GB2289590A

GB2289590A - Combining a compressed audio signal with a video signal

Info

Publication number: GB2289590A
Application number: GB9418905A
Authority: GB
Inventors: Maeda Shigeho; Hanada Haruyoshi
Original assignee: Elbex Video KK
Current assignee: Elbex Video KK
Priority date: 1994-05-19
Filing date: 1994-09-20
Publication date: 1995-11-22
Also published as: JPH07322222A; GB9418905D0; DE4441517A1

Description

d 4-1 - 1 2289590 TRANSMITTING AND RECEIVING APPARATUS FOR VIDEO SIGNAL

WITH COMPRESSED AUDIO SIGNALS

BACKGROUND OF THE INVENTION Field of the Invention:

This invention relates to a transmitting and receiving system for video signals injected with an audio signal, and more particularly to a transmitting and receiving apparatus for video signals injected with an audio signal whose time base is compressed.

Description of the Prior Art:

In a monitoring television system such as a closed circuit television network, the video signal and an audio signal are separately transmitted. Therefore, in such a system, dual transmission lines are required for the video and the audio signals which is costly. Moreover, in such a system, the receiving apparatus becomes duplicated and complex for large monitoring because separate selections and receiving video and audio channels must be used.

There is known system for multiplexing a carrier wave whose frequency is modulated by an audio signal onto the video signal and for transmitting audio-video multiplexed signal via single transmission line. However, in such a system, the carrier signal frequency must be outside the video frequency bandwidth, moreover, the signal level must be minimal in order not to cause noise into the video signal.

This requires an extended bandwidth transmission line and very high gain receiving circuits and this causes crosstalk between channels of the receiving apparatus.

There is another known system for transmitting an image signal and an audio signal through a common transmission line, using different carrier waves, just as a case of television broadcasting. However, in such a system, both a transmitting apparatus and a receiving apparatus become complicated and costly.

SUMMARY OF THE INVENTION

An object of the present invention is to enable the transmission of audio and video signals through a common transmission line without exerting any audio signal influence on the video signal or video signal influence on the audio signal.

An apparatus for transmitting television signals according to the present invention comprises a compression means for compressing a time base of an audio signal, an injection means for injecting the compressed audio signal into the video signal during the vertical blanking interval and a composition means for transmitting video signals injected with the compressed audio signal.

W A An apparatus for receiving television signals according to the present invention receives a composite video signal injected with compressed audio signals and more particularly an audio signal whose time base is compressed and injected during the vertical blanking period of a composite video signal, and comprises an extracting and restoring means for extracting the compressed audio signal from the received composite video signal and for restoring the time base of the extracted audio signal.

According to the present invention, since the time base of the audio signal is compressed and the compressed audio signal is injected during the vertical blanking period of the composite video signal, the audio signal can be transmitted through a video transmission line without exerting any influence on the transmitted image of the composite video signal.

The compression means of the transmitting apparatus preferably includes an analog-to-digital conversion circuit for converting the audio'signal to be transmitted into a digital signal, a memory for storing the digital signal outputted from the analog-to-digital conversion circuit, a control circuit for controlling the writing of the digital signal into the memory and the readout of the signal from the memory and for outputting the signal stored in the memory at a faster speed than the writing speed of the signal into the 1 memory, and a digital-to-analog conversion circuit for converting the digital signal read out from the memory back into an analog signal. Thereby, the time base of the audio signal can be efficiently compressed while the continuous audio signals are written in the memory.

The composition means of the transmitting apparatus preferably includes an output signal changeover circuit which is controlled by the control circuit for alternatingly switching over the composite output signal from a video signal to a compressed audio signal. Thereby, the video signal and the audio signal are sequentially transmitted and do not interfere with each other.

When the transmitted audio signal includes first and second audio signals such as stereo sound transmission, the compression means of the transmitting apparatus preferably includes an analog-to-digital conversion circuit having first and second A/D converters, the first A/D converter converting the first audio signal into a digital signal, and the second A/D converter converting the second audio signal into a digital signal, a memory for storing the digital signals outputted from the first and second A/D converters, a control circuit for controlling the writing of the digital signals into the memory and the readout of the signal from the memory and for outputting the signal stored in the memory at a faster speed than the writing speed of the signal into the memory, a digital-to-analog conversion circuit for converting the digital signal read out from the memory back into an analog signal, and a signal composing portion for composing output signals from both the D/A converters. Thereby, the first and second audio signals for stereo sound can be transmitted through a common transmission line used for video signal without exerting any influence on the image of the video signal or interacting upon each other.

In addition, when the transmitted audio signal includes first and second audio signals such as stereo sound transmission, the compression means of the transmitting apparatus may further include an audio signal changeover circuit for alternately switching over the first and second audio signals and for outputting the same to the analog-to-digital conversion circuit. Thereby, the first and second audio signals for stereo sound can be transmitted through a common transmission line used for video signal without exerting any influence on the image of the video signal and interacting upon each other.

The composition means of the transmitting apparatus preferably further includes a first clamping circuit for clamping the video signal to be fed to the signal changeover circuit at a predetermined voltage level, and a second clamping circuit for clamping an output signal from the digital-toanalog conversion A circuit to be fed to the signal changeover circuit at the same predetermined voltage level set to the video signal level. Thereby, the audio signal and the video signal both clamped to the same level can be alternatingly transmitted through a common transmission line without interacting upon each other.

Preferably, the compression means of the transmitting apparatus compresses the time base of the audio signal at the rate that a time corresponding to one vertical scanning period of the video signal becomes equal to a time corresponding to one horizontal scanning period of the video signal, and the composition means of the transmitting apparatus injects the compressed audio signal onto a single horizontal line period during the vertical blanking period of the video signal. Thereby, the audio signal cannot disturb or influence the image of the video signal.

The compression means of the transmitting apparatus in a preferred embodiment includes an analogto-digital conversion circuit for converting the audio signal to be transmitted into a digital signal, a memory for storing the outputted digital signals, a control circuit for controlling and coordinating the timing and the speed of the writing of the signals into the memory and the time and the speed of the readout of the signals from the memory, so that the readout of the written signal from the memory is compressed and outputted every 1 vertical scanning period and the writing of the audio signal during one vertical scanning period is read out during one horizontal scanning period, thereby the compression ratio is equal to the one vertical scanning time period divided by one horizontal scanning time period, and a digital-to-analog conversion circuit for converting the compressed signal read out from the memory into an analog signal, wherein the composition means of the transmitting apparatus preferably injects an output signal from the digital-to-analog conversion circuit into one horizontal scanning period during the vertical blanking period of the video signal.

