Method and system for multiplex transmission of an audio signal and a video signal through a communication cable.

BACKGROUND OF THE INVENTION:

Field of the Invention:

0001The present invention relates to a system and method for audio-video multiplex transmission through a pair of telephone communication cables by multiplexing simultaneous two-way voice signals and, a frequency-modulated carrier of monochrome/color signal.

Description of the Prior Art:

0002There is known a method of baseband-transmitting simultaneous two-way voice signals through a pair of telephone communication cable and a video signal with power supply through co-axial cable respectively, as employed in a door-phone camera and so forth. As will be understood from the spectrum distribution of NTSC (National Television System Committee) signal as shown in Fig. 1, many times a video signal includes a low frequency spectrum down to a DC component. In this diagram, S1 represents the spectrum of a luminance signal and S2 that of a chrominance subcarrier, respectively.

0003When audio and video signals are directly multiplexed according to the conventional method of transmission, there arises a disadvantage that the low-frequency component of the video signal comes to overlap with the audio signals in baseband transmission. For the purpose of preventing such spectra of audio and video signals from overlapping, a carrier is frequency-modulated by the video signal as to avoid appearance in its low-frequency range of any video component and then such modulated video carrier and the voice signals are multiplexed for simultaneous transmission. Fig. 2 graphically shows a spectrum distribution of a carrier that is directly frequency-modulated by the NTSC signal.

0004At the demodulator output, as shown in Fig. 3, the 2nd harmonic component S4 of the FM carrier spectrum is superposed on the demodulated video signal S3. and the wide- band useless component S4 can be removed by means of a low-pass filter so that the required video signal S3 alone is obtained.

0005However, if the carrier frequency of the video signal is set to be low, the lower side of the 2nd harmonic component S4 of the FM carrier overlaps with the upper-side component of the demodulated video signal at the demodulator output to consequently bring about 'moire' interference. Since it is necessary to set the carrier frequency as high as 4.5 to 5.5 -MHz according to the conventional method of transmission, it follows that the bandwidth necessary for FM carrier transmission becomes wide to eventually raise a problem in multichannel transmission or the like.

0006There is also known a means for baseband transmission of a video signal with a telephone communication cable. Here the telephone communication cable is not only limited to an inhouse cable alone (for example, a communication cable composed of a pair of parallel or twisted wires each having a diameter of 0.65 mm and coated with polyvinyl chloride), but also for interphone it includes parallel vinyl lines generally used to AC power supply (composed of a pair of twisted wires (20/0.18 or 30/0.18)). Hereinafter any of the above communication cables will be referred to simply as cable. There exists a problem that such cable is affected harmfully by external radio waves, and particularly medium waves (550 - 1350 kHz) are induced along the cable to cause "moire" interference and so forth on a reproduced picture at the receiving end. That is, on a television picture, there will appear "moire" pattern if any interference wave superposed on the video signal is more than -50dB, whereby the picture quality is deteriorated. In order to eliminate normal mode" interference, a push-pull transmission with balancers may be preferable, but it is still difficult to completely prevent 'common mode' disturbance caused by medium waves at 'high field-intensity areas. Furthermore, due to the necessity of fixing the cable with staples or threading the cable through an iron pipe and so forth it is unavoidable that some effects are actuated with respect to the environmental condition for the cable, hence causing multipath reflection of the carrier being transmitted through the cable. Particularly, transmission characteristics on a span over 100 meters indicate a uniform descending curve at high frequencies occasionally with some dips of -6 dB or so at medium frequencies (0.5 to 2 MHz) caused by impedance mis-matching at cable terminators. It may be contrived to transmit the video signal by the use of a co-axial cable for the purpose of obtaining satisfactory reproduced picture. However, when replacing an existing equipment for interphone with new one for door phone camera to pick up visitors, for example, there still remains a disadvantage of requiring additional work to install another co-axial cable separately besides the existing one. OBJECTS AND SUMMARY OF THE INVENTION:

0007The present invention has been accomplished in view of the circumstances mentioned above. And its principal object resides in providing an improved method which, in multiplex transmission of audio and video informations, is capable of separating the pass-band necessary for the frequency-modulated carrier of video signal from- that necessary for audio signals and also from broadcast band of medium waves.

0008And another object of the invention is to provide an improved method which ensures high quality reproduction of picture with facility in multiplex transmission of audio and video signals through cables.

0009Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

0010In order to achieve such objects, the present invention is constituted as follows.

