JPH0744666B2 - Television signal transmitter with digital audio - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television signal for multiplexing and transmitting digitized audio while maintaining compatibility with an existing television receiver for receiving terrestrial television broadcasting. The present invention relates to a delivery device.

2. Description of the Related Art Almost all television broadcasts (excluding satellite broadcasts) carried out in the world use the FM subcarrier system as the audio transmission system. In recent years, as seen in satellite broadcasting and video discs, there is an increasing number of examples in which audio is digitized and transmitted or recorded to improve its quality.
However, in television broadcasting, multiplexing of such high-speed digital signals has been extremely difficult because there is no available frequency band.

The configuration shown in FIG. 2 has been announced as a conventional example of a digital audio multiplexing method that does not interfere with (compatibility with) an existing television receiver (Noda et al .: For terrestrial television broadcasting). Digital audio transmission system, Technical Report of Television Society, 11-22, October 1987). The output of the video carrier oscillator 1 is changed by the video signal to the vestigial sideband (VSB):
VSB modulation is performed by the stigial side band) modulation means 2.
On the other hand, in the voice digitized by the encoding means 4,
A carrier orthogonal to the above video carrier is used as a two-phase PSK (Phase Shi
ft Keying) Modulate. Conventional analog FM subcarrier is FM
It is generated by the modulation means 7. The three modulated waves are combined by the combining means 8 and transmitted. In this way, since the quadrature phase near the center frequency of the video carrier is used for multiplexing the digital signal, the frequency spectrum is almost the same as the conventional one. Further, since the quadrature signal component is not detected in the synchronous detection type receiver, compatibility can be maintained.

Problems to be Solved by the Invention Although the above conventional example is ideally compatible with the existing receiver, there are some problems in reality. First, existing receivers include pseudo-synchronous detection type and envelope detection type in addition to (complete) synchronous detection type, and in these types of receivers, the influence from the quadrature component appears in the demodulated video signal. It will be. Secondly, even in the synchronous detection type, the quadrature component is detected by a slight error in the phase of the carrier wave in the synchronous detection. In particular, when a ghost is present, the phase accuracy of this detection axis is reduced, so that the interference with the video signal becomes even greater.

Further, regarding the use of the orthogonal axis of the video carrier, there is a plan to carry information for increasing the definition of the video signal (one of so-called EDTV systems), and therefore, there is no guarantee that it can be used for digital audio transmission. .

Means for Solving the Problems In order to solve the above problems, the present invention uses a dedicated second audio subcarrier for digital audio transmission, and multiplexes it with a conventional video carrier and FM audio subcarrier. It is to be sent out. This second audio subcarrier must be multiplexed in a very narrow band in order to eliminate interference with the video signal of the local station and the analog audio signal and the video signal of the adjacent station. For this reason, the second audio subcarrier is inevitably multi-valued QAM.
(Quadratuve Amplitude Modulation) modulation (for example, 16-value QAM or 64-value QAM) must be used. However, when demodulating such a multi-value QAM modulated wave, the carrier recovery circuit becomes extremely large. However, it was not feasible from the viewpoint of being built into a consumer receiver. Further, it is not easy to obtain a sufficient level in terms of demodulation performance (bit error rate). Therefore, in the present invention, a reference carrier component transmission period (period in which modulation is stopped) is set in the above-mentioned multi-level QAM modulated wave in synchronization with the horizontal synchronizing signal or the vertical synchronizing signal of the video signal of its own station which is simultaneously transmitted. The carrier is reproduced on the receiving side easily and stably. Therefore, the transmission device for a television signal with digital audio according to the present invention is a VS that modulates a video carrier with VSB (Vestigial Side Band) by a video signal to be transmitted.
The B modulation means, the timing generation means for generating a reference timing pulse signal having a low duty ratio by using one or both of the horizontal synchronization signal and the vertical synchronization signal included in this video signal, and the first signal by the audio signal to be transmitted. FM modulating means for FM-modulating the audio subcarrier, encoding means for converting the audio signal into a digital audio signal, and multilevel QAM modulating means for multilevel QAM modulating the second audio subcarrier by the digital audio. The multi-valued QAM modulation means is provided with a synthesizing means for synthesizing and transmitting the modulated video carrier, the first audio subcarrier and the second audio subcarrier, and the multilevel QAM modulating means stops the modulation during the output period of the timing pulse signal. Then, it is controlled to output the reference carrier component of the second audio subcarrier.