Instead of injecting the compressed signal into the video signal, the composition means of the transmitting apparatus may include an output signal changeover circuit for switching over the output signal from the digital-to-analog conversion circuit and the video signal alternatingly, and for outputting the compresse d audio signal during one horizontal scanning period within every vertical blanking period of the video signal.

The extracting and restoring means of the receiving apparatus preferably includes an extracting circuit for extracting the compressed analog audio signal, an analog-to-digital conversion circuit for converting the compressed audio signal extracted from the extraction circuit into a digital signal, a memory 1 for storing the outputted digital signal from the analog-to-digital conversion circuit, a control circuit for controlling the time and the speed of the writing of the signal into the memory and the time and the speed of the readout of the signal from the memory and for outputting the signals stored in the memory at a slower, inverse speed to the writing speed of the signal in the memory, and a digital-to-analog conversion circuit for converting the signal read out from the memory back into an analog signal. Thereby, the audio signal whose time base is compressed is regenerated into continuous audio signal whose time base is restored.

When the received audio signal is an alternating compressed stereo signal of first and second audio signals, the digital-to-analog conversion circuit of the receiving apparatus preferably includes a first D/A converter for converting the first audio signal into an analog signal, and a second D/A converter for converting the second audio signal into an analog signal. Thereby, the first and second audio signals for stereo sound can be regenerated without influencing the image signal or one audio signal influencing the other.

In addition, when the stereo audio signal is an alternately combined signal of first and second audio signals, the extracting and restoring means of the receiving apparatus may further include a first sampleholding circuit for extracting and holding the A first audio signal from the digital-to-analog conversion circuit, and a second sample-holding circuit for extracting and holding the second audio signal from the digital-to-analog conversion circuit. Thereby, the first and second audio signals of a stereo sound can be regenerated without influencing the image signal or one audio signal influencing the other.

The extracting and restoring means of the receiving apparatus preferably further includes a clamping circuit for clamping the signal to be fed to the analog-to-digital conversion circuit at a predetermined voltage level, thereby, the received audio analog signals can be accurately converted into digital signals.

The extracting and restoring means for the receiving apparatus preferably further includes an audio band pass filter for removing any video signal components from the output signals of the digital-toanalog conversion circuit. Thereby, the audio signal can be regenerated without being affected by the video signal.

When the time base of the audio signal in the composite signal is compressed at the rate that a time corresponding to one vertical scanning period of the video signal becomes equal to a time corresponding to one horizontal scanning period of the video signal, the extracting and restoring means of the receiving m 1 9 - 10 apparatus preferably restores the time base of the extracted audio signal at the rate that a time corresponding to one horizontal scanning period of the video signal becomes equal to a time corresponding to one vertical scanning period of the video signal. Thereby, the audio signal is accurately regenerated.

When the compressed audio signal is injected to a specific horizontal line during the vertical blanking period of the video signal, the extracting and restoring means of the receiving apparatus preferably extracts the audio signal from the composite signal by separating the synchronizing signal from the composite video signal and for applying the separated synchronizing pulses to the controller, and timing circuit for extracting the compressed audio signal from the composite video signal and for inputting the extracted signal to the D/A conversion circuit during the specific horizontal line period. Thereby, the audio signal can be accurately extracted and regenerated.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and features of the invention will become apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings, in which:

Fig. 1 is a block diagram showing an electric circuit of a transmitting apparatus as an embodiment of 7 11 - the present invention; Fig. 2 is a view showing the wave form of an electric signal in the transmitting apparatus shown in Fig. 1; Fig. 3 is a block diagram showing an electric circuit of a receiving apparatus for separating a signal composed by the transmitting apparatus shown in Fig. 1 into a video signal and an audio signal as an embodiment of the present invention; Fig. 4 is a block diagram showing an electric circuit of a transmitting apparatus as another embodiment of the present invention; Fig circuit of embodiment Fig circuit of embodiment Fig circuit of embodiment Fig.

is a block diagram showing an electric a transmitting apparatus as another of the present invention; 6 is a block diagram showing an electric a transmitting apparatus as another of the present invention; 7 is a block diagram showing an electric a transmitting apparatus as a further of the present invention; 8 is a view showing the wave form of an electric signal generated by the transmitting apparatuses shown in Figs. 4, 5, 6 and 7; Fig. 9 is a block diagram showing an electric circuit of a receiving apparatus for separating a signal composed by any one of the transmitting apparatuses t shown in Figs. 4, 5 and 7 into a video signal and an audio signal as an embodiment of the present invention; and Fig. 10 is a block diagram showing an electric circuit of a receiving apparatus for separating a signal composed by the transmitting apparatus shown in Fig. 6 into a video signal and an audio signal as an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Fig. 1, a transmitting apparatus 10 is a composition apparatus for composing a video signal and an audio signal.

In the transmitting apparatus 10, a video signal from a video signal generator such as a television camera or a video player is fed to terminal 12, while an audio signal from an audio signal generator such as a microphone or a video player is fed to terminal 14.

In the illustrated embodiment, the video signal fed to the terminal 12 is a well known composite video signal which is fed to a clamping circuit 16 and a synchronizing separator circuit 18. The synchronizing separator circuit 18 extracts the horizontal synchronizing signal and the vertical synchronizing signal from the video signal, and feeds the extracted signals to a timing controller 20. The controller 20 generates various timing signals on the basis of the A 13 - synchronizing signals fed from the synchronizing separator circuit 18.

The audio signal fed to the terminal 14 is an analog signal successively passing through a variable resistor or a volume control 22 and an amplifier 24 and then fed to an analog-to-digital conversion circuit 26. The A/D converter 26 is activated at a predetermined period on the basis of the timing signal fed from the controller 20, and converts the analog audio signal during the activated period into a digital signal.

The output signal from the A/D converter 26 is fed to a memory 28. The memory 28 is also fed with the write timing signal from the controller 20 for controlling the writing speed, therefore, the memory 28 successively writes the signal fed from the A/D converter 26. The timing controller 20 also feeds the readout timing signal to the memory so that the signal written in the memory 28 is read out at a greater speed than the writing speed.

The readout speed of the signal from the memory 28 preferably corresponds to such a speed that a time corresponding to one vertical scanning period of the video signal is equal to a time corresponding to one horizontal scanning period. Otherwise, any other speed may be adapted to the readout speed. The readout timing of the signal from the memory 28 corresponds to one horizontal scanning period of a video signal preferably f a - 14 within the vertical blanking period, more preferably, is a specific horizontal scanning line period during the vertical blanking period.

The memory 28 may be provided with a plurality of memory portions for successively storing audio signals equal to a time portion corresponding to at least one vertical scanning period of the video signal, such memory arrangement permits simple successive sequencing of the memory portion in rotation for successively writing the signal and for successively reading out the signal every one vertical scanning of the video signal.

