JP3128602B2 - Digital transmission equipment - 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 apparatus for serially transmitting a wideband digital signal through a coaxial cable.

[0002]

2. Description of the Related Art A digital transmission apparatus for serially transmitting a wideband digital signal through a coaxial cable is a commercial VTR.
In recent years, it has been used in studio equipment such as cameras and cameras.

Hereinafter, an example of the above-described conventional digital transmission device will be described with reference to the drawings.

FIG. 6 is a block diagram showing the configuration of a conventional digital transmission device. In FIG. 6, reference numeral 15 denotes a parallel
Data input terminal, 16 is parallel clock input terminal, 17
Is a parallel-serial converter, 18 is a PLL circuit, 19 is a transmission signal processing circuit, 20 is a transmission circuit, 21 is a coaxial cable, 22
Is an equalizer, 23 is a received signal processing circuit, 24 is a serial / parallel converter, 25 is a PLL circuit, 26 is a parallel data output terminal, and 27 is a parallel clock output terminal.

[0005] The operation of the digital transmission apparatus configured as described above will be described below.

An input signal to the parallel data input terminal 15 is a 270 Mbps NTSC component signal. In other words, the parallel data input terminal 15
Bit parallel data is input. 10 entered
The bit parallel data is latched in the parallel-serial converter 17 by the clock input from the parallel clock input terminal 16. PLL circuit 18
Now, the phase is locked to the input parallel clock.
A serial clock having a frequency of 10 times is generated and input to the parallel / serial converter 17.

The data converted into serial data by the parallel / serial converter 17 is transmitted to a transmission signal processing circuit 19.
And performs scramble processing and conversion from an NRZ signal to an NRZI signal. This scrambling process is not for encryption but for preventing 0s or 1s from continuing on the transmission path. Transmission signal processing circuit
The output of 19 is input to the transmission circuit 20, amplified by the buffer amplifier, and output to the coaxial cable 21.

The maximum length of the coaxial cable 21 is about 300 m. At this time, around 30 MHz, which is a band required for 270 Mbps serial transmission, the signal is attenuated by about 30 dB, so it is necessary to compensate for the high frequency band. Further, since the group delay in the low frequency band is not flat in the coaxial cable, it is necessary to compensate for this.
For these two reasons, the equalizer 22 is provided at the input stage of the receiver.

The output of the equalizer 22 is input to a PLL circuit 25 to reproduce a serial clock,
The parallel clock is also generated by dividing by 10
Output to clock output terminal 27. In the reception signal processing circuit 23, after the output of the equalizer 22 is identified as the original data by the serial clock reproduced by the PLL circuit 25, the reception signal processing circuit 23 converts the NRZI signal into an NRZ signal, descrambles the signal, and outputs it. The serial / parallel converter 24 converts the input serial data into the original 10-bit parallel data using the serial clock and the parallel clock reproduced by the PLL circuit 25, and outputs the data to the parallel data output terminal 26. (For example, RonWard: “Avoiding t
he Pitfalls in Serial Digital Signal Distributio
n ”, 133rd SMPTE Technical Conference, preprint N
o.133-45, Oct. , 1991).

[0010]

However, in the above configuration, it is difficult to realize a wideband equalizer that compensates for the attenuation of the high frequency band of the coaxial cable and also compensates for the deviation of the group delay of the low frequency band. In addition, there is a problem that the transmission distance is limited to be shorter than the S / N limit of the transmission digital signal by the compensating ability of the equalizer.

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and has as its object to provide a digital transmission apparatus for transmitting a wideband digital signal through a coaxial cable without using an equalizer.

[0012]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems and achieve the object, the present invention divides serial data into a plurality of parallel data uniformly, or hierarchically encodes the serial data in the order of data priority. In this method, each of the parallel data is subjected to quadrature amplitude modulation or vestigial sideband amplitude modulation in an independent channel, and transmitted through a coaxial cable by frequency multiplexing.

