JPH0529981A - Sub modulation circuit for vsb-am communication system - Google Patents

Abstract

PURPOSE:To attain effective use of a frequency band by sampling a modulation signal at a specific frequency, extracting only two kinds of side bands for a specific range, adding them, applying D/A conversion to the sum and applying amplitude limit to the result according to the inverse Nyquist characteristic so as to pass only a side band wave below a specific band. CONSTITUTION:A modulation signal X2 fed to an input terminal I2 is sampled at an A/D converter circuit 12 synchronously with a clock CK1 resulting from 1/9-frequency-dividing a carrier C0 fed to an input terminal I3 at a 1/9 frequency divider circuit 22. Then BPFs 13, 14 pass only side bands apart by 0.5-1fs and 1-1.5fs respectively synchronously with a clock CK2 whose frequency is 4fs being a multiple of four of the sampling frequency fs, the one side polarity is inverted and the result is added to the other at an adder 15 and the amplitude of the sum signal is limited according to the inverse Nyquist characteristic at an inverse Nyquist filter 17. Then the result is D/A-converted and its output is given to a BPF 19, from which only the frequency component of 0.5fs in the carrier C0 is passed and the result is multiplexed onto a main modulation signal and the result is transmitted.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a second VSB-AM communication system applied to a television broadcasting system and the like.
The present invention relates to a modulator circuit.

[0002]

EDs compatible with the NTSC standard system
In the TV signal system, various methods have been proposed in which an additional signal such as a wide screen signal or a high frequency component of a luminance signal is multiplexed with an existing NTSC signal.

As a typical example of such a multiplexing method, there has been proposed a quadrature two-phase modulation system in which a quadrature component of a video carrier is amplitude-modulated with an additional signal and multiplexed with an existing NTSC signal. For details of this quadrature two-phase modulation system, if necessary, a paper by Inoue et al. Entitled "Wide-screen EDTV system study" proposed in the Technical Report of the Television Society on August 25, 1988, or See, for example, the invention of Yasumoto et al. Entitled "High-definition signal generator for television signal" described in Japanese Patent Laid-Open No. 63-1178.

[0004]

The problems to be solved by the present invention are as follows.
It is an object of the present invention to provide a VSB-AM communication system capable of multiplexing an additional signal based on a novel principle which is completely different from the conventional quadrature two-phase modulation system and a sub-modulation circuit suitable for a transmission side device of this communication system. ..

[0005]

According to the present invention, a main modulation circuit that amplitude-modulates a carrier wave with a first modulated signal to generate a first modulated signal including upper and lower sidebands, and the carrier wave is used as the first modulated signal. A second modulation signal having a narrower band than the first modulation signal, amplitude-modulated with a second modulation signal, and synthesizing the upper and lower sidebands so as to have mutually opposite polarities to generate a second modulated signal; Inverse Nyquist filter that limits the amplitude of the modulated signal, and if neither the first modulated signal nor the second modulated signal whose amplitude is limited by the inverse Nyquist filter includes the carrier, the carrier And a synthesis circuit for synthesizing the first and second modulated signals, and before and after the synthesis circuit, for the first modulated signal, one sideband full band and the other sideband one VSB format communication including parts And a band-limiting circuit for limiting the band so that the second modulated signal whose amplitude is narrow-band limited becomes a DSB format signal that includes all of both sidebands, and this synthesized and band-limited A transmission side device including a circuit for transmitting a signal as a multiplexed signal, a carrier recovery circuit for recovering a carrier included in the received multiplexed signal from the received multiplexed signal, and synchronously detecting the multiplexed signal with the reproduced carrier. And a first demodulation circuit for reproducing the first modulated signal, and each of the signal bands including upper and lower sidebands of the second modulated signal in the multiplexed signal are synchronously detected by the reproduced carrier and A VSB-AM communication system including a receiving side device including a second demodulation circuit that synthesizes signals with opposite polarities and removes unnecessary frequency components from the synthesized signal to reproduce the second modulated signal is provided. Provided That.

