WO2008056933A1

WO2008056933A1 - Method for transmitting video signal in half-duplex and voice signal and data signal in full-duplex and modem for transmitting the signals using the method

Info

Publication number: WO2008056933A1
Application number: PCT/KR2007/005584
Authority: WO
Inventors: Jae Wook Chun
Original assignee: Linx Telecom Co., Ltd.
Priority date: 2006-11-09
Filing date: 2007-11-07
Publication date: 2008-05-15
Also published as: KR100813010B1

Abstract

The present invention relates to a method for transmitting video signals in half-duplex and voice signals in full-duplex and a signal transmission modem using the method. According to the present invention, the first modem combines a digitalized voice signal with a data signal, modulates the combined signal, combines the modulated signal with a base band video signal, and transmits the combined signal to the second modem through a communication line. Simultaneously, the second modem combines a digitalized voice signal with a data signal, modulates the combined signal onto a carrier different from that of the first modem, and transmits the modulated signal to the first modem through the communication line. Accordingly, there simultaneously exist a base band video signal and a modulated voice/data synthesis signal transmitted from the first modem and a modulated voice/data synthesis signal transmitted from the second modem on the communication line. The first modem separates the voice/data synthesis signal transmitted from the second modem among the signals received through the communication line using a band pass filter and performs demodulation to remove the carrier used for the modulation by the second modem. In addition, the second modem separates the voice/data synthesis signal transmitted from the first modem among the signals received through the communication line using a band pass filter and performs demodulation to remove the carrier used for the modulation by the first modem.

Description

METHOD FOR TRANSMITTING VIDEO SIGNAL IN HALF- DUPLEX AND VOICE SIGNAL AND DATA SIGNAL IN FULL- DUPLEX AND MODEM FOR TRANSMITTING THE SIGNALS

USING THE METHOD Technical Field

[1] The present invention relates to a communication modem and a private telephone system for a multi-family house using the same, and more particularly, to a method for transmitting video signals in half-duplex and voice signals in full-duplex and a signal transmission modem using the method. In addition, the present invention relates to a private telephone system for a multi-family house using the modem. Background Art

[2] A private telephone system is installed in a multi-family house. Although private telephone systems of the early stage are installed in a per-channel signaling method (hereinafter, referred to as a "star type"), a common channel signaling method (hereinafter, referred to as a "bus type") has been developed to conveniently install a private telephone system and to save installation costs. Recently, with the advancement of information communication technologies, network type private telephone systems are introduced. Although a home network system provides a variety of convenient functions, a star or bus type private telephone system provides only limited simple communication functions.

[3] On the other hand, an automatic door and an interphone (hereinafter, referred to as a

"lobby phone") are installed at a common entrance of a multi-family house in order to control incoming and outgoing persons. A camera is connected to the lobby phone, and an input device is installed in the lobby phone to select a home (hereinafter, referred to as an "individual home") that is connected to the private telephone system and desired to connect a call. If a visitor calls an individual home desired to communicate with using the input device, a resident can identify the visitor and open the automatic door through a home phone (hereinafter, referred to as an "indoor interphone") installed in the called home. A display apparatus for displaying images photographed by the camera of the lobby phone is connected to the indoor interphone. In addition, an interphone is also installed at the entrance of each individual home (hereinafter, referred to as an "outdoor interphone"). A camera is also installed in the outdoor interphone, and when a visitor communicates with a resident on the indoor interphone through the outdoor interphone, images of the visitor photographed by the camera of the outdoor interphone are displayed on the display apparatus connected to the indoor interphone. If the called family is absent, the images photographed by the camera of the lobby phone or the outdoor interphone are displayed on a display apparatus connected to an interphone installed in a management room (hereinafter, referred to as a "management interphone") or stored in a server in the management room. Disclosure of Invention Technical Problem

[4] Recently, a home network system capable of simultaneously transmitting voice and video signals among a lobby phone installed at a common entrance, an indoor interphone, an outdoor interphone, and a management interphone is installed in a newly constructed multi-family house in order to control persons coming in and going out of the multi-family house.

