JPH0564169A - Orthogonal multiplex modulation receiver - Google Patents

Abstract

PURPOSE:To considerably reduce the deterioration in decoded voice signal quality considerably due to nonlinearity of an orthogonal synchronization detection circuit even when a detection circuit having the nonlinearity is used with respect to the orthogonal synchronization detection circuit for the orthogonal multiplex transmission receiver. CONSTITUTION:A detection output of an in-phase detection circuit 205 is led to a modulation circuit 224, in which a recovery carrier is modulated again and a re-modulation signal is obtained. Then the re-modulation signal is added to a signal inputted to an orthogonal detection circuit 220 at an adder circuit 218 to apply orthogonal synchronization detection to the synthesized output. Thus, the video modulation wave component is cancelled in advance from the input to the orthogonal detection circuit and production of crosstalk between the video signal and a multiplex signal at an output of the orthogonal detection is prevented.

Description

Detailed Description of the Invention

[Object of the Invention] (Field of Industrial Application) The present invention relates to a receiving apparatus of a television transmission system that multiplex-transmits a digitally encoded audio signal to a current television broadcast. (Prior art)

In television broadcasting, a new television transmission system capable of transmitting signals of higher image quality and higher sound has been proposed. In particular, in order to improve the sound quality, there is a proposal to multiplex-transmit a digitally encoded voice signal by an orthogonal multiplexing method. The orthogonal multiplexing method is a method compatible with the conventional transmission line and receiver because the digital audio signal can be newly added while maintaining compatibility with the existing signal.

FIG. 2 is a diagram for explaining the principle of this orthogonal multiplex transmission. In the figure, the video signal input from the video input terminal 101 is A
M-modulated. The carrier at this time is a carrier in which the output of the local oscillator (local oscillator) 106 is distributed in the 0 ° phase by the distributor 107. The AM-modulated output is converted into a vestigial side-wave AM-modulated wave by the VSB filter 103. The vestigial sideband is generally defined by a VSB filter in which the passband is from the video carrier frequency fc to 750 KHz and the attenuation band is from 1.25 MHz. In the current method, the analog FM modulated audio signal is frequency-multiplexed with the VSB filter output, but it is omitted in FIG. 2 (orthogonal multiplexing and simultaneous transmission are also possible).

On the other hand, the digitally encoded voice signal is supplied to the multiple signal input terminal 108, and the AM modulation circuit 10
At 9, carrier suppression AM modulation is performed. The carrier used at this time is a signal which is phase-shifted by 90 ° and distributed by the distributor 107. The modulated wave subjected to the carrier suppression AM modulation is spectrally shaped by the inverse Nyquist filter 110. The purpose of the inverse Nyquist filter 110 is to prevent the multiplexed signal from appearing as crosstalk in the video signal during demodulation on the receiving side. That is, if the inverse Nyquist equalizer processing having the characteristics shown in FIG. 2 is performed on the transmitting side in advance, the quadrature modulated wave is completely orthogonal to the image carrier in the output of the Nyquist filter used in the image signal demodulation on the receiving side. Therefore, it becomes possible to separate them. The output of the inverse Nyquist filter 110 is level-adjusted by the attenuator 111 and supplied to the synthesis circuit 104. The video modulated wave from the VSB filter 103 and the quadrature modulated wave from the inverse Nyquist filter 110 are combined by the combining circuit 104 and sent to the transmission line 105. The attenuator 111 is for adjusting the quadrature modulated wave signal level in consideration of incomplete demodulation characteristics of the current television set. The relative level difference between the video modulated wave and the quadrature modulated wave (hereinafter referred to as multiple level) is set to about -25 to -30 dB. Next, the processing on the receiving side will be described.

In FIG. 2, the modulated wave output from the transmission line 105 is branched and one is a reception Nyquist filter 11
2 is supplied. The output of the Nyquist filter 112 is synchronously detected by the in-phase detection circuit 113, and the demodulated video signal is output to the output terminal 114. In-phase detection circuit 113
The carrier supplied to is a video carrier reproduced by the carrier reproducing circuit 115 and distributed by the distributor 116 in a 0 ° phase.