The signal read out from the memory 28 is converted into an analog signal in a digital-to-analog conversion circuit 30. The D/A converter 30 is also fed with a timing signal from the timing controller 20 so that the digital-to-analog conversion of the signal fed from the memory 28 is in synchronization with the readout of the signal from the memory 28.

The frequency of an audio signal shown in Fig. 2(A) is compressed by n times shown in Fig. 2(B), n is the rate of one vertical scanning period 1V of a video signal divided by a time corresponding to one horizontal scanning period 1H of the video signal shown in Fig. 2(B), in case of NTSC system, n can be defined as the number of horizontal scanning lines per one field, i.e., about 262.5. Thereby, the audio signal is compressed timewise, namely, a time base of the audio signal is compressed into l/n, therefore, the compressed audio signal frequency is increased by n times. The compressed audio signal is outputted from the D/A converter 30 during the vertical blanking period of the video signal 1Bv shown in Fig. 2(C), preferably during a specific one horizontal scanning period 1H among the vertical blanking period 1Bv as shown in Fig. 2(D).

An output signal from the D/A converter 30 is fed to a clamping circuit 32, the clamping circuit 32 is also fed by a timing signal from the controller 20. On the basis of the timing signal fed from the controller 20, the clamping circuit 32 clamps the outputted signal from the D/A converter 30 to a predetermined voltage level for a duration corresponding to an injection period of the compressed audio signal into the video signal, i.e., preferably during the 1H period of the 1V period shown in Fig. 2(D).

The timing signal fed from the controller 20 to the clamping circuit 32 is also fed from the controller 20 to a clamping circuit 16. On the basis of the timing signal fed from the controller 20, the clamping circuit 16 clamps the video signal fed from the terminal 12 to a predetermined voltage level during the remaining vertical scanning period 1V shown in Fig. 2(D).

The audio signal whose signal components are present during the 1H period of Figs. 2(B) and 2(D) is fed from the clamping circuit 32 to a signal input c 9 terminal of a signal changeover circuit 34, and a video signal shown in Fig. 2(C) is fed from the clamping circuit 16 to the other signal input terminal of the signal changeover circuit 34.

The timing signals fed from the controller 20 to the clamping circuits 16 and 32 are also fed to the changeover circuit 34 for controlling the changeover timing. The changeover circuit 34 is an electronic switch circuit that outputs alternatively the compressed audio signal during the horizontal scanning period 1H shown in Fig. 2(B), and the video signal during the remainder of the vertical scanning period 1V shown in Fig. 2(C) to terminal 36. Since both signals of the compressed audio signal and the video signal are clamped to the same predetermined level by the clamping circuits 16 and 32, a very smooth switchover can be achieved during the switching of the electronic switch of the changeover circuit 34. As a result, a flawless composite signal shown in Fig. 2(D) is outputted to the terminal 36.

In the transmitting apparatus 10, each of the circuits 18, 20, 26, 28 and 30 operates as one of elements of a compression means for compressing the time base of an audio signal. In addition, each of the clamping circuits 16, 32 and the changeover circuit 34 operates as one of elements of a composition means for composing the compressed audio signal and a video 17 signal. Further, the timing controller 20 operates as a control circuit for controlling the circuits 16, 26, 28, 30, 32 and 34 in the transmitting apparatus 10 together with the synchronizing separator circuit 18. If the video signal source is a video camera incorporating synchronization generator and the transmitting apparatus 10 is connected to the video camera, the synchronizing signals can be driven directly from the video camera and the synchronizing separator circuit 18 is unnecessary.

The timing signals are fed from the timing controller 20 to each of the circuits 16, 26, 28, 30, 32, 34 and 36 and related to one another so as to have a predetermined phase relation to one another and to a video signal. Such a phase relation can be maintained by generating various timing signals on the basis of the synchronizing signals outputted from the synchronizing separator circuit 18.

Referring to Fig. 3, a receiving apparatus 40 separates and restores the audio signal from a composite signal composed of a video signal injected with compressed audio signal.

The receiving apparatus 40 receives a composite signal from the transmitting apparatus 10 shown in Fig. 1 through terminal 42. The composite signal is fed to a synchronizing separator circuit 44 and to the clamping circuit 48.

The synchronizing separator circuit 44 separates 1 1 18 the horizontal and vertical synchronizing signals from the composite signal, and feeds the separated synchronizing signals to a timing controller 50. The timing controller 50 generates various timing signals on the basis of the synchronizing signals fed from the synchronizing separator circuit 44. The timing signals generated by the controller 50 are fed to the clamping circuit 48, to the A/D converter 54, to the memory 56, and to the D/A converter 58.

On the basis of the timing signals fed from the controller 50, the clamping circuit 48 clamps the composite signal fed from the terminal 42 to a predetermined voltage level during the 1H period shown in Fig. 2(D), i.e., during the time the compressed audio signal is injected into the video signal. Thereby, the audio signal components within the composite video signal are clamped to a predetermined voltage level.

An output signal of the clamping circuit 48 is fed to an analog-todigital conversion circuit 54.

The A/D converter 54 converts the audio signal outputted from the clamping circuit 48 during the 1H period shown in Fig. 2(D), on the basis of the timing signal for conversion fed from the controller 50, into a digital signal.

The signal converted in the A/D converter 54 is fed to a memory 56 and written into the memory on the basis of the write timing signal fed from the controller 1 19 50. The memory 56 successively writes the signal fed from the A/D converter 54. The signal written into the memory 56 is read out from the memory 56 at a slower speed than the writing speed on the basis of the readout timing signal fed from the controller 50.

The readout speed of a signal from the memory 56 preferably corresponds to such a speed that a time corresponding to one horizontal scanning period of a video signal becomes equal to a time corresponding to one vertical scanning period of the video signal. Otherwise, any other speed may be adapted for the readout. The readout timing from the memory 56 corresponds to the timing of a specific horizontal scanning period shown in Fig. 2(D) during one vertical scanning period of a video signal, and the readout of the signal from the memory 56 is repeated every one vertical scanning period of the video signal.

Similarly to the memory 28 shown in Fig. 1, the memory 56 may be provided with a plurality of memory portions for successively storing audio signals equal to a time portion corresponding to at least one horizontal scanning period of a video signal, such memory arrangement permits simple successive sequencing of the memory portion in rotation for successively writing the signal and for successively reading out the signal every one vertical scanning of the video signal.