[5] However, in a previously constructed multi-family house, a lobby phone is generally not installed at a common entrance, and a PVC Insulated Indoor Telephone Wire (TIV, one pair) for transmitting only analog voice signals is installed between an indoor interphone installed in a home and a management interphone installed in a management room. In order to install a lobby phone at a common entrance of a previously constructed multi-family house and simultaneously transmit voice and video signals between the lobby phone and an indoor interphone or between an outdoor interphone and a management interphone, a communication line, such as a coaxial cable, capable of simultaneously transmitting voice and video signals or a separate communication line for transmitting video signals should be installed. In addition, if an existing line is used, a separate modem should be used to digitalize and transmit voice and video signals simultaneously.

[6] If a separate line capable of simultaneously transmitting voice and video signals is installed to construct a private telephone system in a previously constructed multi- family house, there is a disadvantage in that the cost for constructing a line is high, and it takes a great deal of time to construct the line. Furthermore, if a private telephone system is installed using an existing line of a previously constructed multi- family house, an Asymmetric Digital Subscriber Line (ADSL) modem, Very-High-Data-Rate Digital Subscriber Line (VDSL) modem, or Power Line Communication (PLC) modem is required, and thus there is a problem in that the price of the private telephone system is increased.

[7] Accordingly, the present invention is conceived to solve the aforementioned problems in the prior art. A primary object of the present invention is to provide a method for transmitting video signals in half-duplex and voice and data signals in full- duplex using TIV communication lines installed in a previously constructed multi- family house.

[8] A secondary object of the present invention is to provide a modem that can transmit video signals, voice signals, and data signals without replacing TIV communication lines installed in a previously constructed multi-family house and can also be manufactured at a low cost.

[9] A tertiary object of the present invention is to provide a private telephone system for a multi-family house, which can provide a variety of services such as a home network using a modem according to the present invention, without replacing TIV communication lines installed in a previously constructed multi-family house. Technical Solution

[10] According to an aspect of the present invention for achieving the objects, there is provided a method of transmitting video signals in half-duplex and voice and data signals in full-duplex between a first modem and a second modem. According to the present invention, the first modem combines a digitalized voice signal with a data signal, modulates the combined signal, combines the modulated signal with a base band video signal, and transmits the combined signal to the second modem through a communication line. Simultaneously, the second modem combines a digitalized voice signal with a data signal, modulates the combined signal onto a carrier different from that of the first modem, and transmits the modulated signal to the first modem through the communication line. Accordingly, there simultaneously exist a base band video signal and a modulated voice/data synthesis signal transmitted from the first modem and a modulated voice/data synthesis signal transmitted from the second modem on the communication line. The first modem separates the voice/data synthesis signal transmitted from the second modem among the signals received through the communication line using a band pass filter and performs demodulation to remove the carrier used for the modulation by the second modem. In addition, the second modem separates the voice/data synthesis signal transmitted from the first modem among the signals received through the communication line using a band pass filter and performs demodulation to remove the carrier used for the modulation by the first modem. In addition, the second modem separates the video signal transmitted from the first modem using a low pass filter.

[11] According to another aspect of the present invention, there is provided a method of transmitting voice and data signals in full-duplex and video signals in half-duplex between a first modem and a second modem, comprising: in the first modem, a voice/ data signal synthesizing step of combining a voice signal converted into a digital signal with a data signal, a first modulation step of modulating the synthesized voice/data signal onto a first carrier, a transmission signal coupling step of combining the modulated signal with a base band video signal, and a first transmission signal transmitting step of applying the coupled signal to a communication line; in the second modem, a voice/data signal synthesizing step of combining a voice signal converted into a digital signal with a data signal, a second modulation step of modulating the synthesized voice/data signal onto a second carrier, and a second transmission signal transmitting step of applying the modulated signal to a communication line; in the first modem, a receiving step of receiving the transmission signal applied to the communication line, a voice/data synthesis signal separating step of separating the voice/ data synthesis signal modulated onto the second carrier from the received transmission signal, and a demodulating step of demodulating the separated voice/data synthesis signal by removing the second carrier; and in the second modem, a receiving step of receiving the transmission signal applied to the communication line, a video signal separating step of separating the base band video signal transmitted by the first modem from the received transmission signal, a voice/data synthesis signal separating step of separating the voice/data synthesis signal modulated onto the first carrier from the received transmission signal, and a demodulating step of demodulating the separated voice/data synthesis signal by removing the first carrier.

