in accordance with claim 27, wherein said third means includes means for receiving the image data only when the specified signal is continuously received for a predetermined time period.

29. A method of transmitting image data on a telephone network, comprising the steps of:

(a) establishing a telephone circuit between an apparatus of a sending party and an apparatus of a receiving party through said telephone network;

(b) sending the image data from the apparatus of the sending party to the apparatus of receiving party regardless of a presence and regardless of an absence of a response from the apparatus of the receiving party to the apparatus of the sending party after sending a specified DTMF signal for a predetermined time period in response to an image data sending command;

(c) starting a modem in the apparatus of the receiving party only when the specified DTMF signal is continuously received for the predetermined time period; and

(d) receiving the image data being sent from the apparatus of the sending party through the modem which has been started in the step (c) in the apparatus of the receiving party.

30. A method of transmitting image data on a telephone network, comprising the steps of:

(a) establishing a telephone circuit between apparatuses of two parties through said telephone network;

(b) sending a specified DTMF signal from any one of the apparatuses;

(c) sending the image data regardless of a presence and regardless of an absence of a response from the other of the apparatuses after sending the specified DTMF signal; and

(d) starting a modem of the other one of the apparatuses only when the specified DTMF signal is continuously received for a predetermined time period.

31. A method of transmitting image data in video or visual telephone system, comprising the steps of:

(a) establishing a telephone circuit between video or visual telephones of a sending party and a receiving party through a telephone network;

(b) sending the image data from the video or visual telephone of the sending party to the video or visual telephone of the receiving party regardless of a presence and regardless of an absence of a response from the apparatus of the receiving party to the apparatus of the sending party after sending a specified DTMF signal for a predetermined time period in response to an image data sending command;

(c) starting a modem of the video or visual telephone of the receiving party only when the specified DTMF signal is continuously received for the predetermined time period; and

(d) receiving the image data sent from the video or visual telephone of the sending party through the modem of the video or visual telephone of the receiving party.

32. A method of transmitting image data in video or visible telephone system, comprising the steps of:

(a) establishing a telephone circuit between video or visible telephones of two parties through said telephone network;

(b) sending a specified DTMF signal from any one of the video or visible telephones prior to sending image data from any one of the video or visible telephones;

(c) sending the image data regardless of a presence and regardless of an absence of a response from the other of the video or visible telephones after sending the specified DTMF signal; and

(d) starting a modem of the other one of the video or visible telephones only when the specified DTMF signal is continuously received for a predetermined time period.

33. A system of transmitting image data on a telephone network, comprising:

means for establishing a telephone circuit between apparatus for two parties through said telephone network;

means for sending a specified DTMF signal from any one of the apparatuses prior to sending image data from any one of the apparatuses;

means for sending the image data from any one of the apparatus regardless of a presence and regardless of an absence of a response from the other of the apparatuses; and

means for starting a modem of the other one of the apparatuses only when the specified DTMF signal is continuously received for a predetermined time period.

34. A system of transmitting image data on a telephone network, comprising

first means for establishing a telephone circuit between apparatus of a sending party and an apparatus of a receiving party through said telephone network;

second means for sending the image data from the apparatus of the sending party to the apparatus of the receiving party regardless of a presence and regardless of an absence of a response from the apparatus of the receiving party after sending a specified DTMF signal for a predetermined time period in response to an image data sending command;

third means for starting a modem of the apparatus of receiving party only when the specified DTMF signal is continuously received for the predetermined time period; and

fourth means for receiving the image data being sent from the apparatus of the sending party through the modem which has been started in the apparatus of the receiving party.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to a system and method for transmitting image data on a telephone network or equivalent. More specifically, the present invention relates to a novel transmission control procedure which is suitable for sending and receiving image data of relatively small data amount through the telephone network or equivalent as a video or visual telephone system.

2. Description of the prior art

Conventionally, a facsimile communication system is well known as a system for sending and receiving the image data through a telephone network or equivalent. The facsimile communication system is adapted to transmit the image data through a transmission control procedure including phases A-E as shown in FIG. 1 in accordance with the T series recommendation of CCITT. The phase A is a phase for setting call up, the phase B is a pre-message phase, and the image data is transmitted in the phase C. The phase D is a post-message phase and the phase E is a phase for putting call down.