The signal read out from the memory 56 is 1 9 - 20 converted into an analog signal in a digital-to-analog conversion circuit 58, at the same speed as the readout speed of the signal from the memory 56 and in synchronization with the readout of the signal from the memory 56 on the basis of the timing signal for conversion outputted from the controller 50.

Thereby, the frequency of the compressed audio signal in the composite signal shown in Fig. 2(D) is reduced at the rate that a time corresponding to one horizontal scanning period 1H of a video signal becomes equal to a time corresponding to one vertical scanning period 1V of the video signal. Thus, the compressed audio signal time base is extended, thereby, the time base of the compressed audio signal is restored to its original real time base.

Since the compression rate of the time base of the audio signal in the transmitting apparatus 10 is l/n, accordingly, the time base of the audio signal i extended by n times in the receiving apparatus 40, therefore, the restored audio frequency in the receiving apparatus 40 is divided by l/n.

An output signal of the D/A converter 58 is fed to a low-pass filter 60 for preventing the passage of a frequency component higher than that of an audio signal, so that only a frequency component of the audio signal is outputted from the low-pass filter 60. An output signal of the low- pass filter 60 is amplified by an S - 21 amplifier 62, and the amplified signal is fed from a terminal 64 to a reproduction apparatus such as a speaker, or to an audio-video signal recording apparatus such as a video tape recorder for recording a separated audio signal together with the video signal which is outputted from a terminal 52.

In the receiving apparatus 40, each of the circuits 44, 48, 50, 54, 56, 58 and 60 operates as one of elements of a means for restoring the time base of the compressed audio signal. In addition, the timing controller 50 operates as a control circuit for controlling the circuits 48, 54, 56 and 58 in the receiving apparatus 40 together with the synchronizing separator circuit 44.

The timing signals fed from the timing controller 50 to each of the circuits 48, 54, 56 and 58 are related to one another so as to have a predetermined phase relation to one another and to a video signal. Such a phase relation can be maintained by generating various timing signals on the basis of the synchronizing signals outputted from the synchronizing separator circuit 44.

Referring to Fig. 4, a transmitting apparatus 70 is a composition apparatus for composing a video signal and two analog audio signals for stereo transmission. The transmitting apparatus 70 receives the firstaudio signal through terminal 14L and the second audio signal through terminal 14L.

The first analog audio signal fed to the terminal 14L passes through a volume control 22L and an amplifier 24L, and thereafter fed to a first analog-to-digital conversion circuit 26L. The second analog audio signal fed to the terminal 14R passes through a volume control 22R and an amplifier 24R, and thereafter fed to a second analog-to-digital conversion circuit 26R.

The A/D converters 26L and 26R are activated at a predetermined period on the basis of the timing signals fed from the controller 20, and convert the analog audio signals during the activated period in digital signals.

The output signals from the A/D converters 26L and 26R are fed to a memory 28. The memory 28 is also fed with the write timing signal from the controller 20 for controlling the writing speed, therefore, the memory 28 successively writes the signals fed from the A/D converters 26L and 26R. The timing controller 20 also feeds the readout timing signal to the memory so that the signal written in the memory 28 is read out at a greater speed than the writing speed.

The readout speed of one of the two signals from the memory 28 preferably corresponds to such a speed that a time corresponding to the half of the vertical scanning period of the video signal is equal to a time corresponding to one half of the horizontal scanning period. Otherwise, any other speed may be adapted to the readout speed. The readout timing of both signals 1 7 1 from the memory 28 corresponds to one horizontal scanning period of a video signal preferably within the vertical blanking period, more preferably, is a specific horizontal scanning line period during the vertical blanking period, with each of the two signals occupies half of the specific horizontal scanning line period as shown in Fig. 8(A).

The memory 28 may be provided with a plurality of memory portions for successively storing the two audio signals during a time portion corresponding to at least one vertical scanning period of the video signal, such memory arrangement permits simple successive sequencing of the memory portion in rotation for successively writing the signals and for successively reading out the signals every one vertical scanning of the video signal.

Referring to Fig. 5, a transmitting apparatus 80 is a composition apparatus for composing a video signal and two analog audio,signals for stereo transmission. The transmitting apparatus 80 receives the first audio signal through terminal 14L and the second audio signal 9 - 24 through terminal 14R.

The first analog audio signal fed to the terminal 14L passes through a volume control 22L and an amplifier 24L, and thereafter fed to first analog-to-digital conversion circuit 26L. The second analog audio signal fed to the terminal 14R passes through a volume control 22R and an amplifier 24R, and thereafter fed to a second analog-to-digital conversion circuit 26R.

The A/D converters 26L and 26R convert the corresponding audio signals at a predetermined period on the basis of the timing signals for conversion fed from the controller 20 into digital signals.

The A/D converters 26L and 26R convert the audio signals alternately. The changeover frequency for conversion is preferably selected to be not less than twice the highest frequency of the transmitted audio signals, for example, if the highest frequency of the audio signals is 8KHz, a 16KHz minimum changeover frequency should be selected. Further, it is preferable to lock the changeover frequency to the horizontal synchronization signal. Therefore, as a changeover frequency, the horizontal synchronization frequency of NTSC system, for example, such as 15,750Hz can be selected, preferably double or triple of the horizontal synchronization frequency, such as 31,500Hz or 47,250Hz can be selected. The timing signals having such changeover or clock frequency are applied to the A/D 1 - converters 26L and 26R for alternatively outputting the converted signals to the signal composing portion 72. The converted signals are fed from the signal composing portion 72 to the memory 28.

The memory 28 is also fed with the write timing signal from the controller 20 for controlling the writing speed, therefore, the memory 28 successively writes the signals fed from the composing circuit 72. The timing controller 20 also feeds the readout timing signal to the memory so that the signal written in the memory 28 is read out at a greater speed than the writing speed.

The readout speed of the two composed signals from the memory 28 preferably corresponds to such a speed that a time corresponding to the vertical scanning period of the video signal is equal to a time corresponding to one horizontal scanning period. Otherwise, any other speed may be adapted to the readout speed. The readout timing of the two composed signals from the memory 28 corresponds to one horizontal scanning period of a video signal preferably within the vertical blanking period, more preferably, is a specific horizontal scanning line period during the vertical blanking period, with each of the two signals alternating at a changeover frequency rate during the specific horizontal scanning line period as shown in Fig. 8(B).

26 - The memory 28 may be provided with a plurality of memory portions for successively storing the two audio signals during a time portion corresponding to at least one vertical scanning period of the video signal, such memory arrangement permits simple successive sequencing of the memory portion in rotation for successively writing the signals and for successively reading out the signals every one vertical scanning of the video signal.

Referring to Fig. 6, a transmitting apparatus 90 is a composition apparatus for composing a video signal and two analog audio signals for stereo transmission. The transmitting apparatus 90 receives the first audio signal through terminal 14L and the second audio signal through terminal 14R.