[12] According to a further aspect of the present invention, there is provided a modem for transmitting video signals in half-duplex and voice and data signals in full-duplex. The modem of the present invention comprises a transmission unit for transmitting a signal to a communication line, and a receiving unit for receiving the signal transmitted to the communication line. The transmission unit comprises a modulator for combining a digitalized voice signal with a data signal and modulating the combined signal, a first switch for selectively switching an input of a base band video signal, a coupler for combining an output signal of the modulator with an output signal of the first switch and outputting the combined signal, and a first amplifier for amplifying an output signal of the coupler and applying the amplified signal to the communication line. The receiving unit comprises a second amplifier for receiving a signal from the communication line and amplifying and outputting the received transmission signal; a video separating filter for receiving an output signal of the second amplifier and separating a base band video signal; a second switch for selectively switching an output of the video separating filter; a band pass filter for receiving the output signal of the second amplifier, blocking the signal modulated by the modulator of the transmission unit and applied to the communication line, and passing through a signal modulated by a modulator of another modem connected to the communication line and applied to the communication line; and a demodulator for demodulating the signal passing through the band pass filter into a synthesized signal of a digitalized voice signal and a data signal by removing a carrier used by the modulator of another modem connected to the communication line from the signal passing through the band pass filter. Further, in the modem of the present invention, the second switch is turned off if the first switch is turned on, and the second switch is turned on if the first switch is turned off.

[13] In the modem of the present invention, the transmission unit assigns a video signal with a largest bandwidth to the base band and modulates a voice signal and a data signal onto a carrier having a frequency higher than the bandwidth of the video signal. The transmission unit combines the modulated signal with the video signal to generate one signal and transmits the generated signal to the communication line. The receiving unit separates the signal synthesized by another modem and transmitted to the communication line therefrom as described above into a video signal, a voice signal, and a data signal using a filter. Since the modem of the present invention has a switch for selecting transmission and reception of a video signal, the modem can transmit the base band video signal in one direction or in half-duplex and the voice signal and the data signal in full-duplex through the modulator and the demodulator.

[14] Particularly, if the modem of the present invention is used, video, voice, and data signals can be transmitted by directly using the existing lines of an interphone system installed in a previously constructed multi- family house, and thus a private telephone system for a multi-family house can be constructed with a low cost. Furthermore, according to the present invention, a modem can be manufactured at a low cost using a band pass filter, instead of an expensive hybrid circuit used for a general interphone system in order to transmit voice signals in full-duplex.

[15] Although a notch filter or a band rejection filter can be used as the video separation filter of the receiving unit, a low pass filter is preferably used as the video separation filter. In addition, in the modem of the present invention, a signal digitalized in pulse code modulation (PCM) is preferably used as the digitalized voice signal, and a frequency shift keying (FSK) modulator is preferably used as the modulator. In addition, the modulation frequency is 5MHz or higher in order to avoid duplication or interference among the video signals and is preferably 20MHz or lower in consideration of the characteristic of the TIV line.

[16] According to a still further aspect of the present invention, there is provided a private telephone system for a multi-family house, comprising a plurality of terminal side modems according to the present invention installed in respective homes of the multi-family house, a plurality of system side modems according to the present invention respectively connected to the plurality of terminal side modems and installed in a control room, a switching apparatus connected to the system side modems, for connecting the system side modems to allow transmission of signals among the terminal side modems, a control apparatus for controlling the switching apparatus, and a digital video/voice storage apparatus connected to the control apparatus. A telephone terminal is connected to the terminal side modem, and a camera and/or image display apparatus is connected to the terminal side modem if necessary. The control apparatus stores video signals and/or voice signals transmitted from the modem to which the camera is connected in the digital video/voice storage apparatus or reads video information stored in the digital video/voice storage apparatus and transmits the video information to the modem to which the telephone terminal is connected if there is a request for transmitting the stored video/voice signals.