Further, according to the present invention, the VSB-A described above is used.
As a suitable sub-modulation circuit that constitutes the receiving side device of the M communication system, the first modulated signal is sampled at a sampling frequency fs that is one-ninth of the frequency of the carrier wave while the sampling frequency and its harmonics An A / D conversion circuit for converting into a digital signal composed of sidebands, and a digital signal output from this A / D conversion circuit is resampled at a frequency 4fs which is four times the sampling frequency, and the frequency of 4fs and its harmonics. Around the wave
A first digital band-pass filter circuit that selectively passes sidebands existing at a distance of 0.5 fs or more and fs or less, and a digital signal output from the A / D conversion circuit at a frequency 4 fs that is four times the sampling frequency. While resampling, f around this 4fs frequency and its harmonics
a second digital band pass filter circuit for selectively passing side band waves existing at a distance of s or more and 1.5 fs or less and having a polarity opposite to that of the side band wave output from the first digital band pass filter circuit; , An adder circuit for adding the outputs of the first and second digital band-pass filtering circuits, a digital filter for limiting the amplitude of the inverse Nyquist characteristic to the output of the adder circuit, and the output of the digital filter to an analog signal. A D / A conversion circuit and an analog bandpass filtering circuit that selectively passes only those frequency components included in the restored analog signal that are present at a distance of 0.5 fs or less around the frequency of the carrier wave. Provision is provided.

[0007]

FIG. 1 is a block diagram showing the structure of a transmitting side apparatus which constitutes a VSB-AM communication system according to an embodiment of the present invention, where I ₁ and I ₂ are first and second modulations, respectively. Input terminals for signals X ₁ and X ₂ , I ₃ is an input terminal for carrier wave C ₀ , 1 is a first modulation circuit (main modulation circuit), 2 is a second modulation circuit (sub-modulation circuit), and 3 is multiplexing. Reference numeral 5 is a circuit, 4 is a band-pass filtering circuit, 5 is an RF converter, 6 is an antenna, and O ₁ is output multiplexing of multiplexed television signals.

The transmission side apparatus of FIG. 1 is exemplified as a transmission side apparatus of a multiplexed television signal which multiplexes an additional signal such as a wide aspect signal having a narrower band than an NTSC standard television signal and sends the multiplexed signal. ing.

That is, the _first terminal supplied to the input terminal I 1.
Modulated signal X ₁ of an NTSC television signal of a base band of the luminance signal Y and the carrier chrominance signal multiplexed by frequency interleaving on the high-frequency portion.
Further, an additional signal such as a wide aspect signal to be multiplexed with the NTSC television signal is supplied to the input terminal I ₂ as a second modulation signal X ₂ having a narrower band than that of the first modulation signal X _1. .. Furthermore, the input terminal I ₃ is connected to NT
The intermediate frequency carrier (IF carrier) of the SC television signal is supplied as the carrier C ₀ .

A carrier wave C ₀ supplied to an input terminal I ₃ ,
The angular frequencies of the first and second modulated signals X ₁ and X ₂ supplied to the input terminals I ₁ and I ₂ are ω ₀ , ω ₁ and ω _{2 in the} same order, and further the carrier wave C ₀ and the first and second Modulation signals X ₁ , X
_{If the} peak level is normalized to “1” for any of the _two , it can be described as C ₀ = cos ω ₀ t X ₁ = cos ω ₁ t X ₂ = cos ω ₂ t.

In the first modulation circuit 1, the first modulation signal X
By amplitude-modulating the carrier wave C ₀ by ₁ in the carrier wave sending format, the first modulated signal Y ₁ including the carrier wave C ₀ and the upper and lower sidebands around the carrier wave C ₀ is created. For convenience of explanation, the modulation index is "1". That is, Y ₁ = X ₁ C ₀ + C ₀ = cos ω ₁ · cos ω ₀ t = 1/2 [cos (ω ₀ + ω ₁ ) t + cos (ω ₀ ─ω ₁ ) t] + cos ω ₀ t ... ( 1)

As shown in FIG. 2 (A), the frequency spectrum of the _first modulated signal Y ₁ is composed of an upper sideband and a lower sideband symmetrically distributed on both sides of the carrier C _0. There is. Although detailed illustration is omitted to avoid complication,
The high band portion of each sideband of the modulated signal Y ₁ is composed of the high band component of the luminance signal and the carrier color signal.