[0013]

According to the present invention, in order to divide a wideband digital signal into a plurality of channels, perform multi-level digital modulation with high transmission efficiency on each of them, and multiplex the frequencies, the bandwidth of each channel is increased. The band can be narrowed, and the amplitude / group delay deviation in each channel is reduced, so that an equalizer is not required.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A digital transmission device according to each embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a digital transmission apparatus according to a first embodiment of the present invention, and FIG. 4 is a quadrature amplitude modulator of FIG.
FIG. 1 is a block diagram showing a configuration of a QAM modulator (hereinafter, referred to as a QAM modulator);
FIG. 5 is a block diagram showing the configuration of the quadrature amplitude demodulator (hereinafter referred to as QAM demodulator) of FIG.

In FIG. 1, 1 is a data input terminal, 2 is a dividing circuit, 3, 4, and 5 are QAM modulators # 1, # 2,.
n and 6 are frequency multiplexing circuits, 7 is a coaxial cable, 8 is a modulation signal distribution circuit, 9, 10, and 11 are QAM demodulators # 1, # 2,.
#N and 12 are synthesis circuits, and 13 is a data output terminal.

In the QAM modulator 3 shown in FIG.
Is a modulation signal processing circuit, 32 is a quadrature modulator, 33 is an up converter, and the other QAM modulators 4 and 5 have the same configuration. In the QAM demodulator 9 in FIG. 5, reference numeral 91 denotes a tuner, 92 denotes a quadrature detector, 93 denotes a demodulation signal processing circuit, and the other QAM demodulators 10 and 11 have the same configuration.

The operation of the digital transmission apparatus configured as described above will be described below with reference to FIGS.

Digital data input to the data input terminal 1 is converted into a 270 Mbps NTSC component signal as in the conventional example. Since a 27 Mbps 10-bit parallel signal is input to the data input terminal 1, the QAM modulator #
When the number of 1 to #n (3 to 5) is 10, the dividing circuit 2 only needs to pass the input signal as it is. If the number of QAM modulators is not 10, the dividing circuit 2 divides the input data according to the number of QAM modulators and the transmission bandwidth of each QAM modulator. In this embodiment, the number of QAM modulators is ten, the transmission bandwidth of each modulator is 6 MHz, and the modulator is a 64 QAM modulator.

Each 64QAM modulator only needs to transmit 27 Mbps data. In actual transmission, an error correction code is added. If this is used as a Reed-Solomon correction code, usually about 10% of the total data amount is allocated to the correction code, so that the total data rate is 30 Mbps. Since 64QAM is a modulation method of 6 bits / symbol, the transmission efficiency is 6 times.
Therefore, to transmit 30Mbps data, 5.0M
Hz bandwidth is required. Since the transmission bandwidth is 6MHz,
The roll-off filter may have a roll-off rate of 0.2 or less.

As the carrier frequencies of the ten 64QAM modulators, for example, according to the channel plan of CATV in the United States, the lower-frequency channels of the coaxial cable 7 with a small attenuation are preferentially used. , 69,
75, 85, 93, 99, 105, 111, and 117 MHz can be selected.

Next, the 64QAM modulator will be described with reference to FIG. The 27 Mbps data input from the dividing circuit 2 is
The signal is input to the modulation signal processing circuit 31. Modulation signal processing circuit 31
Then, an error correction code is added, 64 QAM mapping is performed, roll-off filtering is performed, and DA conversion is performed, and the result is output to the quadrature modulator 32. The quadrature modulator 32 modulates the input two-axis signals of I and Q using two carrier waves having phases different by 90 degrees, adds the signals, and outputs the resultant signals. Here, the carrier used for modulation has a constant intermediate frequency. The up-converter 33 in the next stage multiplies the input modulated wave at the intermediate frequency and the output of the internal local oscillator by an internal multiplier, thereby up-converting the signals into the above-described 10 types of frequencies.

Returning to FIG. 1, the frequency multiplexing circuit 6 performs frequency multiplexing by adding the outputs of the ten 64QAM modulators by an adding circuit and outputs the result to the coaxial cable 7. The adder circuit may be, for example, an adder using a resistor in consideration of impedance matching. In the case of using the carrier waves of the ten frequencies described above, each has a bandwidth of 6 MHz, so that the bandwidth after multiplexing is 66 MHz from 54 to 120 MHz.
z.

The frequency-multiplexed digital modulation wave passing through the coaxial cable 7 of 0 to several hundred meters is input to the modulation signal distribution circuit 8. The modulation signal distribution circuit 8 removes unnecessary band noise through a band-pass filter that passes 54 to 120 MHz, and then amplifies the signal with an amplifier having a good noise figure and distributes the signal equally into ten. 64QAM demodulators # 1 to #n
(9 to 11).

