JP3112576B2 - Digital transmission system and digital modulation transceiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital transmission system for digital transmission of television signals and the like and a digital modulation transmitting / receiving apparatus.

[0002]

2. Description of the Related Art An example of a waveform transmitted by digitally modulating a television signal will be described below with reference to FIGS.

In a certain area, even if an attempt is made to transmit a digital television signal to a vacant channel of a television broadcast frequency band by using phase modulation (hereinafter, referred to as PSK) or quadrature amplitude modulation (hereinafter, referred to as QAM),
In many cases, the channel is already
It is used as television broadcasting of the C system. When a digital television signal is transmitted in such a vacant channel in which NTSC television broadcasting is performed in an adjacent area, as shown in FIG. The transmission spectrum overlaps with the television signal, and co-channel interference occurs on the receiving side.

[0004] When a digital television signal is disturbed by analog broadcasting as described above, there is no effective countermeasure for PSK or QAM, and this disturbance directly appears as deterioration of a video. For this reason, very strong error correction must be used. Further, when the level of interference is very large, even the error correction cannot cope with the problem, and the transmission system is broken.

Further, orthogonal frequency division multiplexing modulation (hereinafter referred to as O
In FDM), equal-amplitude carrier waves modulated by four-phase phase modulation (hereinafter, referred to as QPSK) or QAM or the like are multiplexed into frequencies at which the power of the spectrum of the other carrier becomes "0". Make two transmission frequency bands. The total spectrum obtained by superimposing the respective carrier waves uniformly spreads over the entire frequency band as shown in FIG. FIG. 9A is a diagram illustrating a modulated wave spectrum modulated by QPSK, QAM, or the like and having a center frequency fc of a carrier.

[0006] In OFDM, different data is carried by each of the superposed carriers, so that one channel can simultaneously transmit data by the number of carriers. For this reason, even if interference is caused in a certain partial band, it can be corrected to some extent by error correction. However, since analog broadcasting has a great influence on digital broadcasting in the vicinity of the frequency spectrum used as a carrier by the existing analog modulated broadcasting, most of the error correction capability must be used for reducing the disturbance. For this reason, in digital television broadcasting, interference from existing television broadcasting due to analog modulation in an adjacent area cannot be effectively reduced.

[0007]

[Problems that the Invention is to Solve] In the system for transmitting this way the conventional television signal digitally modulated and, to reduce the interfere harm from an existing television broadcasting by an analog modulation of the adjacent regions effectively There was a problem that can not be. SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the above-mentioned disadvantages of the prior art and to effectively reduce interference from existing television broadcasts due to analog modulation in adjacent areas.

[0008]

In order to achieve the above object, according to the present invention, a supplied video signal is separated into a high-frequency component and a low-frequency component, each of which is encoded with high efficiency, and the high-frequency component is encoded. Is assigned to a spectrum near the frequency of the video carrier, the chrominance subcarrier and the audio carrier of the analog broadcast channel, and the first
, And the low-frequency component is allocated to a spectrum other than the three carrier waves, and is digitally modulated by a second modulation method that is more susceptible to interference than the first, and the digitally modulated signal is transmitted to an analog broadcast channel. Provide a digital transmission system for transmission.

Further, the first modulation method is QPSK (Qu
adrature Phase Shift Keying) in the second modulation method
The formula is 64QAM (Quadrature Amplitude Modulation)
A digital transmission method is provided.