On the other hand, regarding the demodulation of the multiplex signal, the bandpass filter (hereinafter referred to as BPF) 117 extracts the multiplex signal, and then the quadrature detection circuit 118 performs synchronous detection.
The carrier supplied to the quadrature detection circuit 118 is the distributor 11.
It is a carrier of 90 ° phase output from No. 6. BPF
Since 117 extracts only the band in which both sides of the VSB-AM modulated video modulated wave are aligned, the video component is a perfect DSB-AM in the BPF output. That is, since there is only the in-phase component with respect to the video carrier, it is possible to completely separate it from the multiplexed signal.

FIG. 3A shows an example of the spectrum of the video modulated wave, the audio modulated wave, and the orthogonal multiplex modulated wave. Further, FIG. 3 (B) shows the spectrum of the BPF 117 output.

FIG. 3C is a diagram showing a modulated wave vector of the output of the BPF 117. The image modulated wave and the orthogonal modulated wave are orthogonal to each other at 90 ° as shown. Further, as described above, the multiplexing level is about -30 dB and each unmodulated carrier amplitude ratio is 30: 1.

In this way, the quadrature synchronous detection circuit 118 which demodulates the multiplexed signal is supplied with an unnecessary video modulation wave component at a level 30 times or more higher than that of the quadrature modulation wave which is the desired signal. If the quadrature synchronous detection circuit 118 has a completely linear characteristic and there is no phase error of the reproduced carrier, it can be completely separated as described above, but in reality, perfect linearity cannot be obtained and there is also a phase error. To do. Even if there is no phase error, if non-linearity exists as will be described later, a phase error occurs depending on the amplitude level of the modulated wave, and the signals cannot be completely separated.

FIG. 4A is a diagram showing general non-linearity of a quadrature synchronous detector. In the figure, the horizontal axis represents the (modulated wave) input level of the detector, and the vertical axis represents the detection output level. As shown by the broken line, the input / output characteristic generally has a saturation characteristic, and the third-order distortion (IM ³ ) which is important in the nonlinear characteristic becomes the same as the original signal level at a certain input level (this part is I
Called the M ³ intercept point). The input level is set in advance so that the difference between IM ³ and the original signal level is equal to or less than a specified value. However, as described above, the image-modulated wave, which is an unnecessary signal, is about 30 times as large. Since it is large, if the specified value is to be satisfied, the input level for the quadrature signal will be very small. Therefore, in the conventional quadrature synchronous detection circuit, IM ³
However, there is a problem that the detection level of the multiplexed signal must be increased by increasing the input level even if it cannot be secured sufficiently large.

FIG. 4B shows an example in which the nonlinear characteristic of the quadrature synchronous detection circuit is shown by a modulated wave vector. In the same figure, the video modulation wave vector is a vector that moves on the in-phase axis (shown for the output of the BPF 117 in FIG. 2).
Therefore, since only the DSB-AM component of the video modulation wave is extracted, there is no orthogonal component in the video modulation wave). On the other hand, the quadrature-modulated wave is a carrier-suppressed AM modulation of a quadrature carrier with a multiplex signal, and thus has a vector as shown in the figure (actually, it also has an in-phase component due to an inverse Nyquist filter, but here, It is only for detection, and it is not necessary to consider crosstalk in which multiple signals leak to the video signal. Therefore, in the figure, it is shown as a vector that moves on orthogonal axes for simplicity).

In FIG. 4B, the video modulation wave vector amplitude maximum value (unmodulated carrier vector) is about 30 times the quadrature modulation wave vector amplitude maximum value. Therefore, regarding the occurrence of non-linearity in the quadrature synchronous detection circuit, it is sufficient to consider only the video modulation wave level. The nonlinearity of the quadrature synchronous detection circuit can be considered as the distortion of the amplitude component (AM / AM) and the distortion of the phase component (AM / PM), as shown in the figure. That is, the input / output amplitude characteristic and the input / output phase characteristic become non-linear according to the input level. When the AM / AM and AM / PM conversion distortions shown by the broken line in FIG. 7B occur, as is clear from the figure, the video modulation wave component and the quadrature modulation wave component are no longer orthogonal, and the video signal is a multiplexed signal. Crosstalk that leaks occurs.