0011In signal multiplex transmission, video signals to be transmitted unidirectionally or bidirectionally are multiplex with audio signals to be in baseband-transmitted bidirectionally, and such multiplex signals are transmitted simultaneously through a pair of telephone communication cables. At the sending end of a video signal, at first it is separated into a luminance signal and a chrominance signal. Then the carrier is frequency-modulated with the luminance signal while the chrominance signal is down-converted to a lower band, and the audio signals and such frequency-modulated carrier of the luminance signal are transmitted simultaneously after being multiplexed by frequency division. At the receiving end, the luminance signal is reproduced by frequency-demodulation while the chrominance signal is up-converted into the original band and the original video signal is restored by combining the reproduced luminance and chrominance signals with each other.

0012In another embodiment of the present invention, video signals to be transmitted unidirectionally or bidirectionally are multiplexed with audio signals to be transmitted bidirectionally, and then such multiplex signals are transmitted simultaneously. At the carrier's sending end, a carrier of the video signal is frequency-modulated with a band ranging from 1 to 10 MHz, and the carrier thus modulated is multiplexed with the audio signals which are to be baseband-transmitted in a band below 10 kHz. Then the multiplex signals are transmitted to the carriers receiving end through a pair or one of pairs of telephone communication cables. Meanwhile on the receiving end, the multiplex signals are separated into the audio signal and the video carrier, and then the separated video carrier is demodulated after removal of the amplitude-modulated component by limiters.

BRIEF DESCRIPTION OF THE DRAWING:

0013The present invention will be better understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: <ul id="ul0001" list-style="none"><li>Fig. 1 is a spectrum distribution diagram of NTSC signal:</li><li>Fig. 2 is a spectrum distribution diagram obtained when a video carrier is directly frequency-modulated by the NTSC signal:</li><li>Fig. 3 is a spectrum distribution diagram obtained when the carrier is demodulated into the NTSC signal:</li><li>Fig. 4 and 5 show a first embodiment of the present invention, in which Fig. 4 is a block diagram of the outdoor's unit of door-phone camera equipment and Fig. 5 is a block diagram of the indoor's unit ;</li><li>Fig. 6 is a spectrum distribution diagram of down-converted chrominance signal and a frequency-modulated luminance carrier:</li><li>Fig. 7 is a block diagram schematically showing the constitution of a bidirectional video transmission equipment in a second embodiment of the invention:</li><li>Fig. 8 is a spectrum distribution diagram in a bidirectional video transmission mode;</li><li>Fig. 9 is a block diagram schematically showing the constitution of a door-phone camera multiplex transmission system in a third embodiment which carries out the audio-video multiplex transmission method of the invention:</li><li>Fig. 10 graphically shows the transmission characteristics of a PVC cable made of parallel wires of 0.65 mm in each diameter and 50 m in length:</li><li>Fig. 11 is a spectrum distribution diagram of a frequency-modulated video carrier in the embodiment:</li><li>. fig. 12 is a block diagram schematically showing the constitution of a inhouse TV telephone system in a fourth embodiment of the invention:</li><li>Fig. 13 is spectrum distribution diagrams of bidirectional frequency-modulated video carriers in the embodiment of fig. 12: and</li><li>Fig. 14 is a spectrum distribution diagram of a frequency-modulated NTSC carrier transmitted in another embodiment of the invention.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS:

0014Fig. 4 and 5 show transmission processors for an exemplary embodiment of the present invention, in which Fig. 4 is a block diagram of outdoor processors and Fig. 5 is that of indoor processors.

0015A color video signal inputted to the outdoor processors is separated into a luminance signal a and a chrominance signal b (frequency fs) through a low-pass filter 11 and a band-pass filter 12. The luminance signal a is fed to a preemphasis circuit 13 which serves to improve the signal-to-noise ratio, S/N, by emphasizing the high-frequency component of the luminance signal a. The luminance signal c thus emphasized is than fed to a frequency- modulator 14. A carrier's frequency of 3.4 MHz is allocated to the synchronizing-pulse's tip (i.e. sync-tip) and 4.4 MHz, to the white-peak of the video signal. The frequency-modulated luminance carrier d is fed to a mixer 16 through a high-pass filter 15.