The first analog audio signal fed to the terminal 14L passes through a volume control 22L and an amplifier 24L, and thereafter fed to one input of an analog stereo multiplexer circuit 92. The second analog audio signal fed to the terminal 14R passes through a volume control 22R and an amplifier 24R, and thereafter fed to second 1 input of the well known analog stereo multiplexer circuit 92.

The signal outputted from the analog stereo multiplexer circuit 92 is fed to an analog-to-digital conversion circuit 26. The A/D converter 26 is activated at a predetermined period on the basis of the timing signals fed from the controller 20, and converts the analog stereo multiplexed audio signals during the activated period into digital signals.

The output signals from the A/D converter 26 are fed to a memory 28. The memory 28 is also fed with the write timing signal from the controller 20 for controlling the writing speed, therefore, the memory 28 successively writes the signals fed from the A/D converter 28. The timing controller 20 also feeds the readout timing signal to the memory so that the signal written in the memory 28 is read out at a greater speed than the writing speed.

The readout speed of the signals from the memory 28 preferably corresponds to such a speed that a time corresponding to the vertical scanning period of the video signal is equal to a time corresponding to one horizontal scanning period. Otherwise, any other speed may be adapted to the readout speed. The readout timing of signals from the memory 28 corresponds to one vertical scanning period of a video signal preferably within the vertical blanking period, more preferably, is a specific horizontal scanning line period during the vertical blanking period as shown in Fig. 8(C).

The memory 28 may be provided with a plurality of memory portions for successively storing the multiplexed audio signals, during a time portion corresponding to at least one vertical scanning period of the video signal, such memory arrangement permits simple successive sequencing of the memory portion in rotation for successively writing the signal and for successively reading out the signals every one vertical scanning of the video signal.

Referring to Fig. 7, a transmitting apparatus 100 is a composition apparatus for composing a video signal and two analog audio signals for stereo transmission. The transmitting apparatus 100 receives the first audio signal through terminal 14L and the second audio signal through terminal 14R.

The first analog audio signal fed to the terminal 14L passes through a volume control 22L and an amplifier 24L, and thereafter fed to first input of an electronic 9 switch 102. The second analog audio signal fed to the terminal 14R passes through a volume control 22R and an amplifier 24R, and thereafter fed to second input of an electronic switch 102.

The electronic switch 102 is controlled by a timing signal fed from the timing controller 20. The output of the electronic switch 102 is connected to an A/D converter 26. The A/D converter 26 is activated at a predetermined period on the basis of the timing signals fed from the controller 20, and converts the analog audio signals fed from the electronic switch 102 during the activated period into digital signals.

The output signals from the A/D converter 26 are fed to a memory 28. The memory 28 is also fed with the write timing signals from the controller 20 for controlling the writing speed, therefore, the memory 28 successively writes the signals fed from the A/D converter 26. The timing controller 20 also feeds the readout timing signal to the memory so that the signal written in the memory 28 is read out at a greater speed than the writing speed.

The readout speed of one of the two signals from the memory 28 preferably corresponds to such a speed that a time corresponding to the one half of the vertical scanning period of the video signal is equal to a time corresponding to one horizontal scanning period. Otherwise, any other speed may be adapted to the readout - 30 speed. The readout timing of both signals from the memory 28 corresponds to one vertical scanning period of a video signal preferably within the vertical blanking period, more preferably, during specific two horizontal scanning line periods during the vertical blanking period, with each of the two signals occupies one specific horizontal scanning line period as shown in Fig. 8(D).

In the transmitting apparatus 70 shown in Fig. 4, each of the A/D converters 26L and 26R operates as one of elements of a compression means for compressing the time base of a stereo audio signal together with the 9 1 31 - circuits 18, 20, 28 and 30.

In the transmitting apparatus 80 shown in Fig. each of the A/D converters 26L and 26R and the signal composing portion 72 operate as one of elements of a compression means or compressing the time base of a stereo audio signal together with the circuits 18, 20, 28 and 30.

In the transmitting apparatus 90 shown in Fig. 6, the audio multiplexer 92 along with the A/D converter 26 operate as one of elements of a compression means for compressing the time base of a stereo audio signal together with the circuits 18, 20, 28 and 30.

In the transmitting apparatus 100 shown in Fig. 7, the electronic switch 102 along with the A/D converter 26 operate as one of elements of a compression means for compressing the time base of a stereo audio signal together with the circuits 18, 20, 28 and 30.

In each of the transmitting apparatuses 70, 80, 90 and 100, the synchronization separator circuit 18 is not necessary if the transmitting apparatus is connected directly to a video signal generator, such as a video camera, and fed directly with synchronization signals from the video signal generator.

In each of the transmitting apparatuses 70, 80, 90 and 100, the clamping circuits 16 and 32 along with the changeover circuit 34 operate as an element of a composition means for composing the compressed audio 32 signal and a video signal, similar to the composition means of the transmitting apparatus 10 of Fig. 1.

As a result the transmitting apparatuses 70, 80, 90 and 100 can transmit two separate audio signals for stereo sound along with an image signal through a common transmission line without exerting any influence on the image signal and without exerting any influence on each other.

Referring to Fig. 9, a receiving apparatus 74 receives a composite signal from any one of the transmitting apparatuses 70, 80, 90 and 100 shown in Figs. 4, 5 and 7 through the terminal 42. The composite signal is fed to a synchronization separator circuit 44 and to the clamping circuit 48.

The synchronizing separator circuit 44 separates the horizontal and vertical synchronizing signals from the received composite signal, and feeds the separated synchronizing signals to a timing controller 50. The timing controller 50 generates various timing signals on the basis of the synchronizing signals fed from the synchronizing separator circuit 44. The timing signals generated by the controller 50 are fed to the clamping circuit 48, to the A/D converter 54, to the memory 56, and to the D/A converters 58L and 58R.

On the basis of the timing signals fed from the controller 50, the clamping circuit 48 clamps the composite signal fed from the terminal 42 to a 1 predetermined voltage level during the 1H period shown in Fig. 8(A) or 8(B) or during the 2H period of Fig. 8(D), i.e., during the time the compressed audio signals are injected into the video signal. Thereby, the audio signal components within the composite video signal are clamped to a predetermined voltage level.

The A/D converter 54 converts the audio signal outputted from the clamping circuit 48 during the 1H period shown in Fig. 8(A) or 8(B) or during the 2H period shown in Fig. 8(D) on the basis of the timing signal for conversion fed from the controller 50 into a digital signal.