FIG. 4C is a diagram showing an example of the crosstalk. When the video signal has a ramp waveform as shown in the figure, if there is nonlinearity as shown in the figure, crosstalk as shown in the figure occurs in the quadrature demodulation output (multiplexed signal). Only talk is shown).

[0013]

As described above, the orthogonal synchronous detection circuit of the receiving apparatus for orthogonal multiplex transmission has about 3
Only the quadrature modulation wave must be detected while separating the video modulation wave component having a signal level as high as 0 times. As a result,
In order to keep the detection output of the multiplexed signal sufficiently large while limiting the gain of the amplifier in the latter stage, an excessive video modulation wave is simultaneously input to the quadrature synchronous detection circuit, and the nonlinear characteristics of the quadrature synchronous detection circuit are It becomes a problem. That is, due to this non-linearity, crosstalk occurs between the video modulated wave and the quadrature modulated wave that were originally separated by the orthogonality of the carrier phase, and the video signal component leaks into the multiple signal detection output. When such crosstalk exists, there is a problem that when digital data is transmitted as a multiplex signal, the bit error rate of the data increases and the quality of the decoded voice signal deteriorates.

Therefore, the present invention relates to a quadrature synchronous detection circuit of a quadrature multiplex transmission / reception apparatus, even if a detection circuit having a non-linearity of the same performance as the conventional one is used, a decoded voice signal due to the non-linearity of the quadrature synchronous detection circuit. It is an object of the present invention to provide an orthogonal multiplex transmission / reception device capable of significantly reducing the deterioration of quality.

[0015]

According to the present invention, a signal subjected to quadrature multiplex modulation is detected by an in-phase synchronous detection section to demodulate one signal component, and the signal subjected to quadrature multiplex modulation is detected by the quadrature synchronous detection section. In a receiving device that detects and demodulates the other signal component, a means for remodulating a regenerated carrier by a detection output of the in-phase synchronous detection unit to obtain a remodulated signal, and the remodulated signal to the quadrature synchronous detection unit. And a means for synthesizing the input signal and quadrature-coherently detecting the synthesized output.

[0016]

By the above means, the modulated wave re-modulated by the video signal detected in-phase in advance is combined with the input of the quadrature detection circuit before the multiplexed signal is separated from the video signal by the quadrature detection and demodulated. .. The phase of the remodulated output is adjusted at the time of this combination, and the image modulated wave components at the time of combination are canceled. As described above, since the video modulation wave component is canceled in advance by the quadrature detection circuit input, the crosstalk between the video signal and the multiplex signal as described above does not exist in the quadrature detection output, and therefore the digital signal of the multiplex signal is not present. Since the bit error rate does not deteriorate, the decoded voice signal quality does not deteriorate. (Embodiment) An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1A is a block diagram showing an embodiment of an orthogonal multiplex transmission / reception apparatus according to the present invention. In the figure, the modulated wave input to the input terminal 201 is the Nyquist filter 202 and the carrier recovery circuit 203 as in the conventional example.
And a band pass filter (hereinafter referred to as BPF) 204 (this configuration is configured to correspond to the conventional example in order to simplify the description, but this configuration is not particularly premised). .. The output of the Nyquist filter 202 is detected by the in-phase detection circuit 205 using the output of the carrier regeneration circuit 203, and the low-pass filter (hereinafter referred to as LPF) 206 removes the signal outside the required band, and the output terminal 2
It is output to 07 as a demodulation video signal. On the other hand, BPF2
The orthogonal multiplex modulated wave extracted in 04 is added by an adding circuit 2 described later.
After 18, the signal is detected by the quadrature detection circuit 220. In the quadrature detection, the output of the carrier regeneration circuit 203 is used as the phase shifter 2
A carrier that is phase-shifted by 90 ° in 19 is used. The quadrature detection output is also output to the output terminal 222 as a multiplexed signal via the LPF 221 similarly to the video output.