[17] According to the present invention, a private telephone system for a multi-family house capable of transmitting video signals, voice signals, and data signals and providing a variety of additional services can be constructed without changing communication lines of an interphone system installed in a previously constructed multi- family house, using a modem capable of transmitting video signals in half-duplex and voice and data signals in full-duplex. Brief Description of the Drawings

[18] Fig. 1 is a view illustrating bandwidths of video, data, and voice signals to be transmitted using a modem according to the present invention;

[19] Fig. 2 is a spectrum of synthesized video, data, and voice signals;

[20] Fig. 3 is a view illustrating a bandwidth of a signal obtained by synthesizing a voice signal digitalized in pulse code modulation (PCM) with a data signal;

[21] Fig. 4 is a spectrum of a signal obtained by combining a voice/data signal modulated in the method of the present invention with a video signal;

[22] Fig. 5 is a block diagram showing a modem according to an embodiment of the present invention;

[23] Fig. 6 is a spectrum of a signal transmitted from a first modem 10 of Fig. 5 to a line;

[24] Fig. 7 is a spectrum of a signal transmitted from a second modem 10 of Fig. 5 to a line; and

[25] Fig. 8 is a block diagram of another embodiment of a modem according to the present invention; and

[26] Fig. 9 is a block diagram of a private telephone system for a multi- family house using the modem according to the present invention.

[27] <Explanation of Reference Numerals for Major Portions in Drawings>

[28] 10 : First modem 10' : First modem

[29] 20 : Transmission unit 21 : Modulator

[30] 22 : Coupler 23 : First amplifier

[31] 24 : First switch 25 : Oscillator

[32] 30 : Receiving unit 31 : Second amplifier [33] 32 : band pass filter 33 : Demodulator

[34] 34 : Low pass filter 35 : Oscillator

[35] 36 : Second switch

Best Mode for Carrying Out the Invention

[36] Hereinafter, a constitutional principle of a modem according to the present invention and a private telephone system for a multi-family house using the modem will be described in detail with reference to the accompanying drawings.

[37] Time division multiplexing (TDM) and frequency division multiplexing (FDM) are well known as methods for simultaneously transmitting a plurality of signals using one communication line. Generally, TDM is a digital signal transmission method, and signals to be transmitted should be converted into digital signals. Since a wide transmission bandwidth is required to apply the TDM to voice, data, and video signals, it is not easy to apply the TDM to an interphone system. The present invention is conceived to provide a modem that can transmit voice, data, and video signals using one communication line by employing the FDM and can also be manufactured at a low cost.

[38] Fig. 1 is a view illustrating bandwidths of video, data, and voice signals to be transmitted using the modem according to the present invention. As shown in the figure, a video signal (Fig. 1 (a)) transmitted from a camera connected to the modem of the present invention has a bandwidth of 4.5MHz, an analog voice signal (Fig. 1 (c)) created by an interphone has a bandwidth of 3.4KHz, and a data signal of 64Kbps (bits per second) coded in Manchester coding (Fig. 1 (b)) has a bandwidth of 64KHz. When a voice signal is digitalized in pulse code modulation (PCM), the voice signal can be coded at a data rate of 64Kbps to have the same bandwidth of 64KHz as that of a data signal. The video, voice, and data signals created to have such bandwidths can be transmitted in FDM in a variety of methods.

[39] Fig. 2 is a view illustrating a spectrum of video, data, and voice signals synthesized in conventional FDM. The spectrum shown in Fig.2 is a spectrum in which the voice signal is modulated using a carrier of 5.5MHz, and the data signal is modulated onto a carrier of 6.5MHz, and then the voice and data signals are combined with the video signal. If the signals are transmitted in FDM as described above, a receiving unit of the modem can separate the video signal using a low pass filter having a bandwidth of 4.5MHz, the voice signal using a band pass filter having a central frequency of 5.5MHz and a bandwidth of 8KHz (fc±4KHz), and the data signal using a band pass filter having a central frequency of 6.5MHz and a bandwidth of 130KHz (fc±65KHz). Carriers of the signals separated using the band pass filters are removed by a demodulator. In this case, two band pass filters, two modulators, and two demodulators are needed in a modem, and thus the manufacturing costs are increased. In addition, it is difficult to practically implement a band pass filter having a bandwidth of 8KHz in an MHz band when such a modem is intended to be manufactured.

[40] Fig. 3 is a view illustrating a band width of a signal obtained by synthesizing a voice signal digitalized in PCM with a data signal in accordance with a method of the present invention. As shown in Fig. 3, if an analog voice signal is digitalized in PCM, the voice signal digitalized in PCM is combined with a data signal in a base band to create a voice/data synthesis signal, and then one voice/data synthesis signal is modulated, a circuit can be easily and simply implemented. That is, according to the method of the present invention, a voice signal and a data signal can be processed using one band pass filter, one modulator, and one demodulator.