Operation As shown in Fig. 3, the
Approximately between the FM audio subcarrier and the image carrier of the upper adjacent station
There is a gap of 200 KHz (in the case of Japan and the US), and if the capacity of the filter allows a margin, a band of about 150 KHz can be used. On the other hand, if 16-value QAM is used for the modulation of the second audio subcarrier, the relationship between the transmission rate R (bps) and the occupied bandwidth B (Hz) is R = 4B / when a 30% roll-off filter is used. It becomes 1.3. Now, since B = 150 KHz, R = 460 Kbps is given.

If the digital audio to be transmitted is stereo audio with a bandwidth of 15 KHz, the sampling frequency will normally be 32 KHz. Supposing that the audio is quantized by 6 bits by the high-efficiency coding technology (the CD-ROM already reproduces high-quality audio by 4 bits), the required transmission speed S (bp
s) includes S = 32K × 6 × 2 ÷ 0.8 = 480K (bps) including the redundancy of 20%, and RS is established. That is, it is shown that if a digital modulated wave having a transmission efficiency of about 16-level QAM is additionally multiplexed as a second audio subcarrier, stereo digital audio can be transmitted to an existing receiver while maintaining compatibility. As is clear from FIG. 3, the center frequency of the second subcarrier is preferably above 4.65 μHz above the video carrier of the own station.

Next, in the present invention, the synchronization information of the video signal is used in order to facilitate the demodulation of the second audio subcarrier at the receiver. A reference timing pulse signal is generated using a horizontal or vertical synchronizing signal included in the video signal. If the horizontal synchronizing signal were used, the timing pulse signal would also be about 15.7 KHz, which is the same as the horizontal synchronizing signal, and the pulse width would be set to a duty ratio of several percent. The second voice subcarrier is multi-valued QAM-modulated by the digitized voice,
At this time, during the period of the pulse width of the timing pulse signal, this modulation is stopped and only the carrier component of the second subcarrier is output. The receiving side uses this carrier component to reproduce a continuous carrier. This carrier reproduction can use the same method as the case of reproducing a continuous color subcarrier from the color burst signal in the color reproduction of the video signal. This is because the receiving side can easily generate the same reference timing pulse as the transmitting side based on the horizontal or vertical synchronizing signal of the video signal, and thus can know the period in which the modulation of the second subcarrier is stopped. Because.

If the duty ratio of the reference timing pulse is too small, carrier reproduction on the receiving side becomes unstable, and conversely, if it is too large, the transmission efficiency of digital audio decreases. Therefore, it is appropriate to set it to about 5%.

In existing television receivers, the vicinity of the frequency of the second audio subcarrier is a region that is removed or sufficiently attenuated by a bandpass filter (for both video and FM audio).
It is hardly affected by the audio subcarrier. We have experimentally confirmed that if the second audio subcarrier is installed near 4.65 μHz and the power is set to be lower than the video carrier by 25 dB or more, no interference will be detected not only in the own station but also in the upper adjacent station.

Example An example of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of a television signal transmitting apparatus according to the present invention. The point where the video signal is VSB modulated and the analog audio is FM
The point of being modulated is the same as that of the conventional example shown in FIG. In the present invention, the output of the second sub-carrier oscillator 10 having the frequency _s2 is multi-valued QAM-modulated by the audio digitized by the encoding means 4 and synthesized with the video carrier and the first audio sub-carrier. Note that the processing in the encoding means 4 includes not only high-efficiency encoding but also processing such as addition of error correction code and synchronization code. On the other hand, timing generation means
At 12, the horizontal or vertical synchronizing signal component is extracted from the input video signal, and the low-duty ratio reference timing pulse signal synchronized with this component is output. The multi-level QAM modulation means 11 normally performs the modulation operation with the digital voice signal from the encoding means 4, but during the period when the reference timing pulse signal is given, the modulation is stopped and the output of the second subcarrier oscillator 10 is output. The carrier wave component (or a signal whose phase relationship with it is clear) is output as it is.