The signal converted in the A/D converter 54 is fed to a memory 56 and written into the memory on the basis of the write timing signal fed from the controller 50. The memory 56 successively writes the signal fed from the A/D converter 54. The signal written into the memory 56 is read out from the memory 56 at a slower speed than the writing speed on the basis of the readout timing signals fed from the controller 50.

The readout speed of a signal from the memory 56 preferably corresponds to such a speed that a time corresponding to one or two horizontal scanning periods of a video signal becomes equal to a time corresponding to one vertical scanning period of the video signal, 1 otherwise, any other speed may be adapted to the readout speed. The readout of the signal from the memory 56 is repeated every one vertical scanning period of the video signal.

Similarly to the memory 28 shown in Fig. 4, 5 or 7, the memory 56 may be provided with a plurality of memory portions for successively storing audio signals equal to a time portion corresponding to at least one horizontal scanning period of a video signal, such memory arrangement permits simple successive sequencing of the memory portion in rotation for successively writing the signal and for successively reading out the signal every one vertical scanning of the video signal.

A composite signal composed by any one of the transmitting apparatuses 70, 80, 90 and 100 shown in Figs. 4, 5 and 7 can be separated into a video signal and an audio signal by the receiving apparatus 74. Therefore, the receiving apparatus 74 comprises a first and a second D/A converters 58L and 58R for converting the first and second audio signals read out from the memory 56 into analog signals respectively.

The first and second D/A converters 58L and 58R respectively convert the first audio signal and the second audio signal into analog signals at the same speed as the readout speed of the signal from the memory 56 and in synchronization with the readout of the signal from the memory 56 on the basis of the timing signals for conversion outputted from the controller 50.

The frequency and the phase of the timing signal for conversion in each of the D/A converters 58L and 58R are preferably made equal to the frequency and phase of the timing signal for conversion in the A/D converters 26L and 26R of any one of the transmitting apparatuses 70, 80 and 100 shown in Figs. 4, 5 and 7.

For reading out from the memory 56 and separating the audio signals from the composite signal composed by the transmitting apparatus 70 shown in Fig. 4, the timing signals generated by the timing controller 50 of the receiving apparatus 74 preferably correspond to such a speed that a time corresponding to one half of the horizontal scanning period becomes equal to one half of vertical scanning period of the video signal, otherwise, any other speed may be adapted for readout and conversion. The readout timing of both first and second audio signals from the memory circuit 56 and the conversion timing by both D/A converters 58L and 58R corresponds to reading out a specific horizontal scanning period shown in Fig. 8(A) during one vertical scanning period of a video signal with one of the two audio signals is read out and converted during the first half of the vertical scanning period of a video signal and the other audio signal is read out and converted during the second half of the vertical scanning period of a video signal.

- 36 For reading out from the memory 56 and for separating the audio signals from the composite signal composed by the transmitting apparatus 80 shown in Fig. 5, the timing signals generated by the timing controller 50 of the receiving apparatus 74 correspond to the changeover timing of the transmitting apparatus 80, preferably to an alternating changeover frequency selected to be not less than twice the highest frequency of the received audio frequency, for example, if the highest frequency of the audio signals is 8KHZ, a 16KHz minimum changeover frequency should be selected. Furthermore, it is preferable to lock the changeover frequency to the horizontal synchronization signal, therefore, a changeover frequency of the horizontal synchronization frequency of NTSC system, for example, such as 15,750Hz can be selected, preferably, double or triple of the horizontal synchronization frequency, such as 31,500Hz or 47,250Hz can be selected. Such changeover or clock frequency is applied to the D/A converters 58L and 58R for alternatively outputting a converted analog audio signal.

The readout speed of the two alternating audio signals composed by the transmitting apparatus 80 from the memory 56 preferably corresponds to such a speed that a time corresponding to one horizontal scanning period of a video signal becomes equal to a time corresponding to one vertical scanning period of a video i signal. The readout timing of signals from the memory 56 corresponds to reading out one specific horizontal scanning line period, shown in Fig. 8(B), during one vertical scanning period of a video signal.

For reading out from the memory 56 and separating the audio signals from the composite signal composed by the transmitting apparatus 100 shown in Fig. 7, the timing signals generated by the timing controller 50 of the receiving apparatus 74 preferably correspond to such a speed that a time corresponding to one horizontal scanning period becomes equal to one half of vertical scanning period of the video signal, otherwise, any other speed may be adapted for readout and conversion. The readout timing of both first and second audio signals from the memory circuit 56 and the conversion timing by both D/A converters 58L and 58R corresponds to reading out two specific horizontal scanning periods shown in Fig. 8(D) during one vertical scanning period of a video signal, with one of the audio signals is read out and converted during the first half of the vertical scanning period of a video signal and the other audio signal is read out and converted during the second half of the vertical scanning period of a video signal.

The output signals of the A/D converters 58L and 58R of the receiving apparatus 74 shown in Fig. 9 are respectively fed to amplifiers 62L and 62R through lowpass filters 60L and 60R to prevent the passage of a frequency component higher than that of an audio signal, thereby, the filtered and amplified audio signals are outputted from the amplifiers 62L and 62R through terminals 64L and 64R to a reproduction apparatus such as a speaker, or to an audio-video signal recording apparatus such as a video tape recorder for recording an audio signal separated from the video signal which is outputted form terminal 52.

Referring to Fig. 10, a receiving apparatus 84 receives a composite signal from the transmitting apparatus 90 shown in Fig. 6 through the terminal 42. The composite signal is fed to a synchronizing separator circuit 44 and to the clamping circuit 48.

The synchronizing separator circuit 44 separates the horizontal and vertical synchronizing signals from the composite signal, and feeds the separated synchronizing signals to a timing controller 50. The timing controller 50 generates various timing signals on the basis of the synchronizing signals fed from the synchronizing separator circuit 44. The timing signals generated by the controller 50 are fed to the clamping circuit 48, to the A/D converter 54, to the memory 56, and to the D/A converter 58.

On the basis of the timing signals fed from the controller 50, the clamping circuit 48 clamps the composite signal fed from the terminal 42 to a predetermined voltage level during the 1H period shown 1 - 39 in Fig. 8(C), i.e., during the time the compressed audio signals are injected into the video signal. Thereby, the audio signal components within the composite video signal are clamped to a predetermined voltage level.

The A/D converter 54 converts the audio signal outputted from the clamping circuit 48 during the 1H period shown in Fig. S(C) during the 1H period shown in Fig. 8(C) on the basis of the timing signal for conversion fed from the controller 50 into a digital signal.