[0010]

[0011]

[0012]

[0013]

[0014]

According to the above construction, OFD
Of the total spectrum of M, the video carrier of the current analog broadcast, the color sub-carrier, the use of the spectrum of the frequency corresponding to the audio carrier, even if missing, the video breakdown is less particularly important video information is sent, It is possible to reduce video breakage due to co-channel interference that digital broadcasting receives from current analog broadcasting.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to FIGS. FIG. 1 shows a schematic configuration of a digital modulation transmitting apparatus according to an embodiment of the transmitting side of the present invention. In FIG. 1, an analog or digital video signal is supplied to a high-pass filter (hereinafter, referred to as HPF) 2 and a low-pass filter (hereinafter, referred to as LPF) 4 via an input terminal 1. In the HPF 2, high frequency components are extracted from the supplied video signal and supplied to the compression encoder 3. In the compression encoder 3, H
The high-frequency component supplied from the PF 2 is converted into a b-bit parallel code having information on the high-frequency component of the image and supplied to the reordering unit 6. In the LPF 4, a low-frequency component is extracted from the supplied video signal and supplied to the compression encoder 5. In the compression encoder 5, the low-frequency components supplied from the LPF 4 are converted into b-bit parallel codes having information on the low-frequency components of the image, and are rearranged by the rearranger 6.
Supplied to

In the reordering unit 6, the two systems of b-bit parallel codes from the compression encoder 5 and the compression encoder 6 are rearranged and supplied to the error correction encoder 7 as one system of b-bit parallel codes. Is done. In the error correction encoder 7,
An error correction bit is added to each of the b-bit parallel codes from the reordering unit 6 and supplied to the OFDM modulator 8 as an error-correcting code sequence including b'-bit parallel codes. OF
In the DM modulator 8, every other code supplied from the error correction encoder 7 is divided into an in-phase axis component and an orthogonal axis component, and OFDM using an inverse discrete Fourier transform is performed. The signals are supplied to the quadrature modulator 9 as in-phase axis signals and quadrature axis signals composed of parallel codes.
In the quadrature modulator 9, the supplied in-phase axis signal and quadrature axis signal are quadrature amplitude-modulated and output as a transmission signal. The clock C1 is supplied to the reordering unit 6, the frequency dividers 10 and 12. In the frequency divider 10, the supplied clock C1 is N /
{N− (i + j + k)} is divided and supplied as a clock 11 to the reordering unit 6 and the compression encoder 5. In the frequency divider 12, the supplied clock C1 is N / (i + j +
k) The frequency is divided and supplied as a clock 13 to the rearranger 6 and the compression encoder 3.

The configurations of the error correction encoder 7, the OFDM modulator 8, and the quadrature modulator 9 are disclosed, for example, in NHK STRL Open Research Publications, pp. 28-36, 1992. .

Here, out of the number N of modulated carriers transmitted by the OFDM modulator 8 in one channel, the number i is centered on the number n, the number j is centered on the number m, and the number l is centered on the number l. The description will be made on the assumption that k carriers are particularly susceptible to interference by the video analog main carrier, the color subcarrier, and the sound carrier of the current analog broadcast present on the same channel.

The reordering unit 6 receives N codes from the compression encoder 3 and the compression encoder 5 at a fixed time, and (i + j + k) of the N codes are received from the compression encoder 3 by N.
− (I + j + k) are received from the compression encoder 5. Since these code transmission rates are different, the clock 13 is used as the transmission clock of the compression encoder 3 and the reception clock of the rearranger 6, the clock 11 is used as the transmission clock of the compression encoder 5 and the reception clock of the rearranger 6, and The clock C1 is used as a transmission clock of the rearranger 6. Next, the configuration of the rearranger 6 in FIG. 1 will be described in more detail with reference to FIG.

The rearranger 6 comprises serial / parallel converters 21 and 22, a parallel / serial converter 23 and a counter 24. In FIG. 2, all of the signals other than the clocks C1, 11, 13 and the read signal 25 are b-bit parallel signal lines for representing codes.

In the counter 24, the supplied clock 1
3 is counted, and the read signal 25 is supplied to the parallel / serial converter 23 when N− (i + j + k) times are counted.

In the serial / parallel converter 21, the code supplied from the compression encoder 5 via the signal line 15 is read in synchronization with the clock 13, converted, and supplied to the parallel / serial converter 23. . Cereal/
In the parallel converter 22, the code supplied from the compression encoder 3 via the signal line 14 is read in synchronization with the clock 11, converted, and supplied to the parallel / serial converter 23.