A multilevel QAM signal will be described by taking 16QAM as an example. Fourth
The figure is a signal point constellation diagram on the phase plane of 16QAM.
As can be seen from the figure, there are 16 kinds of signal points in total by combining the phase and the amplitude. Since only one of 16 types is transmitted in one clock period (time slot) in this way, parallel transmission of 4 bits becomes possible. The reference carrier wave used for modulation is a thick arrow on the in-phase axis in the figure.

FIG. 5 is a block diagram showing the structure of the multilevel quadrature modulation means 11. A digital signal input (digital audio in the case of the present invention) is first converted into a plurality of parallel signals by the serial / parallel conversion circuit 20. With 16QAM, there are four signals as described above. Next, this parallel signal is converted into a signal point of the modulated wave (for example, the fourth point).
It is given to the code / amplitude conversion circuit 21 in order to be converted into FIG. The output of the conversion circuit 21 is divided into two systems, which are used as information for setting the sub-paths of the carrier components of the in-phase axis and the orthogonal axis, respectively. That is, it can be said that the “coordinates” of each signal point on the phase plane are output. A usual method for realizing the code / amplitude conversion circuit 21 is to use a ROM in which the contents of the conversion are stored in advance. The amplitude information output from the conversion circuit 21 passes through the carrier wave fixing circuits 22 and 23 before being converted into analog signals by the D / A converters 24 and 25. These fixed circuits 22 and 23 work so that the preset values of the output signals are continuously output during the period when the reference timing pulse signal is applied to the control terminals. Needless to say, those values are amplitude information for fixing the finally extracted QAM signal to the reference carrier component. The outputs of the D / A converters 24 and 25 are input to the in-phase axis balanced modulator 29 and the quadrature axis balanced modulator 30 via the waveform shaping low-pass filters 26 and 27, respectively. Finally, the outputs of the two balanced modulators 29 and 30 are added in the synthesizing circuit 31 and output as a multilevel QAM signal.

Next, the function of the timing generating means 12 can be roughly classified into the following two.

(1) A horizontal or vertical synchronizing component is extracted from the input video signal.

(2) A reference timing pulse signal synchronized with the extracted video synchronization component is generated.

Of these, the video synchronization extraction (synchronous reproduction) of (1) is a process that is usually performed in a general television receiver.
It can be easily configured by using (Phase Locked Loop) circuits. The pulse generation of (2) is a function of generating a stable pulse signal in which the time offset T ₁ (may be zero) from the video synchronizing signal and the pulse width T ₂ are defined, as shown in FIG. . Again, this is not a difficult technique if a high-precision monostable multivibrator is used, for example. After all, as shown at the bottom of FIG. 6, the second audio subcarrier becomes the reference carrier component at this pulse width T ₂ .