The readout speed of a signal from the memory 56 preferably corresponds to such a speed that a time corresponding to one or two horizontal scanning periods of a video signal becomes equal to a time corresponding to one vertical scanning period of the video signal, otherwise, any other speed may be adapted to the readout speed. The readout of the signal from the memory 56 is - 40 repeated every one vertical scanning period of the video signal.

Similarly to the memory 28 shown in Fig. 6, the memory 56 may be provided with a plurality of memory portions for successively storing audio signals equal to a time portion corresponding to at least one horizontal scanning period of a video signal, such memory arrangement permits simple successive sequencing of the memory portion in rotation for successively writing the signal and for successively reading out the signal every one vertical scanning of the video signal.

For reading out from the memory 56 and separating the audio signals from the composite signal composed by the transmitting apparatus 90 shown in Fig. 6, the timing signals generated by the timing controller 50 of the receiving apparatus 94 shown in Fig. 10 preferably correspond to such a speed that a time corresponding to one horizontal scanning period becomes equal to one vertical scanning period of the video signal, otherwise, any other speed may be adapted for readout and conversion. The readout timing of the multiplexed first and second audio signals from the memory circuit 56 and the conversion timing by the D/A converter 58 correspond to reading out one specific horizontal scanning period shown in Fig. 8(C) during one vertical scanning period of a video signal.

The multiplex stereo signals outputted from the t D/A converter 58 are fed to a demultiplexer circuit 61 through a low-pass filter 60. The low-pass filter 60 prevents the passage of frequency components higher than that of an audio signal, thereby filtered multiplexed stereo audio signals are fed to the demultiplexer circuit 61. The demultiplexer circuit separates and feeds the first and second audio signals into two audio amplifiers 62L and 62R for outputting the two separated audio signals through terminals 64L and 64R.

In the receiving apparatuses 74 and 94, each of the circuits 44, 48, 50, 54 and 56 operates as one of elements of a means for extracting the compressed audio signal from a composite video signal. In addition, the timing controller 50 operates as a control circuit for controlling the circuits 48, 54 and 56 in the receiving apparatuses 74 and 94 together with the synchronizing separator circuit 44.

In the receiving apparatus 74, each of the D/A converters 58L, 58R and the low-pass filters 60L, 60R operates as one of elements of a means for restoring the time base of the compressed audio signal together with other circuits 44, 48, 50, 54 and 56.

In the receiving apparatus 94, the D/A converter 58, the demultiplexer circuit 61 and the low-pass filters 60L and 60R operate as one of elements of a means for restoring the time base of the compressed audio signal together with other circuits 44, 48, 50, 54 a 42 and 56.

The timing signals fed from the timing controller 50 to each of the circuits 48, 54, 56 and 58 or 58L and 58R are related to one another so as to have a predetermined phase relation to one another and to a video signal. Such a phase relation can be maintained by generating various timing signals on the basis of the synchronizing signals outputted from the synchronizing separator circuit 44.

For receiving the composite signals composed by any one of thetransmitting apparatuses 70, 80 and 100, the memory circuit 56 of the receiving apparatus 74 may further incorporate two readout circuits, each outputted separately to the two D/A converters 58L and 58R, and each readout output may incorporate a delay circuit controlled by the timing controller 50 to coincide the timing of the first and second audio signals perfectly, alternatively an additional delay circuit controlled by the timing controller 50 may be added to each of the D/A converters 58L and 58R.

Since the compression rate of the audio signals is equal to a ratio of the vertical scanning period and the horizontal scanning period, for example, in case of NTSC system, the vertical scanning period is 16.666 m Sec. and the horizontal scanning period is 53p Sec., therefore, the compression ratio becomes 16.666m sec. - 314. 53p sec.

L Accordingly, by using a compression ratio of 314, it is possible to compress an audio signal of 14KHz, for example, and compose it into NTSC video signal having a bandwidth of 4.5MHz (314 x 14KHz = 4.396MHz).

Depending on the audio frequencies used and the video signal bandwidth, it is possible to compress and compose therefore more than two audio signals into a video signal at the transmitting site and extract and restore the more than two audio signals at the receiving site using a common transmission line.

It is also possible, for example, to compose such audio signals during every other vertical scanning period and compress the audio signal into several horizontal scanning periods, or into a portion of a single or multiple horizontal scanning periods. Accordingly, there are limitless combinations of audio signals compression ratios, audio signals compression durations and the audio signals composing timing, similarly, there are limitless combinations of restoring and extracting means of a single or multiple audio signals, in accordance-with the compression ratio, the compression duration or the composing timing used to compose the audio signals and a video signal.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that it is intended to cover all changes and modifications of the example of

L 44 the invention herein chosen for the purpose of the disclosure, which modifications do not constitute departures from the spirit and scope of the invention