On the other hand, in one amplitude modulation circuit 2a of the second modulation circuit 2, the carrier C ₀ is amplitude-modulated by the second modulation signal X ₂ . Further, the other of the amplitude modulation circuit 2b, the second modulated signal carrier -C ₀ antiphase by X ₂ is amplitude-modulated. This carrier of opposite phase-C
₀ is created by passing the carrier wave C ₀ on the input terminal I ₃ through the 180 ^° phase shift circuit 2c.

The upper sideband included in the output of the amplitude modulation circuit 2a is extracted by the bandpass filtering circuit 2d, and the lower sideband included in the output of the amplitude modulation circuit 2b is extracted by the bandpass filtering circuit 2e. The second modulated signal Y ₂ is created by synthesizing and in the synthesizing circuit 2f. That is,

Y ₂ = [X ₂ C ₀ ] _H + [− X ₂ C ₀ ] _L = 1/2 [cos (ω ₀ + ω ₂ ) t + cos (ω ₀ −ω ₂ ) t] _H −1/2 [ cos (ω ₀ + ω ₂ ) t + cos (ω ₀ −ω ₂ ) t] _L = 1/2 [cos (ω ₀ + ω ₂ ) t−cos (ω ₀ −ω ₂ ) t] (2) However, The symbols [] _H and [] _L mean that only the upper and lower sidebands in [] are extracted, respectively.
Further, it is assumed that the extracted upper and lower sidebands do not include the carrier wave itself.

As shown in FIG. 2B, the frequency spectrum of the _second modulated signal Y ₂ has a positive phase upper sideband C ₀ X ₂ and an antiphase lower sideband −C ₀ X. ₂ and ₂ are symmetrically distributed around the carrier frequency ω ₀ / 2π.

The second modulated signal Y ₂ is subjected to amplitude limitation by the inverse Nyquist filter 2g and then the first modulated signal by the multiplexing circuit 3.
Of the modulated signal Y ₁ of FIG.
A multiplexed television signal Z having a frequency spectrum as shown in (C) is created. That is,

Z = Y ₁ + Y ₂ = 1/2 [cos (ω ₀ + ω ₁ ) t + cos (ω ₀ ─ω ₁ ) t] ＋1/2 [cos (ω ₀ + ω ₂ ) t−cos (ω ₀ ─ω ₂ ) t] + cos ω ₀ t (3)

This multiplexed television signal Z is band-pass filtered by the band-pass filtering circuit 4 to become a VSB-AM system multiplexed television signal as shown in FIG. 2 (D). That is, a vestigial sideband (VSB) signal including the entire upper sideband and a part of the lower sideband is generated for the NTSC television signal, and the multiplexed signal is divided into upper and lower sidebands. A double sideband (DSB) signal containing the entire band is generated. The VSB television signal is D
It can be considered that the low frequency part of the SB system and the high frequency part of the single sideband (SSD) system are combined.

Therefore, in the multiplexed television signal shown in FIG. 2D, the low frequency band DSB portion in which the modulated signals Y ₁ and Y ₂ are multiplexed and the high frequency band in which only the modulated signal Y ₁ exists. Can be considered separately from the SSB part of. In the present embodiment, how the modulated signals Y ₁ and Y ₂ are multiplexed in the transmission side device and how the demultiplexing is performed in the reception side device will be described based on mathematical expressions. .. Therefore, in the following mathematical expressions, it should be noted that the modulated signal Y ₁ is a multiplexed portion with the modulated signal Y ₂ , that is, the low-frequency portion of the DSB system.

Here, the multiplexed signal Z is decomposed into a carrier wave C ₀ and upper sidebands [Z] _H and lower sidebands [Z] _L on both sides thereof, and Z = [Z] _H + [Z ] _L・ ・ ・ (4), [Z] _H = 1/2 [cos (ω ₀ + ω ₁ ) t + cos (ω ₀ + ω ₂ ) t] (5) [Z] _L = 1 / 2 [cos (ω ₀ + ω ₁ ) t−cos (ω ₀ −ω ₂ ) t] (6) is obtained.