In the conventional transmission control procedure of the facsimile communication system, time periods required for the phases A, B, D and E are respectively approximate 3 seconds, 8 seconds, 2.5 seconds and 1 second in spite of a length of the image data. Therefore, when the data amount of the image data is small, time period which does not take part in transmission of the image data is relatively increased, thus there is a problem that a transmission efficiency becomes worse. Specially, in the video or visual telephone system in which a still picture or image is transmitted rather than a moving picture or image, the image data to be transmitted is only for one image screen, and therefore, if a complex transmission control procedure as in the conventional facsimile communication system is applied, the transmission efficiency is very bad.

SUMMARY OF THE INVENTION

Therefore, a principal object of the present invention is to provide a system for transmitting image data on a telephone network or equivalent in accordance with a novel transmission control procedure.

Another object of the present invention is to provide a system for transmitting image data oh a telephone network or equivalent, in which a transmission efficiency is good even if a data amount of image data to be transmitted is small.

The other object of the present invention is to provide a system for transmitting image data on a telephone network or equivalent in accordance with a simple transmission control procedure which is suitable for a video or visual telephone system.

In a transmission control procedure in accordance with the present invention, in an apparatus of sending part, a tone signal generator is started to operate in response to an image data sending command so that a tone signal having a specified frequency is sent for a predetermined time period, and succeedingly, in spite of presence or absence of a responsive signal from an apparatus of receiving part, the image data is sent. In the apparatus of receiving part, a modem is brought in a state for waiting reception of the image data being sent from the apparatus of sending part only when the above described specified tone signal is continuously received for more than a predetermined time period. Therefore, in the apparatus of receiving part, a state for receiving the image data is automatically set without returning a ready signal for receiving or acknowledge signal.

In accordance with the present invention, a complex and redundant transmission control procedure as in the conventional facsimile communication system is not applied, and therefore, a transmission efficiency is very good even if a data amount of the image data to be sent is small. In the conventional facsimile communication system, as described above, it is necessary to pass through the phases A, B, D and E of approximate 15 seconds in total other than the phase C for transmitting the image data; however, only a time period when the above described specific tone signal is to be sent, for example, 300-400 milliseconds is required for transmitting other than the image data, and therefore the transmission efficiency is drastically increased.

In the case where the present invention is applied to a video or visual telephone system, since the users sit in the front of the video or visual telephones of sending part and receiving part respectively, if and when the image data being sent one-sidely from the video telephone of sending part is not effectively received by the video telephone of receiving part, the user of sending part may repeat again the image data sending procedure through a telephone conversation or audio communication between the both parties. Therefore, an exceptional disadvantage due to a transmission error does not take place in comparison with the case where an automatic receiving mode is set as the facsimile communication system.

In one embodiment, a plurality of tone signals each having a different frequency are utilized. Then, in the case where the image data is intended to be sent from the apparatus of sending part at a high transmission speed, for example, 4,800 bps, one tone signal is utilized, and in the case where the image data is intended to be sent at relatively slow transmission speed of 2,400 bps, for example, another tone signal is utilized. In the apparatus of receiving part, a frequency of such a tone signal is identified and a modem is switched so as to receive the image data at the transmission speed designated by the tone signal.

Prior to sending the above described specific tone signal or signals from the apparatus of sending part, a transmission mode confirmation procedure for notifying a transmission mode to the apparatus of receiving part may be executed. In such a transmission mode confirmation procedure, a command signal consisting of a tone signal having a specified frequency or coded data is sent from the apparatus of sending part to the apparatus of receiving part. Then, in response to the command signal, the apparatus of receiving part returns an acknowledge signal or negative acknowledge signal to the apparatus of sending part. In the case where the acknowledge signal is returned from the apparatus of receiving part, in the apparatus of sending part, as described previously, the image data is sent in accordance with the transmission mode designated by the command signal immediately after sending a tone signal for a predetermined time period. If the negative acknowledge signal is returned or no responsive signal is returned from the apparatus of receiving, in the apparatus of sending part, immediately after sending the tone signal for the predetermined time period, the image data is sent in accordance with a predetermined transmission mode, for example, a default transmission mode in spite of designation by the command signal.

The objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the embodiments of the present invention when taken in conjunction with accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative view showing a transmission control procedure in a conventional facsimile communication system in accordance with the T series recommendations of CCITT.

FIG. 2 is a block diagram showing one example of a video or visual telephone to which the present invention is applicable.