Next, the 64QAM demodulator 9 will be described with reference to FIG. The frequency multiplexed signal input from the modulation signal distribution circuit 8 is input to the tuner 91. Tuner 91 here
Uses an American CATV tuner.
The tuner 91 selects one channel from ten channels and converts the frequency into an intermediate frequency band of 41 to 47 MHz. In the quadrature detector 92, a 90 MHz signal generated by an internal 44 MHz local oscillator is used.
The original I and Q signals are obtained by multiplying each of the two carrier waves having different phase and the modulation signal. The demodulated signal processing circuit 93 AD-converts the input I and Q signals, performs a roll-off filter process on the receiving side, identifies 64 code points, and performs reverse mapping on the original 6-bit data. Then, the data is subjected to error correction processing and output to the synthesis circuit 12. In addition,
The demodulation signal processing circuit 93 includes a carrier recovery circuit for locking the frequency and phase of the local oscillator of the quadrature detector 92 to the carrier of the modulated wave in the intermediate frequency band, and a clock recovery circuit for recovering the symbol clock.

The 27-Mbps digital data of each of the ten channels decoded by the ten 64QAM demodulators is combined with a combining circuit 12.
And output to the data output terminal 13 as a 10-bit parallel signal. The function of the synthesizing circuit 12 is to form a pair with the splitting circuit 2. If the number of 64QAM demodulators is not 10, they are once synthesized and then re-divided into 10-bit parallel signals.

For transmission of a baseband 270 Mbps digital signal, it is necessary to flatten the amplitude and group delay characteristics within a band of at least 0 to 135 MHz. Since it is sufficient that the amplitude and group delay characteristics are flat in each band of 6 MHz, there is no need to particularly provide an equalizer. The bandwidth used in this embodiment is 6 MHz.
Since 64QAM demodulators are available at low cost with the spread of 64QAM transmission on CATV, multiplex transmission of such wideband digital signals can be easily realized.

In this embodiment, the QAM modem is 64Q
Although AM is used, another QAM method such as 16QAM may be used. However, in this case, the amount of data that can be transmitted at 6 MHz changes, so it is necessary to change the number of QAM modems. Further, the total transmission rate is not limited to 270 Mbps, and the channel plan of the frequency multiplexed signal in RF is not limited to the example of the present embodiment.

Further, in this embodiment, the bandwidth of one channel is set to 6 MHz, but this is set to 27 MHz, and the transmission method is QPS.
K may be used. 40 at 27MHz bandwidth using satellite link.
This is because there is a possibility that a QPSK demodulator that transmits 96 Mbps spreads and is supplied at low cost. In this case, if the coding rate of the error correction code is 3/4, the transmission data amount per channel is 30.72 Mbps, so that 270 Mbps transmission requires 9 channels. The total required bandwidth is 27 MHz × 9 = 243 MHz. Although the bandwidth is increased as compared with the required bandwidth (135 MHz) in the base band, the amplitude and group delay deviation in each channel is small, so that the transmission degradation is reduced without providing an equalizer.

The attenuation [dB / m] per unit length of the coaxial cable is proportional to the square root of the frequency. That is,
When the frequency increases four times, the amount of attenuation doubles. Therefore, within a certain bandwidth, the high band has a smaller amplitude deviation than the low band. By making use of this characteristic to reduce the transmission bandwidth per channel in the low band and increase it in the high band, transmission degradation inside the channel can be reduced.

Further, in anticipation of the attenuation in the coaxial cable, the transmission power is increased for the higher band channel. As a result, the reception limit distance of the high-frequency channel is increased, so that the length of a cable that can be transmitted is increased. At this time, considering the case where the coaxial cable length becomes 0 m, the input AG of the QAM demodulator is considered.
The transmission power must not be increased beyond the range that can be absorbed by the C circuit.

Hereinafter, a digital transmission apparatus according to an embodiment of the second invention will be described with reference to the drawings. FIG.
FIG. 9 is a block diagram showing a configuration of a digital transmission device according to a second embodiment of the present invention. The difference from FIG. 1 is that the QAM modulators # 1, # 2, to #n (3 to 5) are vestigial sideband amplitude modulators (VSB-AM modulators # 1, # 2, to #n) 3.
0, 40, 50, QAM demodulators # 1, # 2, #n (9-11) are vestigial sideband amplitude demodulators (VSB-AM demodulators # 1, # 2,.
#N) It is changed to 90, 100, 110.

