Static image video telephone transmission system.

BACKGROUND OF THE INVENTION

Field of the Invention

0001The present invention relates to a static image video telephone transmission system, and in particular, to a static video telephone transmission system in which a static image signal obtained from a television camera is modulated to an audio signal, the modulated signal is sent to a telephone line while talking to transmit the static image signal, and on the other hand, the static image signal supplied over the telephone line is demodulated and visualized on a display unit.

Description of the Prior Art

0002In recent years there has been a demand for a video telephone that allows to transmit voice and image over a telephone line simultaneously. However,it is difficult to continuously transmit and receive a moving image which requires an enormous amount of data over the existing telephone line. Accordingly, a static image video telephone which transmits a static image to the other party during conversation is practically used.

0003This static image video telephone can transmit any desired static image, for example, a face of a talking person, a photographic picture or drawing matching to the contents of the talking by temporarily interrupting the conversation during the usual talking with voice.

0004According to such a static image video telephone, it is therefore possible to transmit and receive an image over a telephone line which has been impossible heretofore. Furthermore, since only one sheet of static image is transmitted at a time and the amount of data processing is also limited, the realization of the practical video telephone can be facilitated.

0005A static image video telephone apparatus capable of transmitting the static image is disclosed, for example, in "Television Technique", for September, 1988.

0006Fig. 5 is a block diagram of the prior art static image video telephone apparatus used for the static image video telephone transmission.

0007With reference to Fig. 5, the static image video telephone apparatus 1 is provided with an external telephone set terminal 2 for connecting to a telephone set not shown, a telephone line connecting terminal 3, a picture recording terminal 4 for connecting to a tape recorder, a reproducing terminal 5, a telephone/reproduction change-over switch 6, and a key pad 7, thereby to achieve connection between the apparatus 1 and other external equipments.

0008Furthermore, the static image video telephone apparatus 1 is provided with a network control circuit 8, a modem 9, a demodulation timing generation circuit 10, an analog/digital converter 11, a central processing unit (hereinafter, referred to as a CPU) 12, an image controller 13, an image memory 14, a digital/analog converter 15, a display unit 16, an analog/digital converter 17, and a television camera 18.

0009The operation of the prior art static image video telephone apparatus will now be described. Voice conversation is transmitted from the telephone set externally provided to the telephone line terminal 3 through the external telephone terminal 2 and the network control circuit 8. On the other hand, a voice signal from the other party is transmitted to the external telephone set via a reverse route of the above-mentioned signal route, that is, through the telephone line terminal 3, the network control circuit 8, and the external telephone set terminal 2.

0010In the case of an image signal transmission, the CPU 12 detects transmission of the image commanded by the operation of the key pad 7. The CPU 12 transmits an image data stored in the image memory 14 in accordance with a program stored therein by adding control information thereto towards the telephone line terminal 3 through the modem circuit 9 and the network control circuit 8.

0011In the case of an image signal receiving, the image signal transmitted over the telephone line is supplied to the modem 9 through a reverse route of the above-­mentioned signal route. The image data from the other party is demodulated in the modem circuit 9 by a demodulation timing signal generated in the demodulation timing generation circuit 10, and the demodulated signal is stored in the image memory 14 through the analog/digital converter 11, CPU 12, and image controller 13, and then displayed on the display unit 16.

0012When the transmission and receiving of the image is completed, the network control circuit 8 resumes the voice conversation. The image shot by the television camera 18 during conversation is quantized by the analog/digital converter 17, stored in the image memory 14 through the image controller 13, and sequentially read onto the display unit 16.

0013With reference to Figs. 7 and 8, it will be described about a frame format for image transmission and receiving in the prior art static image video telephone transmission system, and about the operation for voice and image signal transmission in the prior art static image telephone transmission system.

0014The frame format used in transmission and receiving of an image is divided into a control information signal and an image data signal as shown in Fig. 7. The control information signal is a signal used to demodulate image data properly, and composed of a frame synchronizing signal, an amplitude correction signal, and ID (information data). The frame synchronizing signal is a signal used to generate a demodulation timing signal as shown in Fig. 8A, and also used to switch between voice and image modes. The amplitude correction signal is a signal used to set an AGC (automatic gain control) and a tone level of image appropriately. The ID is a data signal representing information of mutual capability identification signal, i.e., a data signal having information on a transmission and receiving of a image signal.

0015Since the prior art static image video telephone transmission system has an arrangement as described above, when a large frequency shift occurs in a telephone line as shown in Fig. 8B, a phase-locked-loop (hereinbelow referred to as PLL) for reproducing a carrier wave signal and a PLL for reproducing data of the demodulation timing generator as shown in Fig. 6 cannot track the modulated image signal properly, and thus, the demodulation timing signal as shown in Fig. 8B matches to the image signal with a shift of position to the right. With the result that, a received image signal cannot be properly demodulated.