FIG. 3 is an illustrative view showing a transmission control procedure in accordance with one embodiment of the present invention.

FIG. 4 is a flowchart showing an operation of a video or visual telephone of sending part in accordance with FIG. 3 embodiment.

FIG. 5 is a flowchart showing an operation of a video or visual telephone of receiving part in accordance with FIG. 3 embodiment.

FIG. 6 is an illustrative view showing a transmission control procedure in accordance with another embodiment of the present invention.

FIG. 7A and FIG. 7B are flowcharts showing an operation of a video or visual telephone of sending part in accordance with FIG. 6 embodiment.

FIG. 8A and FIG. 8B are flowcharts showing an operation of a video or visual telephone of receiving part in accordance with FIG. 6 embodiment.

FIG. 9 is an illustrative view showing a transmission control procedure in accordance with a still another embodiment of the present invention.

FIG. 10 is a flowchart showing an operation of a video or visual telephone of sending part in accordance with FIG. 9 embodiment.

FIG. 11 is a flowchart showing an operation of a video or visual telephone of receiving part in accordance with FIG. 9 embodiment.

FIG. 12 is a circuit diagram showing a major portion of a video or visual telephone utilized in the other embodiment in accordance with the present invention.

FIG. 13 is an illustrative view showing a transmission control procedure in accordance with the other embodiment of the present invention.

FIG. 14 is a flowchart showing an operation of a video or visual telephone of sending part in accordance with FIG. 13.

FIG. 15 is a flowchart showing an operation of a video or visual telephone of receiving part in accordance with FIG. 13 embodiment.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, a description will be made on the case where the present invention is embodied in a video or visual telephone system. However, it is pointed out in advance that the present invention is also applicable any apparatus or system for transmitting image data by utilizing the telephone network or equivalent other than such a video or visual telephone system.

With referring to FIG. 2, a video or visual telephone 10 which can be utilized in this embodiment includes a CCD camera 12. An image taken by the CCD camera 12 is converted into image data of a digital fashion and stored into a video memory 18 under the control of a microcomputer 16. In the video memory 18, image data which is sent from a video or visual telephone of the other party can be stored. The image data which has been stored in the video memory 18 is sequentially read and converted into a video signal by a D/A converter 19 so that the image in accordance with the image data is displayed on a display 20 which may be a CRT or an LCD, for example.

In addition, although not shown, the microcomputer 16 includes a central processing unit (CPU), a ROM which is connected to the CPU and stores a program in accordance with a transmission control procedure to be described later, and a RAM which is connected to the CPU and includes a timer area or region for a time-counting operation and flag areas or regions necessary for controlling.

To the microcomputer 16, a key pad 22 is further connected. The key pad 22 includes a ten-key 24 similar to a normal or conventional audio telephone, an image data sending key 26, and a transmission speed changing key 28. A transmission mode changing key 30 and a polling key 31 may be provided on the key pad 22 as necessary. In addition, each of the transmission mode changing key 30 and the polling key 31 is a key an operated state of which is held until the same key is operated again.

The video telephone 10 further includes a network control unit 32 which includes a relay 34. The relay 34 is controlled by the microcomputer 16 so that a telephone circuit 36 is selectively connected to an audio telephone 38 or a modem 40. When the telephone circuit 36 is switched to the audio telephone 38 by the relay 34, it is possible to make a telephone conversation or audio communication with the other party by the audio telephone 38 through the telephone circuit 36 as in the normal or conventional telephone. Inversely, if the telephone circuit 36 is switched by the relay 34 so as to be connected to the modem 40, it is possible to send or receive the image data through the telephone circuit 36.

In addition, the modem 40 includes a DTMF (Dual Tone Multi-Frequency) signal generator 40a for generating DTMF signals. The DTMF signal generator 40a can generate sixteen kinds of DTMF signals each having a different frequency as in the conventional telephone. Twelve kinds of the DTMF signals out of sixteen kinds of DTMF signals are utilized for the ten-key 24, each of which corresponding to respective one of twelve keys of the ten-key 24. A portion or all of the remaining four DTMF signals, that is, DTMF-A signal, DTMF-B signal, DTMF-C signal and DTMF-D signal can be utilized in a transmission control procedure of the embodiment.