Here, the output of the in-phase detection circuit 205 is LP
The LPF 226 is also branched to the F 226, and this LPF 226 is a filter that extracts only the frequency component corresponding to the DSB-AM band from the demodulated video signal. The output of the LPF 226 is supplied to an amplifier circuit 223 having a DC offset, and the amplifier circuit 223 has a specified modulation factor (8) for video modulation.
Give a DC offset equal to 7.5%). The output of the amplifier circuit 223 is supplied to the modulation signal input terminal of the AM modulation circuit 224. A reproduction carrier is supplied to the carrier input terminal of the AM modulation circuit 224. Hence AM
The output of the modulation circuit 224 is a signal obtained by re-modulating the reproduced carrier with the low-frequency component of the demodulated video signal. Next, the re-modulated output is supplied to one input terminal of the adder circuit 218 via the phase shift circuit 225.

The output of the above-mentioned BPF 204 is given to the other input terminal of the adder circuit 218. By combining these inputs, the video modulated wave and the orthogonal modulated wave of the output of the BPF 204 are combined (multiplexed). Of the signal, the video modulation wave is canceled by the remodulation output, and only the quadrature modulation wave component remains. The phase shift circuit 225 is for setting the amount of phase shift so that the phase difference between the remodulated output and the image modulated wave output from the BPF 204 becomes 180 °.

Using the embodiment of the present invention as described above,
Only quadrature modulated waves that should be originally detected are input to the quadrature synchronous detection circuit 220, and crosstalk between video modulated waves and quadrature modulated waves is significantly reduced.

Since the remodulation output using the low-frequency component of the video detection output is affected by at least the delay of the LPF 226, etc., the addition circuit 218 cannot necessarily completely cancel the video modulation wave. It is not necessary to completely cancel the talk improvement, but a sufficient crosstalk improvement effect can be obtained by suppressing the image modulated wave to some extent. FIG. 1B is a block diagram showing another embodiment of the present invention. In the following description, FIG.
Only points different from the embodiment of (A) will be described.

In FIG. 1B, the in-phase detection circuit 205
The video signal detected by is output through the LPF 206. The LPF 206 is a filter that passes the video signal band (4.2 MHz for NTSC). This LP
The output of F206 is branched and supplied to the amplifier circuit 223. In the same manner as in the previous embodiment, the signal is re-modulated by the modulation circuit 224, passes through the phase shift circuit 225, and then the BPF2
The input signal is input to the adder circuit 218 in the preceding stage of 04 and is combined with the input IF signal.

The re-modulated output is not the VSB-AM but the DSB-AM like the video modulated wave of the input IF signal, but the BPF 204 uses the band pass for extracting only the DSB area of the VSB-AM modulated wave. Because it is a filter,
It is not necessary for the adder circuit 218 to suppress the video modulation wave over the entire band, and the remodulation output may be a DSB-AM different from the input IF as described above.

As described above, after the addition circuit 218 suppresses at least the DSB-AM region of the video modulation wave, the BPF
204 in its DSB-AM region (ie, quadrature modulation band)
Is extracted and input to the quadrature detection circuit 220. Therefore,
The input signal of the quadrature detection circuit 220 is the same as in the previous embodiment.
The conventional image-modulated wave and the quadrature-modulated wave are not in a state where there is a very large level difference, but the level of the image-modulated wave among these components is significantly suppressed.

As a result, crosstalk from the video signal to the multiplex signal caused by the non-linearity of the quadrature detector is greatly reduced, the code error rate of digital audio data which is the multiplex signal is improved, and the quality of the decoded audio signal is improved. Is improved.