0016MeanwhIle, the chrominance signal b (frequency fs) separated by the band-pass filter 12 is fed to a balanced modulator 17, to which is also fed a carrier of a frequency from a local oscillator 18. As the chrominance signal b is modulated with the carrier in a balanced manner, it is frequency-converted to a signal having frequency components which correspond to the sum and the difference (f + fs, f -fs). Such components are fed to a band-pass filter 19, from which is obtained the difference component g between the two frequencies f and fs. In this embodiment, the chrominance signal around 3.58 MHz is down-converted to a signal around 0.629 MHz. The down-converted component g is fed to the mixer 16.

0017The mixer 16 is supplied also through low-pass filter 20 with two-way audio signals to be in baseband-transmitted from mike amplifier 22 or to speaker amplifier 21. As a result, the mixer 16 produces an output by multiplexing the frequency-modulated luminance carrier, the down-converted chrominance signal and the two-way audio signals, and then such output through a pair of telephone communication cables 30.

0018The multiplex signals transmitted through the telephone communication cable 30 are inputted to the indoor processors shown in Fig. 5. The input multiplex signals are introduced to high-pass filter 41, band-pass filter 43 and low-pass filter 42 disposed in the first stage of the indoor processors, whereby the multiplex signals are separated into frequency-modulated luminance carrier b, two-way audio signals i and down-converted chrominance signal J. Through low-pass filter 42, the two-way audio signals are fed to speaker amplifier 28 or from mike amplifier 29 and the frequency-modulated luminance carrier b is fed to a limiter 44, where the amplitude-modulated component is removed. The output of the limiter 44 is fed to a deemphasis circuit 45 which suppresses the high-frequency component emphasized on the transmitter (outdoor) side. The output of the deemphasis circuit 45 is fed to a frequency-demodulator 49 for demodulation, and the luminance signal k is reproduced through a low-pass filter 47 in the next stage.

0019Meanwhile, the down-converted chrominance signal j separated by the band-pass filter 43 is fed to a balanced modulator 49, which then modulates the output (frequency f) of a local oscillator 50 and the down-converted chrominance signal to produce a signal 1 having frequency components fs and 2f - fs.- This output signal 1 is fed to a band-pass filter 51, from which a chrominance signal m (frequency fs ) is restored.

0020The chrominance signal m thus restored is fed to a mixer 48, which then mixes the chrominance signal m with the luminance signal k to reproduce the color video signal.

0021Due to conversion of the chrominance signal (frequency fs; 3.58 MHz) to a lower band around e.g. 0.629 MHz as described above, a deviation of the carrier of the luminance signal can be set into a range as low as 3.4 to 4.4 MHz. This is obvious from the fact that, with removal of the chrominance signal component from the NTSC signal, the luminance signal bandwidth of merely 2MHz or more seems to be practically enough as well as home VTRs. Fig. 6 shows a spectrum distribution obtained when the chrominance signal is down-converted to a lower band and the sync-tip and white-peak frequencies of the luminance carrier are allocated to 3.4 and 4.4 MHz. In this diagram, S5 and S6 represent the down-converted chrominance signal and the frequency-modulated luminance carrier, respectively. As will be clear from comparison of Fig. 6 with Fig. 2, the bandwidth of the luminance carrier transmitted is extremely narrowed in this embodiment. It signifies that multichannel transmission of video signals becomes possible.

0022An example of such multichannel transmission is shown in Fig. 7, which is a block diagram of an embodiment for bidirectional transmission of a color video signal. In this diagram, carriers transmitters T1, T2 and carrier's receivers R1, R2 such shown in Fig. 4 and 5 respectively are connected to the ends of a telephone communication cable 30, wherein bidirectional transmission of a color video signal is rendered possible by properly allocating the luminance carrier's modulation frequency and the chrominance signal conversion frequency in such a manner as to avert the spectra of individual channel from overlapping. The spectra obtained in this example are shown in Fig. 8, wherein S7, S8 and S9, S10 represent the spectra of down-converted chrominance signals and those of frequency-modulated luminance carriers; respectively.

0023Fig. 9 is a block diagram schematically showing the constitution of another embodiment according to the present invention. This diagram illustrates a door-phone camera multiplex transmission system consisting of a outdoor unit 110 and a indoor unit 120. The outdoor unit 110- comprises a speaker 111, a mike 115 and a CCD camera 112 for picking up visitors' figure and voice and also internally includes an FM modulator 113 for frequency-modulating a carrier with the video signal outputted from the CCD camera 112, and a multiplexer 114 for multiplexing the two-way audio signals and the frequency-modulated video carrier.