The multiplexed television signal of FIG. 2D is
The signal is frequency-converted in the RF converter and is transmitted to the receiving side device by being radiated from the antenna 6 or being output from the output terminal O ₁ onto the cable.

In a receiving side device such as a television receiver, an RF band multiplexed television signal received by an antenna or the like is converted into an intermediate frequency band multiplexed television signal Z in a tuner. The multiplexed television signal Z in the intermediate frequency band is subjected to amplitude limitation by the Nyquist filter and becomes a multiplexed television signal having a frequency spectrum as shown in FIG. In this frequency spectrum, the second modulated signal Y ₂ is symmetrical about the carrier C ₀ because the _second Nyquist filter on the transmitting side and the Nyquist filter on the receiving side limit the amplitudes of the slopes opposite to each other. Subject to comprehensive amplitude limits. The arrangement of the signal frequencies is inverted due to the down conversion in the tuner. A carrier C _{0 of an} intermediate frequency is reproduced from the multiplexed television signal of the intermediate frequency band as shown in FIG. 3, and the reproduced carrier C ₀ and the multiplexed television signal Z form the first and second modulated signals X _1. , X
₂ demodulation is performed.

FIG. 4 is a block diagram showing the structure of the demodulator on the receiving side, where I ₄ is the multiplexed television signal Z.
Input terminal, 7 is a carrier recovery circuit, 8 is a first demodulation circuit, 9 is a second demodulation circuit, O ₂ is an output terminal for the reproduced first modulated signal X ₂ , and O ₃ is a reproduced first signal. 2 is an output terminal for the modulated signal X ₂ .

In the carrier reproducing circuit 7, the carrier C ₀ is reproduced from the multiplexed television signal Z supplied to the input terminal I ₄ . The carrier recovery circuit 7 is realized by a phase locked loop (PLL) circuit, a tank circuit, or the like.

In the coherent detection circuit 8a of the first demodulation circuit 8, the multiplexed television signal Z is coherently detected (multiplication product detection, product demodulation) by the reproduced carrier C ₀ , and the high frequency part of this detection output and the direct current The first modulated signal x ₁ is demodulated by removing the component in the low pass filter circuit 8b. That is, x ₁ = [ZC ₀ ] _L = [Y ₁ C ₀ + Y ₂ C ₀ + C ₀ ² ] _L (7)

Calculating each term in the equation (7), Y ₁ C ₀ = [cos (ω ₀ + ω ₁ ) t · cos ω ₀ t + cos (ω ₀ −ω ₁ ) t · cos ω ₀ t] = 1 / 2 [2cos ω ₁ t + cos (2ω ₀ + ω ₁ ) t + cos (2ω ₀ −ω ₁ ) t] (8) Y ₂ C ₀ = [cos (ω ₀ + ω ₂ ) t ・ cos ω ₀ t−cos ( ω ₀ −ω ₂ ) t · cos ω ₀ t] = 1/2 [cos (2ω ₀ + ω ₂ ) t + cos (2ω ₀ −ω ₂ ) t] (9) C ₀ ² = 1/2 [1 + cos 2ω ₀ t] ... (10)

Here, if ω ₀ >> ω ₁ ω ₂ ,
Cos (2ω ₀ + ω ₁ ) t, cos (2 in equations (8) to (10)
ω ₀ −ω ₁ ) t, cos (2ω ₀ + ω ₂ ), cos (2ω ₀ −ω
₂ ) t and cos 2ω ₀ t are both removed as harmonic components. Further, since the direct current component in the equation (10) is also removed, x ₁ = [ZC ₀ ] _L = cos ω ₁ t (11) is obtained from the equations (7) to (10).

This is nothing other than that in which the NTSC television signal (first modulated signal X ₁ ) modulated and transmitted according to the VSB-AM communication system and received is demodulated by the synchronous detection of the reproduced carrier C ₀ .