[41] Fig. 4 is a spectrum of a signal obtained by combining a voice/data synthesis signal modulated in the method of the present invention with a video signal. The spectrum shown in Fig. 4 corresponds to a signal obtained by digitalizing an analog voice signal, combining the digitalized signal with a data signal, modulating the combined signal onto a carrier of 5.5MHz, combining the modulated signal with a video signal of a base band, and processing this combined signal in FDM. A receiving unit receives a signal having such a spectrum shown in Fig. 4, and separates the video signal therefrom using a low pass filter having a bandwidth of 4.5MHz and also separates the signal modulated after the synthesis of the digitalized voice signal with the data signal (the modulated voice/data synthesis signal) using a band pass filter having a central frequency of 5.5MHz and a bandwidth of about 260KHz(fc+130KHz). If the carriers are removed from the separated modulated signals, a signal in which the data signal has been synthesized with the voice signal digitalized in PCM is obtained. In order to separate the video signal, a notch filter or a band rejection filter may be used in addition to the low pass filter.

[42] In the present invention, a variety of method can be used to modulate or demodulate a signal. Methods of a modulator and a demodulator known in the art include amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). If an AM modulator is used, a video signal may be affected by the amplitude of AM due to a low pass filter, and thus it is desirable to use FM or PM. Generally, in case of a television (TV), video signals are modulated in vestigial side band (VSB) modulation, and voice signals are modulated in FM. In applying the method according to the present invention, a signal obtained by synthesizing a voice signal digitalized in PCM with a video signal is preferably modulated in FM, more preferably in frequency shift keying (FSK) modulation. When a voice signal and a video signal are modulated in FM, a frequency that does not affect the video signal should be selected as a frequency of a carrier. In applying the method of the present invention, since a video signal has a bandwidth of 4.5MHz, a frequency of 5 MHz or higher is preferably selected as a frequency of the carrier.

[43] A modem used in a private telephone system that can directly use communication lines of a previously constructed multi-family house should communicate in full- duplex through one communication line. Accordingly, a general private telephone system uses a hybrid circuit used for a general telephone communication system to combine and offset transmission signals and receiving signals. The hybrid circuit is a circuit that directly passes a transmission signal of a counterpart and transfers its transmission signal to the counterpart while offsetting its transmission signal not to be returned to the transmission side. If the method according to the present invention is applied, a band pass filter is used instead of such a hybrid circuit, thereby simply configuring a circuit that blocks reception of its transmission signal and accepts only signals transmitted from the counterpart. If a circuit is configured as described, manufacturing costs of the modem can be reduced.

[44] Fig. 5 shows an embodiment of a modem manufactured by employing the method of the present invention. Referring to Fig. 5, a first modem 10 according to the present invention comprises a transmission unit 20 and a receiving unit 30.

[45] The transmission unit 20 comprises a modulator 21, a coupler 22, a first switch 24, and a first amplifier 23. The modulator 21 receives a signal Vt+Dt obtained by combining a voice signal Vt with a data signal Dt and performs frequency modulation. An oscillator 25 for generating a carrier is connected to the modulator 21. If the first switch 24 is turned on, a video signal It' of a base band inputted from an apparatus such as a camera is provided to the coupler 22. If the first switch 24 is turned off, the video signal It' provided to the coupler 22 is blocked. The coupler 22 combines an output signal of the modulator 21 with an output signal of the first switch 24 and provides the combined signal to the first amplifier 23, and the first amplifier 23 amplifies the inputted signal and applies the amplified signal to a communication line 40.