As in the first embodiment, the input signal is 270M
Consider a case in which bps data is transmitted in 10-bit parallel at 27 Mbps. Although 8-level VSB-AM transmission has a transmission efficiency of 3 bits / symbol, the transmission efficiency is equivalent to 6 bits / symbol because the band is effectively used by the VSB-AM system. Accordingly, digital data that has been converted to 30 Mbps by adding an error correction code to 27 Mbps data requires a transmission bandwidth of at least 5 MHz. If the roll-off rate is 20% or less, the bandwidth of one channel is 6 MHz.
z or less. Therefore, 270 Mbps data can be transmitted using an 8-level VSB-AM modem instead of the 64QAM modem.

As described above, according to this embodiment, since transmission is performed by dividing into a plurality of channels, an equalizer is not required in each channel. -The advantage of using the AM modulator / demodulator is that since the quadrature modulation / demodulation is not performed, the bit error rate does not deteriorate due to crosstalk between the orthogonal I and Q axes caused by distortion of the amplitude / group delay characteristics of the transmission line. It is.

As in the first aspect, the vestigial sideband amplitude modulator can be implemented with a transmission bandwidth of 6 MHz or 27 MHz.

Further, similarly to the first aspect of the present invention, when the transmission bandwidth of the low frequency channel is made smaller than the transmission bandwidth of the high frequency channel, and the carrier power is increased in the higher frequency channel. , The transmission distance can be further extended.

Hereinafter, a digital transmission apparatus according to an embodiment of the third invention will be described with reference to the drawings. FIG.
FIG. 9 is a block diagram showing a configuration of a digital transmission device according to a third embodiment of the present invention. The difference from FIG. 1 is that the dividing circuit 2 is changed to a hierarchical encoder 200 and the combining circuit 12 is changed to a hierarchical decoder 120. Also, the N modulators and demodulators arranged in parallel have a QAM
System or the VSB-AM system, either digital modulators # 1, # 2 to #n (310, 410, 510) and digital demodulators # 1, # 2 to #n (910, 101, 111)
And

The digitized video signal input to the hierarchical encoder 200 is divided into a signal important for restoring a video such as a low frequency band of a synchronizing signal and a luminance signal, and a signal not so important. As a method for dividing the frequency component of the luminance signal, D
CT (discrete cosine transform).

In the N-divided signal, the higher the importance of the signal, the lower the channel is allocated. This is because the lower the band, the less the attenuation in the coaxial cable, and the longer the transmission distance.

The data of each channel digitally modulated and demodulated using the N channels is converted to a hierarchical decoder.
Entered into 120. The hierarchical decoder 120 performs the reverse process of the hierarchical encoder 200 to restore the original digitized video signal and output it to the data output terminal 13.

As described above, according to the present embodiment, since the signal is divided into a plurality of channels and transmitted, an equalizer is not necessary in each channel. Since the classified video signals are classified in the order of importance and assigned to the low-frequency channels in order from the one with the highest priority, there is an advantage that fatal image quality hardly occurs even when transmitted over a long distance.

As in the first aspect, the digital modulator can be implemented with a transmission bandwidth of 6 MHz or 27 MHz.

Similarly to the first invention, the combination of reducing the transmission bandwidth of the low frequency channel to be smaller than the transmission bandwidth of the high frequency channel and increasing the carrier power for the higher frequency channel is also used. , The transmission distance can be further extended.

[0043]

As described above, in the digital transmission apparatus of the present invention, a wideband digital signal is divided into a plurality of channels, each of which is subjected to QAM or VSB-AM digital modulation, frequency-multiplexed, and then subjected to coaxial cable transmission. Is transmitted, and the signals of the respective channels are demodulated and then combined, whereby a long distance can be transmitted without using an equalizer.

In addition, instead of the wideband digital signal dividing circuit and the synthesizing circuit, a hierarchical encoder and a hierarchical decoder are used to classify the video signals in the order of importance, and to sort the video signals in the order of higher priority to lower priority signals. By assigning to a channel, the transmission distance can be further extended.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a digital transmission device according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a digital transmission device according to a second embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration of a digital transmission device according to a third embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of the QAM modulator of FIG.