On the other hand, in the second demodulation circuit 9, the upper sideband and the lower sideband of the multiplexed television signal Z are extracted by the bandpass filtering circuits 9a and 9b, respectively. The extracted upper sideband is subjected to synchronous detection by the reproduced carrier wave C ₀ in the synchronous detection circuit 9c. Further, the extracted lower sideband is subjected to synchronous detection by the reproduced carrier wave (-C ₀ ) of opposite phase in the synchronous detection circuit 9d.
The reproduced carrier wave (−C ₀ ) having the opposite phase is the reproduced carrier wave C _0.
Is passed through a 180 ^° phase shift circuit 9e.
The detection outputs output from the synchronous detection circuits 9c and 9d are combined by the combining circuit 9f, and the high frequency component and the direct current component included in this combined signal are removed by the low pass filtering circuit 9g, whereby the second modulated signal X ₂ is played.

That is, x ₂ = [[Z] _H C ₀ − [Z] _L C ₀ ] _L (12) By calculating each term in the equation (12), [Z] _H C ₀ = 1 / 2 [cos (ω ₀ + ω ₁ ) t ・ cos ω ₀ t + cos (ω ₀ + ω ₂ ) t ・ cos ω ₀ t] = 1/4 [cos (2ω ₀ + ω ₁ ) t + cos ω ₁ t + cos (2ω ₀ + Ω ₂ ) t + cos ω ₂ t] (13) − [Z] _L C ₀ = 1/2 [−cos (ω ₀ −ω ₁ ) t · cos ω ₀ t + cos (ω ₀ −ω ₂ ) t・ Cos ω ₀ t] = 1/4 [−cos (2ω ₀ −ω ₁ ) t−cos ω ₁ t + cos (2ω ₀ −ω ₂ ) t + cos ω ₂ t] (14) (12) to (12) From the equation (14), x ₂ = cos ω ₂ t (15), and the second modulated signal X ₂ is demodulated.

In the transmission side apparatus illustrated in FIG. 1, the amplitude modulation circuits 2a and 2b may be composed of carrier suppression double sideband modulation circuits such as ring modulators. The amplitude modulation circuit 2
It is also possible to configure a and 2b by a carrier transmission double sideband modulation circuit such as a well-known collector modulation circuit or base modulation circuit and remove the carrier by the band pass filtering circuits 2d and 2e in the subsequent stage. The amplitude modulation circuits 1, 2a and 2b can also be realized by an analog multiplication circuit using an operational amplifier.

The synchronous detection circuit 8 of the receiving side device shown in FIG.
As an example of a, 9c, and 9d, “N
A synchronous detection circuit having the structure as described on page 150 of the HK television technical textbook [above] can be used. Alternatively, these synchronous detection circuits may be realized by an analog purification circuit using an operational amplifier. Furthermore, a multiplication circuit for amplitude modulation or synchronous detection can be configured by a combination of an A / D conversion circuit, a digital multiplication circuit and a D / A conversion circuit.

As described above, (a) the normal VSB is used in the first modulator of the transmitting side apparatus.
The modulated signal X ₁ having received −AM is reproduced by adding the signal components of the respective sidebands by being subjected to normal synchronous detection in the first demodulation unit of the receiving side device. (B) The second modulated signal X ₂ obtained by amplitude-modulating the carrier having the opposite phase in the second modulator of the transmitting side device and then combining the sidebands on the opposite side is the first modulated signal X ₂ of the receiving side device. By receiving the normal synchronous detection in the demodulation unit, the signal components of the respective sidebands are canceled and erased. (C) Normal VSB-AM in the first modulator of the transmitting side device
The received modulated signal X ₁ is subjected to synchronous detection by carrier waves of opposite phases in the first demodulation unit of the receiving side device, and is then synthesized to obtain a signal component of each side band. Are offset and erased. (D) The second modulation signal X ₂ obtained by subjecting the first modulation unit of the transmission side device to amplitude modulation with the carrier of the opposite phase and then combining the sidebands on the opposite side is the second modulation signal X ₂ of the reception side device. In the demodulator, the upper and lower sidebands are subjected to synchronous detection by carrier waves of opposite phases and then combined, whereby the signal components of the respective sidebands are added and reproduced.

Therefore, the combination of the vertical relationship of the sidebands and the direct relationship of the phases of the carrier waves can be reversed from that in the case of FIG.