As described above, according to this apparatus, there is a transmission method of orthogonal modulation multiplex transmission of two signals having a large level difference, such as multiplex transmission of digitally encoded audio signals in current television broadcasting by orthogonal modulation. In the receiving device, crosstalk from the other signal caused by the non-linearity of the detection circuit that detects a signal having a small relative level can be significantly reduced. Therefore, it is possible to improve the reception quality of the signal, for example, if the signal is a digitally encoded speech signal as described above, the decoding quality can be improved.

If the above-mentioned crosstalk reduction is attempted without using the present invention, generally, the detector input level must be greatly attenuated, and the detection sensitivity is lowered, so that noise is generated and the amplifier gain of the subsequent stage is increased. It is difficult to solve such problems at low cost. Therefore, using the present invention,
The above problem can be solved only by adding a very simple circuit, and the performance of the receiving apparatus of the orthogonal modulation multiplex transmission system can be improved at low cost, which is a great industrial effect.

[0028]

As described above, according to the present invention, regarding the quadrature synchronous detection circuit of the quadrature multiplex transmission / reception apparatus, even if the detection circuit having the nonlinearity of the same performance as the conventional one is used
It is possible to significantly reduce the deterioration of the decoded voice signal quality due to the non-linearity of the quadrature synchronous detection circuit.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 1 (A) is a first embodiment, and FIG. 1 (B) is a second embodiment.

FIG. 2 is a block diagram illustrating the principle of a digital voice transmission system that is an example of orthogonal multiplex transmission.

FIG. 3 is a diagram showing an example of a spectrum of orthogonally multiplexed modulated waves, a spectrum diagram of components extracted by a BPF from the modulated waves, and a diagram illustrating a modulated wave vector of a BPF output.

FIG. 4 is a diagram showing a non-linear example of a synchronous detection mixer of an orthogonal multiplex demodulator, a diagram explaining a crosstalk of a video signal into a multiple signal due to the non-linearity, and a diagram showing a crosstalk example.

[Explanation of symbols]

202 ... Nyquist filter, 203 ... Carrier reproduction circuit, 204 ... Band pass filter (BPF), 205 ... In-phase detection circuit, 206, 221, 226 ... Low pass filter, 218 ... Addition circuit, 219 ... Phase shift circuit, 220 ... Quadrature detection circuit, 223 ... Amplification circuit, 224 ... AM modulation circuit, 225 ... Phase shift circuit.

Claims (4)

[Claims] 1. A quadrature multiplex modulated signal is detected by an in-phase synchronous detection section to demodulate one signal component, and the quadrature multiplex modulated signal is detected by an quadrature synchronous detection section to detect the other signal component. In the receiving device for demodulation, means for re-modulating the regenerated carrier by the detection output of the in-phase synchronous detection unit to obtain a re-modulated signal, and combining the re-modulated signal with the signal input to the quadrature synchronous detection unit, and combining A quadrature multiplex modulation receiver comprising means for making output quadrature synchronous detection. 2. The quadrature-multiplexed modulated signal is a video modulated wave in which the in-phase signal is VSB-AM modulated from the video signal, and the quadrature signal is an audio modulated wave in which carrier suppression modulation is performed on the digital audio signal, and 2. The orthogonal multiplex modulation receiver according to claim 1, wherein the audio modulated wave is orthogonally multiplexed with the video modulated wave only in the DSB-AM region. 3. The re-modulated signal is an in-phase synchronous detection output D
It is constructed by a signal from a means for extracting only the component transmitted by SB-AM and a reproduction carrier, and the re-modulated signal is combined with the output of the band pass filter of the input stage of the quadrature synchronous detection section. 2. The method according to claim 2, wherein
A quadrature multiplex modulation receiver described. 4. The re-modulated signal is an in-phase synchronous detection output D.
The re-modulated signal is composed of a signal in an arbitrary band equal to or higher than the SB-AM band and a regenerated carrier, and the re-modulated signal is combined with a signal in the preceding stage of the band pass filter in the input stage of the quadrature synchronous detection unit. The quadrature multiplex modulation receiver according to claim 2.