[46] The receiving unit 30 comprises a second amplifier 31 for receiving an attenuated signal from the communication line 40, amplifying the attenuated signal, and outputting the amplified signal, a band pass filter 32, a demodulator 33, a low pass filter 34, and a second switch 36. The low pass filter 34 connected to the second amplifier 31 receives an output signal of the second amplifier 31, separates a video signal of a base band applied to the communication line 40, and outputs the video signal. If the first switch 24 is turned on, the second switch 36 is turned off to block the video signal transmitted from the transmission side not to be transmitted to an externally connected image display apparatus through the low pass filter. If the first switch 24 is turned off, the second switch 36 is turned on to transmit the video signal of a base band transmitted from a second modem 10' to the externally connected image display apparatus. In addition to the low pass filter, a notch filter or a band rejection filter may be used as a filter for separating the video signal from the output signal of the second amplifier 31. The band pass filter 32 receives the output signal of the second amplifier 31, blocks a signal of the frequency modulated by the modulator 21 of the transmission unit 20 and applied to the communication line, and passes a signal of the frequency modulated by a modulator 21' of the second modem 10' connected to the communication line 40 and applied to the communication line. That is, the band pass filter 32 separates a voice/data signal modulated by the modulator 21' of the second modem 10' connected through the communication line 40. In addition, the demodulator 33 removes a carrier used by the modulator 21' of the second modem 10 connected to the communication line 40 from the signal, which has passed through the band pass filter 32, and outputs a voice/data synthesis signal Vs+Ds in a synthesized form before being modulated by the second modem 10' Although the first and second switches 24 and 36 can be configured as physical switches, they may also be implemented using a logical switch.

[47] Hereinafter, the operation of the modem according to the present invention will be described in detail with reference to Figs. 4 to 7. In the embodiment, the second modem 10' transmits video, voice, and data signals, and the first modem 10 transmits a voice signal and a data signal.

[48] In the first modem 10 and the second modem 10' shown in Fig. 5, the frequency of the modulator is symmetrical to the frequency of the demodulator and the band pass filter. A voice and data signal Vs+Ds transmitted from the second modem 10' to the first modem 10 is modulated onto a carrier frequency fcs through the modulator 21'. Here, the fcs is a frequency of a video bandwidth of 4.5MHz or higher, and the central frequency is 5.5MHz in the embodiment. A signal modulated by the modulator 21' is combined with a video signal of a base band that has passed through the first switch 24' by the coupler 22' to be a signal ® having the spectrum shown in Fig. 4. The signal ® combined by the coupler 22' is applied to the communication line 40 through the amplifier 23'. In this case, since the second modem 10 applies a video signal to the communication line, the first switch 24 of the first modem 10 is turned off to block a video signal from being inputted into the communication line 40 from a camera connected to the first modem 10, and only the voice and data signal Vs+Ds modulated onto the frequency fct by the modulator 21 is applied to the communication line through the amplifier 23. At this time, if the first switch 24 of the first modem 10 is turned on and a video signal is applied to the communication line 40 from the first modem 10, the video signal transmitted from the first modem 10 is combined with the video signal transmitted from the second modem 10', and thus an original signal cannot be restored. Accordingly, the first switch 24 of the first modem 10 is turned off to block the video signal not to be transmitted from the first modem 10. However, the second switch 36 of the first modem 10 is turned on to receive video information transmitted from the second modem 10'. The on/off states of the first and second switches 24, 36, 24' and 36' of the respective modems 10 and 10' are controlled to transmit a video signal from one of the modems to the other using a protocol between the first modem 10 and the second modem 10'. In the embodiment, the first modem 10 transmits only voice and data signals, and the modulator 21 of the first modem 10 performs modulation on the voice and data signal using the frequency fct of a carrier. Here, fcs and fct are different frequencies and should have a difference to such an extent that a signal modulated by the first modem 10 does not overlap with a signal modulated by the second modem 10' so that the band pass filter 32 or 32' of one of the modems 10 and 10' can separate a signal transmitted from the counterpart modem 10 or 10'. In the embodiment, the central frequency of fct is about 6.5MHz. The frequency difference can be changed depending on characteristics of the band pass filters. Since the bandwidth of the voice and data signal is 128KHz if Manchester coding is applied, the frequency difference should be at least 128KHz. Practically, the frequency difference is preferably 500KHz or more in consideration of characteristics of the band pass filters. The spectrum © of an output signal outputted from the amplifier 23 of the first modem 10 is shown in Fig. 6. Since the signal of Fig. 6 outputted from the first modem 10 and the signal of Fig. 4 outputted from the second modem 10' are simultaneously applied to the communication line 40, a signal having the spectrum shown in Fig. 7 is applied to the communication line 40.