FIG. 5 is a block diagram showing the configuration of another embodiment of the second modulation circuit (sub-modulation circuit) of FIG.
Is a low pass filter circuit, 12 is an A / D conversion circuit, 13, 1
4 is a digital band pass filter circuit, 15 is an adder circuit, 1
6 is a D flip-flop, 17 is an inverse Nyquist filter, 18 is a D / A conversion circuit, 19 is a band pass filter circuit,
Reference numeral 21 is an amplifier circuit, 22 is a frequency dividing circuit, 23 is a quadrupling circuit, I ₂ is an input terminal for the _second modulated signal X ₂ , and I ₃ is an input terminal for the carrier wave C ₀ .

The second modulated signal X ₂ supplied to the input terminal I ₂ is converted into a digital signal by the A / D conversion circuit 12 after the high frequency component is removed by the low pass filtering circuit 11. The A / D conversion, the clock signal ck1 of the sampling frequency fs that is created by passing a carrier C ₀ of the frequency f ₀ supplied to the input terminal I ₃ to 9 divider circuit 21 (= f _0/9) It is done synchronously. Prior to this A / D conversion, in order to prevent aliasing noise, a signal component in a frequency range higher than 0.5 fs is removed by the low-pass filter circuit 11, and its output (a) is as shown in FIG. 6 (a). It becomes a spectrum signal. Along with this, the output (b) of the A / D conversion circuit 12 becomes a double sideband wave existing over a frequency range of 0.5 fs around the sampling frequency fs and its harmonics as shown in the spectrum of FIG. 6 (b). It consists of groups.

The digital signal containing the above harmonic component is
In the digital band pass filtering circuits 13 and 14, the signals are filtered while being resampled in synchronization with the clock signal ck2 having the sampling frequency of 4 fs, and the filtered signals are shown in FIGS.
The signal has a spectrum as shown in. That is, the digital band pass filter circuit 13 selectively allows only the side band wave existing around the sampling frequency of 4 fs and its harmonics at a distance of 0.5 fs or more and fs or less. Further, the digital band-pass filtering circuit 14 selectively passes only the side band wave existing around the frequency of 4 fs and its harmonics at a distance of fs or more and 1.5 fs or less. Further, by inverting the polarities of the coefficients of the digital band-pass filtering circuits 13 and 14, the polarities of the signals (c) and (d) are inverted. The clock signal ck2 is created by passing the clock signal ck1 (frequency fs) output from the 9-divider circuit 22 through the 4-multiplier circuit 23.

The signals (c) and (d) output from the digital band-pass filtering circuits 13 and 14 are added by the adding circuit 15 and then synchronized with the clock signal ck2 in the D flip-flop 16 so that the signal shown in FIG. It becomes a synthesized signal having a spectrum as shown in e). The synthesized signal is subjected to amplitude limitation of the inverse Nyquist characteristic by the inverse Nyquist filter 17 and becomes a signal having a spectrum as shown in FIG. 6 (f). The signal (f) whose amplitude is limited is restored to an analog signal again in the D / A conversion circuit 18 in synchronization with the clock signal ck2, and then, in the analog band pass filtering circuit 19, a signal of 0.5 fs around the frequency of the carrier wave. By removing the low-frequency component and high-frequency component excluding the frequency range, a signal having a spectrum as shown in FIG. 6 (g) is supplied to one input terminal of the adder circuit 3 in FIG.

[0040]

Since the VSB-AM communication system according to the present invention is configured as described above, additional information such as a wide aspect signal is added to an NTSC television signal based on a novel method different from the conventional method. Multiplexing is possible, and effective use of frequency bands becomes possible.

Further, the preferred sub-modulation circuit which constitutes the transmission side device of this VSB-AM communication system has the generation of the harmonic component associated with A / D conversion and the periodic frequency characteristic associated with the digital band pass filtering process. By utilizing it, it is possible to realize it substantially only by a digital filter. As a result, it is possible to provide a simple, inexpensive, and highly accurate sub-modulation circuit.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a transmission side apparatus that constitutes a VSB-AM communication system according to an exemplary embodiment of the present invention.

2 is a sub-modulation circuit 2 for an NTSC television signal (first modulated signal Y ₁ ) generated by a main modulation circuit 1 of FIG.
FIG. 3 is a conceptual diagram for explaining an example of the configuration of the frequency spectrum of a multiplexed television signal in which the second modulated signal Y ₂ generated in 1 is multiplexed.