SUMMARY OF THE INVENTION

0016The object of the present invention is to solve the above-mentioned problem, and to provide a static image video telephone transmission system capable of demodulating a received static image signal properly even when a large amount of frequency shift occurred in a telephone line by adding a reference frequency signal to the control information signal of the frame format.

0017To achieve the above-described object, the present invention is characterized in that a reference frequency signal is added at a predetermined cycle to the control information signal of the static image signal to be transmitted in order to correct a frequency shift, which will occur while being transmitted through the telephone line and, at a receiving side, a demodulation timing signal is corrected based on the amount of the frequency shift which is obtained by counting the reference frequency signal of the received signal by a given clock to be compared to a predetermined count value.

0018Therefore, according to the present invention, it becomes possible to correct a frequency shift beyond one cycle shift which the PLLs in the conventional system were hard to follow the image signal by adding a reference frequency signal to a control information signal of the static image signal to be transmitted at a predetermined cycle, and by counting the reference signal of the received signal by a given clock and comparing the obtained value to a predetermined count value so that the amount of the frequency shift can be obtained.

0019The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

0020<ul id="ul0001" list-style="none"><li>Fig. 1 is a block diagram of a static image video telephone apparatus used in a video telephone transmission system in an embodiment of the present invention;</li><li>Fig. 2 is a diagram for explaining a frame format or image transmission and reception in an embodiment of the invention;</li><li>Fig. 3 is a block diagram of the demodulation timing generation circuit in Fig. 1;</li><li>Figs. 4A and 4B are demodulation timing characteristics diagrams respectively in a normal image signal and in an image signal having frequency shift;</li><li>Fig. 5 is a block diagram of a static image video telephone apparatus used in a prior art video telephone transmission system;</li><li>Fig. 6 is a block diagram of a demodulation circuit used in a prior art video telephone transmission system;</li><li>Fig. 7 is an explanatory diagram of a prior art frame format; and</li><li>Figs. 8A and 8B are demodulation timing characteristics diagrams in the prior art static image video telephone transmission system.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0021Preferred embodiments of the invention will now be described with reference to Figs. 1 to 4.

0022With reference to Fig. 1, a static image video telephone apparatus 1 used in a video telephone transmission system in an embodiment of the present invention, similar to the prior art apparatus shown in Fig. 5, includes an analog/digital converter 11, an image controller 13, an image memory 14, a digital/analog converter 15, a display unit 16, and an analog/digital converter 17. On the front portion of the static image video telephone apparatus 1, there are provided with an external telephone set terminal 2, a telephone line connection terminal 3, a picture recording terminal 4, a reproduction terminal 5, a telephone/reproduction change-over switch 6, a key pad 7, etc.

0023Furthermore, the apparatus 1 comprises a network control circuit 8 connected to a telephone line, a modem circuit 9 for demodulating a static image signal supplied from the network control circuit 8 and for modulating a signal including a static image signal obtained from a television camera 18 and a control information signal added to the static image signal by a CPU 12 thereby to send the modulated static image signal to the network control circuit 8. The apparatus 1 further comprises a demodulation timing generation circuit 10 for providing a demodulation timing signal to the modem circuit 9, and a frequency correction circuit 23 for detecting the amount of frequency shift caused by the telephone line based on the reference frequency signal supplied from the network control circuit 8, and for providing frequency shift correction data 25 to the demodulation timing generation circuit 10.

0024Next, with reference to Figs. 2 to 4, a frame format for transmitting and receiving an image in the present invention, and the transmission and receiving of an image signal using the transmission system of the invention will be described.

0025The frame format for the image transmission and reception is formed as shown in Fig. 2. In this frame format, a reference frequency signal 24 which is a characteristic feature of the present invention is added to the control information signal.

0026The operation of transmission and reception of an image signal utilizing the system of the present invention will be described.

0027The operation for transmitting an image signal is performed in a similar manner as in the prior art. More specifically, when the transmission of an image is commanded by the key pad 7 shown in Fig. 1, the CPU 12 detects this command and adds a control information to image data stored in the image memory 14 in accordance with a program stored in the CPU 12, and sends it to the telephone line terminal 3 through the modem circuit 9 and the network control circuit 8. Here, the control information added to the image data includes the frequency correction signal 24 which is characteristic in the present invention as mentioned above.