FIG. 7 shows an example of the configuration of the main part of a television receiver corresponding to the above-mentioned sending device. The television carrier frequency signal including the video signal and the two audio subcarriers is detected by the synchronous detection circuit 31 by the video carrier reproduced by the video carrier reproducing circuit 30. The video signal in the detected signal is a low-pass filter for video.
It is supplied to the terminal 40 via 32. The FM-modulated first audio subcarrier is the first bandpass filter 37 (center frequency).
_s1 is normally 4.5 MHz) and is detected by the FM detection circuit 38. In addition to the configuration shown in FIG. 7, the first audio subcarrier is often extracted by a so-called intercarrier method using intermodulation with a video carrier. On the other hand, the multi-valued QAM-modulated second audio sub-carrier is input to the sub-carrier regeneration circuit 34 and the multi-valued QAM synchronous demodulation circuit 35 through the second bandpass filter 33 (center frequency _s2 is, for example, 4.65 MHz). It The subcarrier reproducing circuit 34 takes in the second subcarrier during the period in which the reference timing pulse signal generated by the timing generating means 12 (the same as that of the transmitting device) using the synchronizing signal in the video signal is input, and The carrier component is reproduced. This operation is similar to the reproduction of the continuous color subcarrier from the color burst, which is performed in the color signal processing. For example, by using a PLL circuit, the circuit is normally operated while the reference timing pulse is input, and the loop is held in the other period. Using the continuous carrier component of the second audio subcarrier obtained in this way, multi-valued QA
The M demodulator 35 synchronously detects the second audio subcarrier.
The digital audio extracted from the demodulator 35 is decoded by the decoding means 36.
Converted to analog voice by. This voice and the voice obtained by the FM detector 38 are selected by the viewer's operation,
Alternatively, it is supplied to the output of the switch 39 by automatic selection according to the reception status (status such as input strength and ghost).

Generally, in the television receiver, in the video signal processing after the terminal 40, the horizontal and vertical sync separation circuits perform the same processing as the timing generating means 12 of FIG. It is also possible to use the function of the sync separation circuit. The "voice signal" in the above description includes stereo voice, bilingual voice, and voice including additional data service (facsimile etc.).

As described above, according to the present invention, it is possible to transmit high-quality digital audio in the same channel while maintaining complete compatibility with existing television receivers. Based on the premise that multi-valued QAM is used to transmit a high-speed digital signal in a narrow band, by utilizing the video synchronization signal that is transmitted at the same time for this carrier reproduction on the receiving side, Brings the possibility. Further, in the present invention, since the carrier wave of the reference phase is sent out for reproducing the carrier wave, the differential conversion of data which is usually necessary is not necessary, which is also advantageous in the bit error rate during reproduction.

Although the above description has been made for the television broadcast of the NTSC system with the channel interval of 6 MHz, the conditions under which the present invention can be used are (1) that the video signal includes a synchronizing signal (2) the channel used by the television , Slight (eg
Since there are two gaps in the frequency (100 KHz or more), it can be said that it can be used for terrestrial television broadcasting in any country in the world.

[Brief description of drawings]

1 is a block diagram of a television signal transmitting apparatus according to an embodiment of the present invention, FIG. 2 is a block diagram of a conventional example of a transmitting apparatus for multiplexing digital audio, FIG. 3 is a spectrum diagram of a television signal, and FIG. Is the constellation diagram of 16-level QAM signal, No. 5
FIG. 6 is a block diagram of a multi-level QAM modulating means in the present invention, FIG. 6 is a timing diagram of a second audio subcarrier in the present invention, and FIG. 7 is a block diagram of a main part of a television signal receiver of an embodiment in the present invention. It is a figure. 2 ... Residual sideband modulation means, 4 ... Encoding means, 7 ...
FM modulation means, 11 ... Multi-valued QAM modulation means, 12 ... Timing generation means, 22,23 ... Carrier fixing means, 30 ... Video carrier reproducing means, 34 ... Second subcarrier reproducing means, 35 ... Multilevel QAM demodulation means, 36 ... Decoding means, 38 ... FM demodulation means.

Claims (1)

[Claims] 1. A video carrier is VS based on a video signal to be transmitted.
Depending on the VSB modulating means for B modulation, the timing generating means for generating the reference timing pulse signal of the low duty ratio by using one or both of the horizontal synchronizing signal and the vertical synchronizing signal included in this video signal, and the audio signal to be transmitted. FM modulation means for FM-modulating the first audio subcarrier, encoding means for converting the audio signal into a digital audio signal, and multilevel QAM modulation for multilevel QAM modulation of the second audio subcarrier by this digital audio. Means and a synthesizing means for synthesizing and transmitting the modulated video carrier, the first audio subcarrier and the second audio subcarrier, and the multilevel QAM modulating means during the output period of the timing pulse signal. An apparatus for transmitting a television signal with digital audio, which is controlled to stop modulation and output a reference carrier component of a second audio subcarrier.