FIG. 3 is a conceptual diagram showing a frequency spectrum in a receiving side device of the multiplexed television signal of FIG.

FIG. 4 is a block diagram showing a configuration of a receiving side device which constitutes a VSB-AM communication system according to an embodiment of the present invention.

5 is a block diagram showing another embodiment of the second modulation circuit of FIG. 1. FIG.

FIG. 6 is a conceptual diagram exemplifying a spectrum of a signal of each unit in FIG.

FIG. 7 is a conceptual diagram illustrating a spectrum of a signal of each unit in FIG.

[Explanation of symbols]

I ₁ Input terminal for the first modulation signal X ₁ I ₂ Input terminal for the second modulation signal X ₂ I ₃ Input terminal for the carrier C ₀ 1 Main modulation circuit (first modulation circuit) 2 Sub modulation circuit (second Modulation circuit of 3) Multiplexing circuit I ₄ Input terminal of reception multiplexed signal 7 Carrier recovery circuit 8 First demodulation circuit 9 Second demodulation circuit 12 A / D conversion circuit 13,14 Extract one of sidebands Digital filter for band pass filter 15 Adder circuit 17 Digital filter for inverse Nyquist filter 18 D / A converter circuit 19 Analog filter for band pass filter

Claims (1)

Claim: What is claimed is: 1. A main modulation circuit for amplitude-modulating a carrier wave with a first modulated signal to generate a first modulated signal including upper and lower sidebands, and the carrier wave for the first modulated signal. A second modulation signal whose amplitude is narrower than that of the second modulation signal, and upper and lower sidebands are combined so as to have opposite polarities to each other to generate a second modulated signal; If the inverse Nyquist filter that limits the amplitude of the signal and the first modulated signal and the second modulated signal whose amplitude is limited by the inverse Nyquist filter do not include the carrier, include the carrier. However, a combining circuit for combining these first and second modulated signals, and before and after this combining circuit, the first modulated signal includes the entire band of one sideband and a part of the other sideband. To be a VSB format signal And a band limiting circuit for limiting the band so that the second modulated signal with the narrow band amplitude limited becomes a DSB format signal including all of both sidebands, and the synthesized and band limited signal is multiplexed. A device for transmitting as a multiplexed signal, a carrier recovery circuit for recovering a carrier included in the received multiplexed signal from the received multiplexed signal, and the multiplexed carrier is synchronously detected by the recovered carrier to detect the multiplexed signal. 1 for reproducing modulated signal 1
Demodulation circuit and each signal band including upper and lower sidebands of the second modulated signal in the multiplexed signal are synchronously detected by the reproduced carrier wave and are synthesized so as to have mutually opposite polarities. A sub-modulation circuit of a transmission side device of a VSB-AM communication system, comprising: a reception side device including a second demodulation circuit that removes unnecessary frequency components and reproduces the second modulated signal. An A / D conversion circuit for sampling the first modulated signal at a sampling frequency fs that is one ninth of the frequency of the carrier wave and converting it into a digital signal composed of sidebands around this sampling frequency and its harmonics. While resampling the digital signal output from the A / D conversion circuit at a frequency 4fs, which is four times the sampling frequency, the frequency of 4fs and its harmonics A first digital bandpass filter circuit for selectively passing sidebands existing at a distance of 0.5 fs or more and fs or less, and a digital signal output from the A / D conversion circuit at a frequency 4 fs which is four times the sampling frequency. While resampling, sidebands existing around the frequency of 4fs and their harmonics at a distance of fs or more and 1.5fs or less are selectively and have opposite polarities with respect to the sidebands output from the first digital bandpass filtering circuit. Second to pass so that
Digital band pass filter circuit, an adder circuit for adding the outputs of the first and second digital band pass filter circuits, a digital filter for limiting the amplitude of the inverse Nyquist characteristic to the output of the adder circuit, and the digital filter D / A conversion circuit that restores the output of the above to an analog signal, and selectively passes only those frequency components included in the restored analog signal that are present at a distance of 0.5 fs or less around the frequency of the carrier wave. A sub-modulation circuit for a VSB-AM communication system comprising an analog band-pass filtering circuit.