0028On the other hand, when receiving a static image signal, the image signal is supplied to the modem circuit 9 through a reverse signal route of the transmission route. The received image signal is input to a PLL for carrier wave signal reproduction and also a chopper demodulator shown in Fig. 6. The PLL for carrier wave signal reproduction detects a frame synchronizing signal included in the received image signal and outputs a demodulation clock signal synchronized with a carrier wave frequency. The chopper demodulator demodulates the modulated image signal to produce analog image data by the use of the demodulation clock signal delivered from the PLL for reproducing carrier wave signal. The demodulated analog image signal is converted into a digital signal based on a demodulation timing clock output from a PLL for data reproduction. If a frequency shift is such a less amount as shown in Fig. 8A, the static image signal can be properly converted into digital image data by the demodulation timing signal which can properly track the image signal. However, if a large amount of a frequency shift as shown in Fig. 8B occurs, the PLL for data reproduction cannot follow the frequency shift in the analog image signal well and the produced demodulation signal therefrom cannot synchronize with the analog signal, so that a demodulation timing signal shifts to the position (b) instead of the expected position (a) as shown in Fig. 8B. Accordingly, the received image cannot be reproduced properly. However, according to the present invention, in the modem circuit 9, the control information signal is demodulated up to a frame synchronizing signal 20 in the same manner as in the prior art, that is, demodulated without correcting the demodulation timing by the frequency correction circuit 23, and the modem circuit 9 sends data to the CPU 12. The CPU 12, upon detecting the end of the frame synchronizing signal 20, actuates the frequency correction circuit 23. The frequency correction circuit 23 detects to what extent the frequency is shifted in the existing telephone line by counting the reference frequency signal added to the control information signal of the received signal by a given clock to compare to a predetermined reference count value, and sends the frequency correction data 25 to the demodulation timing generation circuit 10.

0029Here, the frequency shift correction data 25 will be described. Assume that a frequency signal of 1747.8 Hz which is the TTC Transmission Standards is sent .for 256 cycles as the reference frequency signal.

0030At the receiving side, a count of a reference clock (14,31818MHz) is conducted during the period of 1747.8 HZ, 256 cycles.

0031When there is no frequency shift, the count of the reference clock (14,31818 MHz) during this period is expressed by 14.31818 (MHz) / 1747.8 (Hz) x 258 = 2097152 (counts)      (1) That is, the count value of (1) becomes the reference value to be compared to.

0032However, when a frequency shift occurs, this count value is deviated. In other words, when the frequency signal of 1747.8 Hz is delayed to 1737 Hz due to the frequency shift, the count is expressed by 14.31818 (MHz) / 1713 (Hz) x 258 = 2110221 (counts)      (2)

0033Accordingly, from formulas (1) and (2), a deviation of frequency per one cycle is expressed by (2110221 - 2097152) / 256 = 51 (counts)

0034Therefore, this result can be used as the frequency shift correction data 25.

0035Next, with reference to Fig. 3, a correction method of frequency shift which is characteristic in the present invention will now be described.

0036A zero point of an input static image signal is detected by a zero point detection circuit 27, and subsequently, a deviation between a demodulation timing signal 26 which is directly obtained from the input signal and the zero point of the image signal is detected in a zero point correction circuit 28, and zero point correction data 29 for this deviation is supplied from the zero point correction circuit to an adder 30.

0037The zero point correction circuit 28 performs correction of a minute deviation less than per one cycle of an image signal, and it is difficult to correct a deviation such a large frequency shift as exceeds one cycle.

0038On the other hand, the frequency shift correction data 25 detected by the frequency correction circuit 23 is supplied to the adder 30.

0039The adder 30 adds up these two correction data 25 and 29, and the obtained data is supplied to the demodulation timing signal generation circuit 31.

0040In the demodulation timing signal generation circuit 31, a corrected demodulation timing signal 26 is generated based on the inputted data.

0041The demodulation of image data is performed by the demodulation timing signal 26 corrected in the aforementioned manner, and the demodulated image data is stored in the image memory 14 through the analog/digital converter 11, CPU 12, and image controller 13, and then it is displayed on the display unit 16.

0042Therefore, according to this embodiment, it is possible to transmit the image data which can be properly demodulated without fail at the receiving side even when a large frequency shift occurs in the input image signal because the frequency shift can be detected based upon the reference frequency signal of the image signal to correct the demodulation timing signal.

0043In the above embodiments, although it is described about the image signal in the AM-PM (amplitude modulation phase modulation) system which is the present TTC standards, the present invention is not limited to these embodiments, and the present invention is applicable, for example, to the QAM system (quadrature amplitude modulation system) and other communication systems widely.

0044While there has been described what are at present considered to be preferred embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.

0045A static image video telephone transmission method in which a static image signal obtained from a camera is modulated into an audio signal, and the modulated static image signal is transmitted and received over a telephone line. A reference frequency signal is added to a control information signal of a static image signal to be transmitted at a predetermined cycle in order to correct a frequency shift which will occur in the telephone line. The reference frequency signal of the static image signal received over the telephone line is counted by a given clock and the obtained count value is then compared to a predetermined value to detect a quantity of the frequency shift. A demodulation timing signal is corrected based on this frequency shift data.