[49] Accordingly, both the first and second modems 10 and 10' receive a signal having the spectrum shown in Fig. 7 from the communication line 40 through the respective receiving units 30 and 30'. Video signals are separated from the signals inputted through the amplifiers 31 and 31' of the receiving units 30 and 30' of the first and second modems 10 and 10' by the low pass filters 34 and 34'. Although the receiving unit 30' of the second modem 10' receives a video signal transmitted by the second modem, the video signal is not externally outputted since the second switch 36' is turned off. The frequency of the band pass filter 32 of the receiving unit 30 of the first modem 10 is set to separate only the modulated voice and data signal transmitted from the second modem 10' and to block the voice and data signal transmitted by the first modem, and the frequency of the band pass filter 32' of the receiving unit 30' of the second modem 10' is set to separate only the modulated voice and data signal transmitted from the first modem 10 and to block the voice and data signal transmitted by the second modem. In addition, the frequency of each of the demodulators 33 and 33' is set to demodulate only a signal that has passed through each of the band pass filters 32 and 32'. Although the modulator of each of the modems uses a modulator of frequency modulation in the embodiment, a modulator of AM or PM can also be used.

[50] When a pair of modems according to the present invention are used, a video signal can be transmitted in half-duplex and voice and data signals can be transmitted in full- duplex using a pair (Ip) of TIV lines installed in a previously constructed multi-family house.

[51] Fig. 8 is a block diagram of another embodiment of a modem according to the present invention. The embodiment shown in Fig. 8 is different from the embodiment shown in Fig. 5 in that although the amplifier connected to the communication line in the embodiment shown in Fig. 5 drives the communication line with a single output, the communication line can be driven with a differential output as shown in Fig. 8 in an environment where the wiring condition of the communication line is poor or noisy.

[52] Fig. 9 is a block diagram of a private telephone system for a multi- family house using the modem according to the present invention.

[53] A private telephone system for a multi-family house according to the present invention comprises a plurality of terminal side modems 10' according to the present invention installed in respective homes of the multi-family house, a plurality of system side modems 10 according to the present invention respectively connected to the plurality of terminal side modems 10' and installed in a control room, a switching apparatus 110 connected to the system side modems 10, for connecting the system side modems 10 to allow transmission of signals among the terminal side modems 10', a control apparatus 120 for controlling the switching apparatus 110, and a digital video/ voice storage apparatus 130 connected to the control apparatus 120. As shown in Fig. 5, the demodulator of the terminal side modem 10' has the same frequency as the modulator of the control room side modem 10, and the demodulator of the control room side modem 10 has the same frequency as the modulator of the terminal side modem 10'. Telephone terminals 111, 113, 115, and 117 are connected to the respective terminal side modems 10'. In the embodiment, reference numeral 111 denotes a lobby interphone, reference numeral 113 denotes an indoor interphone, reference numeral 115 denotes an outdoor interphone, and reference numeral 117 denotes a management interphone. A camera 112 is connected to the modem 10 to which the lobby interphone is connected, and an image display apparatus is connected to the modem 10 to which the management interphone is connected.

[54] All of the indoor interphone 113, the outdoor interphone 115, an indoor display apparatus 114, and an entrance hall camera 116 are connected to the modem 10 through a switching box 119. The switching box connects the entrance hall interphone 115 to the indoor interphone 113 when a visitor desires to communicate with the indoor interphone 113 through the entrance hall interphone 115. If a resident is absent, the switching box connects the entrance hall interphone to the modem 10 so that the visitor can communicate with the management interphone 117 or to the modem 10 so that video signals photographed by the entrance hall camera 116 can be stored in the digital video/voice storage apparatus 130. In addition, the switching box 119 also connects the entrance hall camera 116 to the indoor display apparatus 114 so that video signals of the entrance hall camera 116 can be transmitted to the indoor display apparatus 114 when the visitor communicates with the resident through the outdoor interphone 115 and the indoor interphone 113. The control apparatus 120 stores video signals and/or voice signals transmitted from the modem 10 to which the camera 112 is connected in the digital video/voice storage apparatus 130, or reads video information stored in the digital video/voice storage apparatus 130 and transmits the video information to the modem to which the indoor display apparatus 114 and the indoor interphone 113 are connected if there is a request for transmitting the stored video/ voice signals from the indoor interphone 113.