FIG. 5 is a block diagram illustrating a configuration of a QAM demodulator of FIG. 1;

FIG. 6 is a block diagram showing a configuration of a conventional digital transmission device.

[Explanation of symbols]

1 ... digital signal data input terminal, 2 ... division circuit, 3, 4, 5 ... QAM modulators # 1, # 2,-# n,
6: frequency multiplexing circuit, 7: coaxial cable, 8: modulation signal distribution circuit, 9, 10, 11 ... QAM demodulators # 1, # 2,
~ # N, 12: synthesis circuit, 13: data output terminal of digital signal, 30, 40, 50 ... VSB-AM modulator # 1,
# 2 to #n, 90, 100, 110 ... VSB-AM demodulator #
1, # 2 to #n, 120: hierarchical decoder, 200: hierarchical encoder, 310, 410, 510: digital modulators # 1, # 2,
~ # N, 910, 101, 111 ... Digital demodulators # 1, # 2,
~ #N.

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Tad Bowser, 1006 Odakadoma, Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. (72) Inventor Hisaya Kato 1006 Kazuma, Kadoma, Osaka Pref. Matsushita Electric Industrial Co., Ltd. (56) References JP-A-5-218978 (JP, A) JP-A-60-180261 (JP, A) JP-A-4-112579 (JP, U) JP-A-57-98077 (JP, U) "Long-span digital microwave system", IEICE Technical Report, April 21, 1983, Vol. . 83, No. 3, p. 1-8, CS83-1 (58) Field surveyed (Int. Cl. ⁷ , DB name) H04J 1/00-1/20 H04L 27/00-27/38

Claims (3)

(57) [Claims] 1. A dividing circuit for dividing an input digital signal into N parallel data, and N dividing circuits for quadrature amplitude modulating each digital signal divided by the dividing circuit so that transmission bands do not overlap. A quadrature amplitude modulator;
A transmission unit configured by a frequency multiplexing circuit that combines the outputs of the N quadrature amplitude modulators; a modulation signal distribution circuit that distributes a received signal to N signals after transmission by a coaxial cable;
A receiving unit comprising: N quadrature amplitude demodulators; and a synthesizing circuit for synthesizing parallel data outputs of the N quadrature amplitude demodulators and restoring an original digital signal ,
For quadrature amplitude modulators, those with lower carrier frequencies are not transmitted.
If the transmission bandwidth is narrow and the carrier frequency is
A digital transmission device characterized by widening a transmission bandwidth . 2. The method according to claim 1, wherein the input digital signal is divided into N parameters.
A dividing circuit for dividing the data into real data;
Ensure that the transmission bandwidth of each split digital signal does not overlap
N residual sideband amplitudes, each of which is amplitude modulated
The width modulator and the outputs of the N vestigial sideband amplitude modulators.
A transmitter composed of a frequency multiplexing circuit to be synthesized and a coaxial
After transmitting the signal through a cable, the received signal is divided into N signals.
Tuning signal distribution circuit, N vestigial sideband amplitude demodulators,
Parallel data output of N vestigial sideband amplitude demodulators
And a combining circuit that restores the original digital signal by combining
Comprising a receiving unit, wherein the vestigial sideband amplitude modulator
Means that if the carrier frequency is in the low band, the transmission bandwidth is narrow.
If the carrier frequency is high, the transmission bandwidth is wide.
Features and to Lud Ijitaru transmission device to. 3. The method according to claim 1, further comprising the steps of:
Hierarchical coding that divides into N parallel data in order of preference
And each digital signal divided by the hierarchical encoder
Digitally modulated so that transmission bands do not overlap
N digital modulators, and the N digital modulators
It consists of a frequency multiplexing circuit that synthesizes the output of the modulator
After transmitting the signal through a coaxial cable,
And N modulated digital signal distribution circuits.
And the parallel data of the N digital demodulators.
Hierarchy that restores the original digital signal by combining the
And a receiving unit comprising an encoder.
If the carrier frequency is in the lower band , the modulator has a transmission bandwidth.
And the carrier frequency is in the high band, the transmission bandwidth is
It features and to Lud Ijitaru transmission apparatus to wide.