Furthermore, the DTMF signal from a video telephone of the other party is received by a DTMF signal receiver 42 through the telephone circuit 36 and the network control unit 32, and frequencies of a received DTMF signal are identified in the DTMF signal receiver 42. A signal is outputted correspondingly to respective one of the received DTMF signals from the DTMF signal receiver 42 and applied to the microcomputer 16.

In the embodiment described later, the video or visual telephones 10 as shown in FIG. 2 are used as an apparatus of sending part and an apparatus of receiving part, respectively.

With referring to FIG. 3, a transmission control procedure in accordance with one embodiment of the present invention will be roughly described. In the video telephone of sending part, when the image data sending key 26 is operated, in response thereto, a signal DTMF-DTA, that is, DTMF-A signal or DTMF-B signal is sent for a predetermined time period which is slightly longer than a time period necessary for receiving a signal being sent from the video telephone of sending part by the video telephone of receiving part and for prevent an erroneous operation due to an audio or sound signal, for example, 300 milliseconds. Thereafter, after forming of a non-signal state for a predetermined time period which is slightly longer than a time period necessary for changing a transmission speed of the modem 40 and/or for changing-over the relay 34, for example, 75 milliseconds, the image data to which a training signal TR is added at the leading head thereof is sent at a transmission speed of 4,800 bps or 2,400 bps. On the other hand, in the video telephone of receiving part, if the DTMF-A signal or DTMF-B signal being sent from the video telephone of sending part is continuously received or detected for a predetermined time period which is slightly longer than a time period necessary for determining by the microcomputer 16 whether or not the received signal is an effective DTMF signal, for example, 250 milliseconds, in response thereto, the modem is brought in a ready state for receiving at a transmission speed of 4,800 bps or 2,400 bps so that the image data being sent from the video telephone of sending part is received.

More specifically, in the state where the telephone circuit 36 is established in association with the video telephone of receiving part, when the image data sending key 26 is operated in the video telephone of sending part, a sending routine as shown in FIG. 4 is executed. In the first step S1, in the video telephone of sending part, the relay 34 included in the network control unit 32 is enabled by the microcomputer 16 in response to an operation of the image data sending key 26. Accordingly, a modem 40 is connected to the telephone circuit 36.

In the next step S3, a transmission speed being set by the user at that time is recognized by the microcomputer 16 in accordance with a state of the transmission speed changing key 28 included in the key pad 22. Then, when the set transmission speed is 4,800 bps, in the step S5a, the DTMF signal generator 40a included in the modem 40 is started by the microcomputer 16, and the aforementioned DTMF-A signal is sent for 300 milliseconds as the signal DTMF-DTA which is to be sent prior to the image data. Succeedingly, after the non-signal state for 75 milliseconds as shown in FIG. 3 is formed in the step S7a, the modem 40 is set at a transmission speed of 4,800 bps and started in the step S9a. On the other hand, if the transmission speed is set at 2,400 bps, likewise the previous step S5a, the DTMF signal generator 40a is started and the DTMF-B signal as the signal DTMF-DTA is sent for 300 milliseconds by the microcomputer 16 in the step S5b. Then, after forming the non-signal state for 75 milliseconds in the step S7b, in the step S9b, the modem 40 is set at the transmission speed of 2,400 bps and started.

After the previous step S9a or S9b, in the step S11, the image data which has been taken by the CCD camera 12 and stored in the video memory 18 is sent to the telephone circuit 36 through the modem 40 under the control of the microcomputer 16. If it is determined that the end of the image data is completed in the step S13, in the steps S15 and S17, the microcomputer 16 stops the modem 40 to operate and disables the relay 34 included in the network control unit 32.

On the other hand, in the video telephone of receiving part, in the state where the image data sending key 26 is not operated, the microcomputer 16 always watches an output of the DTMF signal receiver 42. Then, if the DTMF signal is received or detected in the first step S21 as shown in FIG. 5, in the succeeding steps S23 and S25, the microcomputer 16 determines based upon the output of the DTMF signal receiver 42 whether or not the detected DTMF signal is the signal DTMF-DTA, that is, DTMF-A signal or DTMF-B signal.