0024The indoor unit 120 comprises.a separator 121 for separating the multiplex signals transmitted from the outdoor unit 110 into the audio signal and the video carrier, a handset 122, a limiter 123 for removing the amplitude-modulated component from frequeny-modulated-video carrier separated by the separator 121, an FM demodulator 124 for demodulating the frequency-modulated carrier posterior to removal of its amplitude-modulated component, and a television monitor 125 fed with the demodulated video signal. The outdoor unit 110 and the indoor unit 120 are connected mutually by means of a inhouse cable 30. The cable 30 is composed of, for example, a pair of parallel PVC-coated wires of 0.65 mm in each diameter.

0025In the embodiment of the constitution mentioned above, the following operation is performed.

0026Fig. 10 graphically shows the measured characteristic curve of the cable 30 (50 m in length) having a characteristic impedance of 140 ohms and employed in the embodiment. Dips at 650 kHz and 3 MHz in this graph result particularly from impedance mis-matching at the cable terminators. And the rugged portion of the curve at frequencies above 10 MHz results from multi-reflection generated at positions stapling the cable on walls and beams. The above is inferred from the fact that the dip frequencies are changed by the span of the cable. As will be understood from Fig. 10, the cable employed in this embodiment has transmission characteristics within -4 dB up to 10 MHz. Therefore, when a carrier is frequency-modulated in the FM modulator 113 with the video signal outputted with a bandwidth of 2 to 3 MHz from the CCD camera 112 in the outdoor unit 110, the carriers spectrum is extended up to a range of 1.5 to 6.5 MHz as shown in Fig. 11, but the cable 30 is still capable of transmitting the frequency-modulated video carrier properly with slight attenuation.

0027Meanwhile, the audio signal outputted from the mike amplifier in the handset 111 is multiplexed with the frequency-modulated video carrier by the multiplexer 114. In this stage, crosstalk between the audio signal and the video carrier can be prevented by selectively allocating the audio channel in a frequency range below 10 kHz. The audio signal multiplexed with the video carrier can be bidirectionally transmitted as in the conventional interphone line due to its baseband-transmission performed usually in any ordinary interphone or the like.

0028The multiplex signals thus produced are transmitted from the outdoor unit 110 to the indoor unit 120 through the cable 30.

0029The cable 30 used in this embodiment is not furnished with an electro-magnetic shield function unlike a co-axial cable so that in case its transmission span is relatively- long, there arises a problem with regard to leakage electric field to external instruments. This problem will be discussed in a paper to be issued; 'Door Phone Camera's Multiplex Transmission System` , IEEE Transaction on Consumer Electronics.

0030In Fig. 9, the multiplex signals transmitted through the cable 30 are inputted to the separator 121 of the indoor unit 120, where the video carrier is separated with the audio signals by means of the high-pass filter (HPF) as shown in Fig. 5. Since the high-pass filter is so set as to perform a enough attenuation of the medium waves and the limiter 123 to perform a complete limiting action of those, it becomes possible, in any place except the vicinity of a medium wave broadcasting station where the electric field intensity is extremely high, to achieve complete suppression of interference by some external radio waves during transmission through the cable 30. Multipath reflection is prone to occur in the carrier transmission through the cable due to some change in the environmental condition of the cable as it is fixed with staples at predetermined intervals or is covered with an iron pipe or the like. However, the video carrier, even if subjected to the rugged attenuation resulting from such multipath reflection, can be kept free from harmful influence of the multipath reflection and so forth by the limiter function.

0031The frequency-modulated video carrier with its ruggedly attenuated component that is suppressed in the limiter 123, is demodulated in the FM demodulator 124 and then is fed to the television monitor 125.

0032Since the audio signal and the video signal are different in pass-band from each other as described previously, there occurs no crosstalk therebetween and, by the function of the high-pass filter and the limiter 123, which sufficiently suppresses the amplitude-modulated component resulting from external radio waves and so forth, a remarkably high quality is attainable in the picture reproduced on the television monitor 125. Furthermore, when replacing the interphone system in use with a TV door-phone system (including such as CCD camera 112 in the embodiment), there is no necessity of newly installing a co-axial cable for transmission of the video signal, and the already installed telephone cable can be converted to consequently save the installation cost.

0033By using a pair of parallel PVC-coated wires as a cable 30 in the above embodiment, it is also possible, as shown in Fig. 12, to realize a inhouse television telephone system for bidirectional transmission of the video signals.