In the parallel / serial converter 23, i of n−i / 2 to n + i / 2 out of N inputs, m−j /
The output of the serial / parallel converter 22 is supplied for j of 2 to m + j / 2 and k of l−k / 2 to l + k / 2, and serial / parallel conversion is performed for the other N− (i + j + k). The output of the vessel 21 is provided. These supplied signals are read by the parallel / serial converter 23 in synchronization with the read signal 25, and
It is output in synchronization with the supplied clock C1. next,
The configuration of the compression encoders 3 and 5 in FIG. 1 will be described in more detail with reference to FIG. Since the configurations of the compression encoders 3 and 5 are the same, the compression encoder 3 will be described below. In FIG. 3, a high frequency component of the video signal supplied from the HPF 2 is supplied to a quantizer 31. In the quantizer 31, the supplied high frequency component is quantized in accordance with the quantization control signal 35 from the buffer capacity controller 34 and supplied to the variable length encoder 32. In the variable-length encoder 32, the quantized high frequency components are variable-length coded,
3. In the buffer 33, the variable-length encoded code is read in synchronization with the clock C2, and is output to the signal line 14 in synchronization with the clock 13.
Further, a signal corresponding to the amount of the code in the buffer 33 is supplied to the buffer capacity controller 34. In the buffer capacity controller 34, a quantization control signal 35 corresponding to the signal supplied from the buffer 33 is supplied to the quantizer 31. Thus, control is performed such that the amount of codes in the buffer 33 is always within a certain range.

Next, the receiving side will be described. FIG. 4 shows a schematic configuration of an embodiment of the digital modulation receiving apparatus according to the present invention. In FIG. 4, a received signal that is a quadrature amplitude modulated analog signal is a quadrature demodulator 41.
Supplied to In the quadrature demodulator 41, the supplied received signal is demodulated in the in-phase axis and the quadrature axis, and each of the received signals is converted into an in-phase axis signal and a quadrature axis signal having a b ′ bit parallel code.
The signal is supplied to the FDM demodulator 42. In the OFDM demodulator 42, the supplied in-phase axis signal and quadrature axis signal are subjected to orthogonal frequency division multiplex demodulation by discrete Fourier transform, and this demodulated code is supplied to an error correction decoder 43.
The configurations of the quadrature demodulator 41 and the OFDM demodulator 42 are, for example, those shown in NHK STRL Open Research Announcement Proceedings, pp. 28-36.

The error correction decoder 43 detects the error of the code supplied from the OFDM demodulator 42 and, when the error can be corrected, corrects the error of the code and then corrects the b-bit code. It is supplied to the distributor 44. When error correction is not possible, the b-bit code obtained by removing the error correction bits from the code is supplied to the distributor 44, and the error detection signal is supplied to the error rate calculator 45. The error rate calculator 45 estimates the error rate at that time based on the supplied error detection signal, and when the estimated error rate exceeds a prescribed value, an error signal is supplied to the high-frequency side decoder 46. You. The clock C1 is supplied to the distributor 44 and the frequency dividers 49 and 51, respectively. In the frequency divider 49, the clock C 1 is frequency-divided by N / {N− (i + j + k)} and supplied as the clock 50 to the distributor 44. In the frequency divider 51, the clock C1 is frequency-divided by N / (i + j + k) and supplied to the distributor 44 as the clock 52.

In the distributor 44, the code supplied from the error correction decoder 43 is read in synchronization with the clock C 1, and in synchronization with the clock 52, the high frequency side decoder 46,
The signals are supplied to the low-frequency side decoder 47 in synchronization with the clock 50. In the low-frequency side decoder 47, the signal supplied from the distributor 44 is decoded into a low-frequency component of the original image signal by a procedure reverse to that of the compression encoder 5 in FIG. 1 and supplied to the mixer 48. In the high-frequency side decoder 46, the signal supplied from the distributor 44 is decoded into a high-frequency component of the original image signal by a procedure reverse to that of the compression encoder 3 in FIG. This decoded signal is output to mixer 48 when no error signal is supplied.
When the error signal is supplied to the mixer 48
Not supplied to In the mixer 48, the high frequency side decoder 46
And the signal supplied from the low-frequency side decoder 47 are synthesized and output as an image signal. Next, the distributor 4 of FIG.
4 will be described in more detail with reference to FIG.