L - 45

Claims

What is claimed is: 1. An apparatus for transmitting television signals

including a video signal and an audio signal, comprising: a compression means for compressing a time base of an audio signal to be transmitted; and a composition means for composing the compressed audio signal into a video signal and for transmitting the composed signal through a common transmission line.
2. An apparatus according to claim 1, wherein said compression means includes: an analog-to-digital conversion circuit for converting said audio signal into a digital signal; a memory for storing output signals from the analog-to-digital conversion circuit; a control circuit for controlling the writing of the signal in the memory and the readout of the signal from the memory and for outputting the signal stored in the memory at a faster speed than the writing speed; and a digital-toanalog conversion circuit for converting the signal read out from said memory into an analog signal.
3. An apparatus according to claim 2, wherein said composition means includes: an output signal switching circuit controlled by said control circuit and for injecting an output signal from said digital-to-analog conversion circuit into said video signal during a 1 1 46 - switchover period in accordance to a control timing by said control circuit.
4. An apparatus according to claim 2 or 3, wherein said audio signal includes first and second audio signals for stereo sound, and wherein said analog-todigital conversion circuit includes: a first A/D converter for converting said first audio signal into a digital signal; a second A/D converter for converting said second audio signal into a digital signal; and a feeding means for feeding the converted digital signals of said first audio signal and said second audio signal independently to said memory.
5. An apparatus according to claim 2 or 3, wherein said audio signal includes first and second audio signals for stereo sound, and wherein said analog-todigital conversion circuit includes: a first A/D converter for converting said first audio signal into a digital signal; a second A/D converter for converting said second audio signal into a digital signal; and a signal composing portion for composing output signals from both A/D converters and for feeding the same to said memory.
6. An apparatus according to claim 2 or 3, wherein said audio signal includes first and second audio 1 a signals, and wherein said compression means further includes a stereo audio signal multiplexer circuit for multiplexing said first and second audio signals and for outputting the same to said analog-to-digital conversion circuit.
7. An apparatus according to claim 2 or 3, wherein said audio signal includes first and second audio signals, and wherein said compression means further includes an audio signal changeover circuit for alternately changing over said first and second audio signals and for outputting the same to said analog-todigital conversion circuit.
8. An apparatus according to claim 3, wherein said composition means further includes: a first clamping circuit for clamping said video signal to be fed to said output signal switching circuit to a predetermined voltage level; and a second clamping circuit for clamping said compressed audio signal outputted from said digital-to-analog conversion circuit to be fed to said output signal switching circuit to said predetermined voltage level.
9. An apparatus according to any one of claims 1 through 7, wherein said compression means compresses the time base of said audio signal at the rate that a time A 9 - 48 corresponding to one vertical scanning period of said video signal becomes equal to a time corresponding to one horizontal scanning period of said video signal.
10. An apparatus according to any one of claims 4 through 7, wherein said compression means compresses the time base of said first audio signal and said second audio signal at the rate that a time corresponding to one vertical scanning period of said video signal becomes equal to a time corresponding to one horizontal scanning period of said video signal.
11. An apparatus according to claim 4, wherein said compression means compresses the time base of said first audio signal and said second audio signal at the rate that a time corresponding to one vertical scanning period of said video signal becomes equal to a time corresponding to two horizontal scanning periods of said video signal.
12. An apparatus according to any one of claims 1 through 11, wherein said composition means injects said compressed audio signal during the vertical blanking period of said video signal.
13. An apparatus according to claim 1, wherein said compression means includes: an analog-to-digital 1 9 S - 49 conversion circuit for converting said audio signal into a digital signal; a memory for storing output signals from the analog-to-digital conversion circuit; a control circuit for controlling the writing of the signals in said memory and the readout of the signals from said memory, said control circuit writing said audio signal in said memory and outputting the written signal from said memory every vertical scanning period, at the rate that a time corresponding to one vertical scanning period of said video signal becomes equal to a time corresponding to one horizontal scanning period of said video signal; and a digital-to-analog conversion circuit for converting the signal read out from said memory into an analog signal, and wherein said composition means injects an output signal from said digital-to-analog conversion circuit during one horizontal scanning period of the vertical blanking period of said video signal.
14. An apparatus according to claim 11, wherein said compression means includes: a memory for storing output signals from said first A/D converter and said second A/D converter; a control circuit for controlling writing of signals in said memory and readout of signals from said memory, said control circuit writing said converted digital signals of said first audio signal and said second audio signal into said memory and outputting the written signal from said memory every one vertical m 0 9 - 50 scanning period at the rate that a time corresponding to one vertical scanning period of said video signal becomes equal to a time corresponding to two horizontal scanning periods of said video signal; and a digital-toanalog conversion circuit for converting the signal read out from said memory into an analog signal, and wherein said composition means injects an output signal from said digital-to-analog conversion circuit during two horizontal scanning periods of the vertical blanking period of said video signal.
15. An apparatus according to claim 13, wherein said composition means injects an output signal from said digital-to-analog conversion circuit during one specifi horizontal scanning period among the vertical blanking period of said video signal.
16. An apparatus according to claim 14, wherein said composition means injects an output signal form said digital to-analog conversion circuit during two specific horizontal scanning periods among the vertical blanking period of said video signal.
17. An apparatus for receiving television signals including composite signals injected with an audio signal whose time base is compressed and transmitted via a common transmission line, comprising:

0 1 IF an extracting and restoring means for extracting the compressed audio signal from said composite signal and for restoring the time base of the extracted audio signal.
18. An apparatus according to claim 17, wherein said extracting and restoring means includes: an analog-todigital conversion circuit for extracting said audio signal from said composite signal and for converting it into a digital signal; a memory for storing an output signal from said analog-to-digital conversion circuit; a control circuit for controlling writing of the signal in said memory and readout of the signal from said memory and for outputting the signals stored in said memory at a slower speed than the writing speed of the signal in said memory; and a digital-to-analog conversion circuit for converting the signal read out from the memory into an analog signal.
19. An apparatus according to claim 18, wherein said audio signal is an alternating signal of first and second audio signals for stereo sound, and wherein said digital-to-analog conversion circuit includes: a first D/A converter for converting said first audio signal into an analog signal; and a second D/A converter for converting said second audio signal into an analog signal.

0.

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20. An apparatus according to claim 18, wherein said audio signal is an alternately combined signal of first and second audio signals for stereo sound, and wherein said digital-to-analog conversion circuit includes: a first D/A converter for converting said first audio signal into an analog signal; and a second D/A converter for converting said second audio signal into an analog signal.
21. An apparatus according to claim 18, wherein said audio signal is a multiplexed signal of first and second audio signals for stereo sound multiplex, and wherein said extracting and restoring means further includes a demultiplexer circuit for decoding the multiplexed signals outputted from said digital-to-analog conversion circuit and for outputting said first audio signal and said second audio signal separately.
22. An apparatus according to any one of claims 18 through 21, wherein said extracting and restoring means further includes a clamping circuit for clamping a composite signal to be fed to said analog-to-digital conversion circuit to a predetermined voltage level.
23. An apparatus according to any one of claims 18 through 22, wherein said extracting and restoring means further includes a low-pass filter for removing signal - 53 component having a frequency higher than the audio signal frequency outputted from said digital-to-analog conversion circuit.
24. An apparatus according to any one of claims 17 through 23, wherein the time base of said audio signal is compressed at the rate that a time corresponding to one vertical scanning period of the video signal becomes equal to a time corresponding to one horizontal scanning period of said video signal, and wherein said extracting and restoring means restores the time base of said extracted audio signal at the rate that a time corresponding to one horizontal scanning period of said video signal becomes equal to a time corresponding to one vertical scanning period of said video signal.
25. An apparatus according to claim 18, wherein the time base of said audio signal in the composite signal is compressed at the rate that a time corresponding to one vertical scanning period of a video signal becomes equal to a time corresponding to periods of said video signal, and extractina and restorina means two horizontal scanning wherein said restores the time base of said extracted audio signal at the rate that a time corresponding to two horizontal video signal becomes equal to a one vertical scanning period of scanning periods of a time corresponding to said video signal.

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26. An apparatus according to any one of claims 17 through 25, wherein said audio signal is injected to said composite signal during the vertical blanking period of a video signal, and wherein said extracting and restoring means extracts said audio signal from said composite signal by separating the synchronising signal from the view signal and applying the separated synchronising signal to a control circuit for outputting a precise timing to control the extracting period of said audio 10 signal.
27. Apparatus for transmitting television signals, substantially as hereinbefore described with reference to the accompanying drawings.
28. Apparatus for receiving television signals, substantially as hereinbefore described with reference to the accompanying drawings.

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