0034In Fig. 12, a terminal unit 140 comprises a handset 141, a CCD camera 142 for picking up a speaker, an FM modulator 143 for frequency-modulating a carrier with the video signal outputted from the CCD camera 142, a multiplexer/separator 144 for multiplexing the audio signal from mike amplifier and the frequency-modulated video carrier while separating the transmitted multiplex signals into the audio signal and the video carrier, a limiter 145 for removing the amplitude-modulated component from the separated video carrier, an FM demodulator 146 for demodulating the video carrier posterior to the limiter 145, and a television monitor 147 supplied with the demodulated video signal. Another terminal unit 150 has the same constitution as that of the aforesaid terminal unit 140. These terminal units 140 and 150 are connected with each other through a single cable 60, which has the same transmission characteristics as that of the cable 30 shown previously in Fig. 10.

0035If the video carriers transmitted from the two terminal units 140 and 150 are so selected that the carriers' spectra thereof are distributed within a range of 1 to 13 MHz as shown in Fig. 13 and allocated without mutual overlapping, then it becomes possible for the cable 60 to transmit the video carriers properly to the receiving ends without substantial attenuation, whereby the cable characteristic deterioration of the carrier at high frequencies can be compensated by a cable compensator to consequently attain proper reproduction of picture without substantially deteriorating the signal-to-noise ratio.

0036Such composite signals as NTSC (National Television System Committee) signal or PAL (Phase Alternation Line) signal are composed of the chrominance and luminance signals , and the carrier directly frequency-modulated with the composite signal can be transmitted through the above-mentioned cable. In an exemplary case of transmission of NTSC signal, Fig, 14 shows the frequency distribution of an FM carrier on "low-band color mode' formerly adopted in four-head videotape recorders for broadcasting stations. As will be clear from comparison between the two cable transmission characteristics shown in Figs. 10 and 14, the directly frequency-modulated carrier spectrum as shown in Fig. 14 ranges substantially within a band of 2 to 11 MHz so that the high frequency component of the carrier is slightly deteriorated by the cable transmission, but such deterioration is so small as be compensable with facility. Although such 'low-band color mode" was adopted in the initial-stage color videotape recorders, it was replaced shortly later with "high-band color mode" on the basis of improvements in both tape and head performances as the former was disadvantageous with regard to tape noise and so forth due to a small modulation index. However, since noise source in this cable are principally interference induced by external radio waves and usually very small, the low-band color mode is sufficiently effective in practical use because, when the cable length is less than a few of hundreds meters, a considerable high C/N (ratio of carrier to noise) is attainable in most cases.

0037As another means to transmit a color signal there is a so-called TCI (Time Compression Integration) signal transmission system which multiplexes a time-compressed color-difference signal during the horizontal blanking period of a luminance signal by line alternation.

0038The TCI signal in which the luminance signal is supposed to have a bandwidth of 3 MHz, can comprise the color-difference signals, (R - Y) and (B - Y), of 0.4 MHz in bandwidth, which effective duration of a horizontal scan, 53 s, is time-compressed to one-seventh, 7.5 s. Since the horizontal blanking duration of NTSC signal amounts to about 11 s, the time-compressed color-difference signals are able to accommodated along-with a horizontal narrower pulse of 2 s in pulsewidth during the blanking period in line alternation.

0039As shown in S5 of Fig. 6 and in S7 of Fig. 8, the passband of the down-converted chrominance signal is unfortunately overlapped with those of medium waves for broadcasting, so that in case subject to suffer interference by broadcasting waves, the transmission using the carrier frequency-modulated with the TCI signal may be preferable rather than the transmission using the down-converted chrominance signal.

0040The method of audio-video multiplex transmission according to the present invention is applicable to PAL, SECAM or TCI signal as well as to the aforementioned NTSC signal.. Furthermore, in case the signal-to-noise ratio (S/N) of the chrominance signal is deteriorated by interference of some external radio waves induced along the telephone communication cable, the S/N can be improved by use of a comb filter in the NTSC or PAL system where there exists a line interrelation. Also in the SECAM system where the chrominance signal is frequency-modulated, the S/N can be improved by the provision of a limiter.

0041The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.

0042There are described above novel features which the skilled man will appreciate give rise to advantages. There are each independent aspects of the invention to be covered by the present application, irrespective of whether or not they are included within the scope of the following claims.