The distributor 44 comprises a serial / parallel converter 55, parallel / serial converters 56 and 57, and a counter 58. In FIG. 5, except for the clocks C1, 50, and 52 and the read signal 59, all are b-bit parallel signal lines for representing codes.

In the counter 58, the supplied clock C
The read signal 59 is supplied to the parallel / serial converters 56 and 57 when 1 is counted and N times.

In the serial / parallel converter 55, the code supplied from the error correction decoder 43 is read in synchronization with the clock C1, and converted and supplied to the parallel / serial converter 56 and the parallel / serial converter 57. Is done. These N serial / parallel converters 5
5, i of n−i / 2 to n + i / 2, m−j
/ 2 to j of m + j / 2, k of l−k / 2 to l + k / 2
The number is supplied to the parallel / serial converter 56, and the other N− (i + j + k) are supplied to the parallel / serial converter 57.

In the parallel / serial converter 56, the code supplied from the serial / parallel converter 55 is read in synchronization with the read signal 59, and is output to the high-frequency side decoder 46 in synchronization with the clock 52. In the parallel / serial converter 57, the code supplied from the serial / parallel converter 55 is read in synchronization with the read signal 59, and is output to the low-frequency side decoder 47 in synchronization with the clock 50. Next, the configurations of the high-frequency side decoder 46 and the low-frequency side decoder 47 in FIG. 4 will be described in more detail with reference to FIG.

The high-frequency side decoder 46 includes a variable-length code decoder 6
1, a buffer 62, a switch 63, and an inverse quantizer 64. The low frequency side decoder 47 includes a variable length code decoder 65, a buffer 66, and an inverse quantizer 67.

In the variable length code decoder 61, the code supplied from the distributor 44 is decoded and supplied to the buffer 62. In the buffer 62, the signal supplied from the variable length code decoder 61 is temporarily stored, and supplied to the inverse quantizer 64 via the switch 63 at a timing synchronized with the supplied clock C2. The switch 63 disconnects the buffer 62 and the inverse quantizer 64 when the error signal is supplied, and the buffer 6 when the error signal is not supplied.
2 and the inverse quantizer 64 are made conductive. In the inverse quantizer 64, the signal supplied from the switch 63 is inversely quantized,
The image signal is supplied to the mixer 48 as a high frequency component. Thus, when an error signal is supplied, the inverse quantizer 6
4 does not receive the signal from the buffer 62, and the dequantizer 64 can receive the signal from the buffer 62 when no error signal is provided.

The code supplied from the distributor 44 is decoded by the low-frequency side decoder 65 and supplied to the buffer 66. In the buffer 66, the signal supplied from the variable length code decoder 65 is temporarily stored, and supplied to the inverse quantizer 67 at a timing synchronized with the supplied clock C2. In the inverse quantizer 67, the signal supplied from the buffer 66 is inversely quantized and supplied to the mixer 48 as a low frequency component of the image signal. Next, a transmission waveform in the digital modulation transmission / reception device configured as described above will be described with reference to FIG.

FIG. 7A is a diagram showing the transmission spectrum of the current NTSC broadcasting station, and FIG. 7B is a diagram showing the spectrum of the OFDM system using the present invention. FIG.
7 (b) near the video carrier f _v , the color sub-carrier f _sc , and the audio carrier f _A shown in FIG. 7 (a). , The signal of relatively high importance is transmitted. The digital modulation receiver detects an error in the signal in the hatched portion in FIG. 7B, and does not use a relatively insignificant signal in the hatched portion when there is an error.