[55] According to the present invention, a private telephone system for a multi-family house capable of transmitting video signals, voice signals, and data signals can be constructed without changing communication lines of an interphone system installed in a previously constructed multi- family house, using a modem capable of transmitting video signals in half-duplex and voice and data signals in full-duplex. Particularly, even though the present invention is applied to a multi-family house where an existing star type private telephone system is constructed, it is possible to communicate with a visitor while viewing images of the visitor when the visitor comes to a private entrance or a common entrance, in addition to voice communications, without changing communication lines. If a visitor comes while a resident is absent, images of the visitor are stored in a digital video recorder (DVR) or other image storage apparatuses, and the resident may confirm the visitor after returning to home. Industrial Applicability

[56] When the transmission method according to the present invention, which can transmit video signals in half-duplex and voice signals and data signal in full-duplex, is used, a modem can be manufactured at a low price. In addition, in addition to reduction in the manufacturing costs of the modem, video, voice, and data signals can be transmitted without changing TIV communication lines installed in a previously constructed multi-family house, and thus there are advantages in that construction time and costs can be reduced when a private telephone system capable of transmitting video, voice, and data signals is constructed in the previously constructed multi-family house. Furthermore, when a private telephone system for a multi-family house is constructed using the modem according to the present invention, a management apparatus can provide a variety of useful information through a display apparatus or an interphone connected to the multi-family house.

[57] The embodiments of the present invention described above and illustrated in the drawings should not be construed as limiting the technical spirit of the present invention. The scope of the present invention is defined only by the appended claims and those skilled in the art can make various modifications and changes without departing from the technical spirit and scope of the present invention. The modifications and changes will fall within the scope of the invention so far as they are apparent to those skilled in the art.

Claims

[1] A method of transmitting voice and data signals in full-duplex and video signals in half-duplex between a first modem and a second modem, the method comprising: in the first modem, a voice/data signal synthesizing step of combining a voice signal converted into a digital signal with a data signal, a first modulation step of modulating the synthesized voice/data signal onto a first carrier, a transmission signal coupling step of combining the modulated signal with a base band video signal, and a first transmission signal transmitting step of applying the coupled signal to a communication line; in the second modem, a voice/data signal synthesizing step of combining a voice signal converted into a digital signal with a data signal, a second modulation step of modulating the synthesized voice/data signal onto a second carrier, and a second transmission signal transmitting step of applying the modulated signal to a communication line; in the first modem, a receiving step of receiving the transmission signal applied to the communication line, a voice/data synthesis signal separating step of separating the voice/data synthesis signal modulated onto the second carrier from the received transmission signal, and a demodulating step of demodulating the separated voice/data synthesis signal by removing the second carrier; and in the second modem, a receiving step of receiving the transmission signal applied to the communication line, a video signal separating step of separating the base band video signal transmitted by the first modem from the received transmission signal, a voice/data synthesis signal separating step of separating the voice/data synthesis signal modulated onto the first carrier from the received transmission signal, and a demodulating step of demodulating the separated voice/data synthesis signal by removing the first carrier.

[2] A modem including a transmission unit for transmitting a signal to a communication line, and a receiving unit for receiving the signal transmitted to the communication line, wherein: the transmission unit comprises: a modulator for combining a digitalized voice signal with a data signal and modulating the combined signal, a first switch for selectively switching an input of a base band video signal, a coupler for combining an output signal of the modulator with an output signal of the first switch, and outputting the combined signal, and a first amplifier for amplifying an output signal of the coupler and applying the amplified signal to the communication line; the receiving unit comprises: a second amplifier for receiving a signal from the communication line and amplifying and outputting the received transmission signal, a video separating filter for receiving an output signal of the second amplifier and separating a base band video signal, a second switch for selectively switching an output of the video separating filter, a band pass filter for receiving the output signal of the second amplifier, blocking the signal modulated by the modulator of the transmission unit and applied to the communication line, and passing a signal modulated by a modulator of another modem connected to the communication line and applied to the communication line, and a demodulator for demodulating the signal passing the band pass filter into a synthesized signal of a digitalized voice signal and a data signal by removing a carrier used by the modulator of another modem connected to the communication line from the signal passing the band pass filter; and the second switch is turned off if the first switch is turned on, and the second switch is turned on if the first switch is turned off.

[3] The modem according to Claim 2, wherein the video separation filter is any one selected from a low pass filter, a notch filter, and a band rejection filter.

[4] The modem according to Claim 3, wherein the digitalized voice signal is a signal digitalized in pulse code modulation (PCM).

[5] The modem according to Claim 4, wherein the modulator is a frequency shift keying (FSK) modulator.

[6] The modem according to Claim 5, wherein a modulation frequency is in a range of 5MHz to 20MHz.