When the DTMF-A signal is detected, in the next step S27a, it is determined whether or not the DTMF-A signal is continuously received or detected for a predetermined time period, for example, 250 milliseconds. If the DTMF-A signal is continuously detected for 250 milliseconds, in the next step S29a, the relay 34 included in the network control unit 32 is enabled by the microcomputer 16. Also, in the next step S31a, the modem 40 is brought in a state capable of receiving the image data at the transmission speed of 4,800 bps. If the DTMF-B signal is detected in the above described steps, after the steps S27b and S29b similar to the previous steps S27a and S29a, the modem 40 is set at the transmission speed of 2,400 bps by the microcomputer 16 in the step S31b.

After the step S31a or S31b, in the step S33, the microcomputer 16 determines whether or not the image data is received within 300 milliseconds after starting of the modem 40. If a carrier signal of the image data is detected within the time period, the image data is received through the modem 40 in the step S35. The received image data is stored in the video memory 18 and displayed on the display 20 (FIG. 2) as an image.

Then, in the step S37, if it is determined that no carrier signal of the image data is received due to the end of the image data being sent from the video telephone of the sending part, the relay 34 is disabled by the microcomputer 16 in the next step S39. Thus, in the embodiment as shown in FIG. 4 and FIG. 5, it is possible to transmit the image data in accordance with the transmission control procedure as shown in FIG. 3. In this case, the DTMF-A signal or DTMF-B signal is utilized for designating the transmission speed of 4,800 bps or 2,400 bps.

In addition, when sending of the image data is completed in accordance with FIG. 4 and FIG. 5, thereafter, it is possible to make a normal telephone conversation utilizing the respective audio telephones 38 between the video telephones of the both parts, whereby confirmation of presence or absence of transmission error can be made through such telephone conversation. If it is confirmed through the telephone conversation that the transmission error takes place at the transmission speed of 4,800 bps, the transmission speed is changed at 2,400 bps through an operation of the transmission speed changing key 28 by the user who operates the video telephone of sending part, and thereafter the image data may be sent again in accordance with the same procedure as described above.

With reference to FIG. 6, a transmission control procedure in accordance with another embodiment of the present invention will be simply described. The embodiment shown is suitable for sending and receiving a color image data between video telephones of the both parties each of which is capable of displaying color image. Then, in taking compatibility between the video telephone which can receive only monochrome image data as shown in FIG. 3 through FIG. 5 into consideration, prior to sending the signal DTMF-DTA, that is, DTMF-A signal or DTMF-B signal and the image data as shown in FIG. 3, a command signal for designating a transmission mode which represents that the color image data will be sent is sent from the video telephone of sending part by utilizing a specified tone signal, for example, DTMF-C signal. If the video telephone of receiving part is the color video telephone, a further specified tone signal, for example, DTMF-D signal is returned from the video telephone of receiving part so as to notify to the video telephone of sending part that the color image data can be received. Thereafter, a transmission control procedure similar to FIG. 3 embodiment will be executed.

More specifically, in the video telephone of sending part, when the image data sending key 26 as shown in FIG. 2 is operated, a sending routine as shown in FIG. 7A and FIG. 7B is executed. After the relay 34 included in the network control unit 32 is enabled by the microcomputer in the first step S101, in the step S103, the microcomputer 16 determines whether or not confirmation of the transmission mode by utilizing the DTMF-C signal and DTMF-D signal as shown in FIG. 6 has been made in association with the video telephone of receiving part. Such determination can be made by setting a suitable flag, for example, a transmission mode confirmation flag described later in the RAM (not shown) of the microcomputer 16.

If the confirmation of the color image data transmission mode has not been completed, in the succeeding step S105, the DTMF signal generator 40a is started by the microcomputer 16 and the DTMF-C signal representing the transmission mode for transmitting the color image is sent for 300 milliseconds. When the DTMF-C signal is thus sent from the video telephone of sending part, the acknowledge signal, that is, the DTMF-D signal is to be returned from the video telephone of receiving part if the same is the color video telephone. Therefore, in the next step S107, the microcomputer 16 determines whether or not the DTMF-D signal is returned from the video telephone of receiving part based upon the output of the DTMF signal receiver 42. Then, if the DTMF-D signal is detected, in the next step S109, it is determined whether or not the DTMF-D signal is continuously received for 250 milliseconds. If the DTMF-D signal is continuously detected for 250 milliseconds, in the next step S111, the microcomputer 16 of the video telephone of sending part sets the transmission mode confirmation flag in the RAM on the assumption that the confirmation of the transmission mode had been made.

Thereafter, the process proceeds to the st