As described above, an image signal is divided into a relatively low importance high frequency component and a relatively high importance signal, and the divided signal is reduced in the susceptibility to interference by the current analog broadcast carrier. Sorts the spectrum according to the OF
Since DM is performed, it is possible to suppress disturbance of an image at the time of reception.

Actually, when QPSK is simply used, the code error rate due to co-channel interference is about 2% at C / I = 6 dB. However, by using the transmitting / receiving device according to the present invention, it is possible to reduce failures caused by co-channel interference in empty channels. And even when there is no co-channel interference, OFDM in the channel
Since all carriers can be used, image data can be transmitted efficiently. For this reason, the problem of digital TV broadcasting using a terrestrial vacant channel is solved, and a digital broadcasting system that can be used in a wider range can be realized. Further, particularly when the interference due to the transmission spectrum of the current NTSC broadcasting station is large, the bit allocation to the spectrum affected by the interference may be changed in order to reduce the influence of the interference. For example, assuming that the number of bits to which the signal 14 in FIG. 1 is allocated is 1 bit and the number of bits allocated to the signal 15 is 3 bits, the data of the high frequency component of the image signal is QPSK and the low frequency component of the image signal is 64 bits.
It is transmitted by QAM. Since QPSK is more resistant to interference than 64QAM, higher frequency components of the image signal are transmitted more reliably than when simply transmitting the spectrum. Further, in the case of further interference, it is possible to perform OFDM without using the interfered spectrum to transmit and receive.

[0037]

According to the present invention, an image signal is divided into a high-frequency component having a relatively low importance and a signal having a relatively high importance, and the divided signal is susceptible to interference by the current analog broadcast carrier. Since the spectrum is transmitted according to the spectrum, disturbance of the image at the time of reception can be suppressed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of a digital modulation transmitting apparatus according to the present invention.

FIG. 2 is a diagram showing a configuration of a rearranger 6.

FIG. 3 is a diagram showing a configuration of compression encoders 3 and 5;

FIG. 4 is a diagram showing a configuration of an embodiment of a digital modulation receiving apparatus according to the present invention.

FIG. 5 is a diagram showing a configuration of a distributor 44.

FIG. 6 is a diagram showing a configuration of high frequency side decoders 46 and 47.

FIG. 7 is a diagram showing a transmission waveform of the digital modulation transmitting / receiving apparatus according to the present invention.

FIG. 8 is a diagram showing the interference that a transmission waveform of a conventional digital modulation transmitting apparatus receives from an existing analog modulation method.

FIG. 9 is a diagram showing a transmission waveform of a conventional digital modulation transmission device.

[Explanation of symbols]

1 input terminal, 2 HPF, 3, 5 compression encoder, 4 ...
LPF, 6: rearranger, 7: error correction encoder, 8 ...
OFDM modulator, 9: quadrature modulator, 10, 12, ... frequency divider.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Noboru Taga 3-3-9, Shimbashi, Minato-ku, Tokyo Inside Toshiba Abu E Co., Ltd. (72) Susumu Komatsu 3-3-1, Shimbashi, Minato-ku, Tokyo No. 9 Toshiba Abu E Co., Ltd. (56) References JP-A-6-164665 (JP, A) JP-A-5-218978 (JP, A) European Patent Application Publication 448492 (EP, A1) (58) Fields surveyed (Int. Cl. ⁷ , DB name) H04J 11/00 H04J 1/00

Claims (2)

(57) [Claims] 1. A supplied video signal is separated into a high-frequency component and a low-frequency component, and each of the high-frequency components is subjected to high-efficiency encoding. And modulates the spectrum in the vicinity of the frequency of the voice carrier and modulates the spectrum with the first modulation method, allocates the low-frequency component to a spectrum other than the three carriers, and is more susceptible to interference than the first.
A digital transmission method in which digital modulation is performed using a second modulation method, and the digitally modulated signal is transmitted to an analog broadcast channel. 2. The method according to claim 1, wherein the first modulation scheme is QPSK (Quadra
digital phase shift keying), wherein the second modulation scheme is 64 QAM (Quadrature Amplitude Modulation).