JP2991509B2 - Digital VSB modulator - 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 VSB modulator for generating a VSB (remaining sideband) modulation signal from a digitized baseband signal.

[0002]

2. Description of the Related Art As is well known, video modulating means used in a conventional television transmitter converts an analog baseband signal to AM and converts it into a DSB (double-sided band) modulated signal. The VSB modulated signal is obtained by band-limiting the signal with a band-pass filter in the intermediate frequency circuit or the transmission frequency circuit. As the band-pass filter, a surface acoustic wave filter is generally used. However, the surface acoustic wave filter is economically disadvantageous because it is necessary to prepare a separate filter for each used frequency, and
Due to the structure, the ripple of the group delay time characteristic is large, and there is a problem that the group delay time characteristic of the transmission device itself is affected by the surface acoustic wave filter.

Recently, there has been a strong demand for digitizing the above-mentioned VSB modulator.
However, it is extremely difficult to simply replace each circuit part constituting the conventional VSB modulation means with a digital circuit having the same function. For example, if the above-mentioned band-pass filter is to be realized by a digital filter, high-speed circuit operation is required and the number of taps becomes very large due to a steep attenuation characteristic of a stop band, thereby increasing the circuit scale. Is caused.

[0004]

As described above, the VSB
When the modulator is digitized, simply replacing each circuit portion with a digital circuit having the same function has a problem that the configuration is complicated and large, and it is difficult to realize.

Accordingly, the present invention has been made in view of the above circumstances, and provides a very good digital VSB modulation device capable of generating a VSB modulation signal from a digitized baseband signal with a simple configuration. With the goal.

[0006]

According to the present invention, there is provided a digital VSB modulator according to the present invention, comprising a separating means for separating a digitized baseband signal into a low-frequency component and a high-frequency component; DSB modulating means for multiplying a low frequency component of the baseband signal by a carrier signal to obtain a DSB modulated signal, and a high frequency component of the baseband signal separated by the separating means
Is orthogonally distributed, and the phase is shifted by 90 degrees to one side
Processing, and delay processing corresponding to the orthogonal transformation processing time
And both signals have a 90 degree phase difference from each other.
Are multiplied by the respective carrier signals having
SSB modulating means for obtaining an SSB modulated signal by addition , DS
A combination means for combining the output of the B modulation means and the output of the SSB modulation means to obtain a VSB modulation signal is provided.

[0007]

According to the above arrangement, a low-frequency DSB modulated signal and a high-frequency S
By generating an SB-modulated signal and combining them, V
Since the SB modulation signal is obtained, the configuration can be greatly simplified and the miniaturization can be promoted as compared with the case where each circuit portion is simply replaced with a digital circuit having the same function as in the related art. Also generated from the same baseband signal
To combine the obtained DSB modulation signal and the SSB modulation signal.
The frequency spectrum of the combined seam
Can be smoothed.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing an embodiment of the present invention, the principle of the present invention will be described.
That is, FIG. 1A shows a frequency spectrum of the VSB modulation signal. This VSB modulation signal is 0-f
DSB modulation is applied to baseband low frequency components up to L Hz, and SSB (single sideband) modulation is applied to baseband high frequency components up to fL to fHHz.

For this reason, the VSB modulated signal can be expressed as shown in Equation 1 when viewed as a function S (t) of time t.

[0010]

(Equation 1)

Where k is a modulation index, f
c is the carrier frequency, f1 and f2 are the modulation signal frequencies, and 0
<F1 ≦ fL, fL ≦ f2 <fH. The first term on the right side of Equation 1 corresponds to the DSB modulation signal, and its frequency spectrum is shown in FIG. 1B. The second term corresponds to the SSB modulation signal, and its frequency spectrum is the same. This is shown in FIG. That is, from the above equation 1, it can be seen that the VSB modulated signal shown in FIG. 1A is a composite signal of the DSB modulated signal shown in FIG. 1B and the SSB modulated signal shown in FIG.

An embodiment of the present invention based on the above principle will now be described in detail with reference to the drawings. In FIG. 2, reference numeral 11 denotes an input terminal to which a baseband signal digitized at a sampling frequency fs and having a band from 0 to fH Hz as shown in FIG. 3A is supplied. The sampling frequency fs satisfies the following relationship: fs> 2fH (fH is the highest frequency of the baseband signal) fs> 2fc.

The digitized baseband signal supplied to the input terminal 11 is divided into two by a distribution circuit 12, and one of the outputs is supplied to an LPF (low-pass filter) 13 so that the baseband signal has a high frequency. The band components are cut off, and low band components from 0 to fL Hz are extracted as shown in FIG. The output of the LPF 13 is divided into two by the distribution circuit 14, and one output is supplied to the negative input terminal − of the subtraction circuit 15.

The other output of the distribution circuit 13 is
The signal is delayed by the delay circuit 16 by the processing time of the LPF 13 and supplied to the positive input terminal + of the subtraction circuit 15. Then, the subtraction circuit 15 converts the original baseband signal shown in FIG. 3A supplied to its positive input terminal + into the baseband signal shown in FIG. By subtracting the low-frequency component of the signal, the high-frequency component of the baseband signal from fL to fH Hz is extracted as shown in FIG.

The high-band baseband signal output from the subtraction circuit 15 is multiplied by the modulation index k (constant) shown in Expression 1 by the multiplication circuit 17 and further multiplied by the constant 0.5 by the multiplication circuit 39. Thereafter, the signal is supplied to the SSB modulation circuit 18. The SSB modulation circuit 18 includes a distribution circuit 19,
Orthogonal transformation circuit 20, delay circuit 21, multiplication circuits 22, 23
And an addition circuit 24. The output of the multiplication circuit 17 is first divided into two by a distribution circuit 19, and one of the outputs is supplied to an orthogonal transformation circuit 20.

The orthogonal transform circuit 20 has a function of shifting the phase of a signal of all target frequency bands by 90 degrees, as is known, for example, by the Hilbert transform. That is, when the input of the orthogonal transform circuit 20 is cos2π · f2 · t, the output is converted into sin2π · f2 · t. The other output of the distribution circuit 19 is given by the delay circuit 21 the same delay time td as the processing time of the orthogonal transform circuit 20.

For this reason, assuming that the input of the distribution circuit 17 is (k / 2) · cos2π · f2 · t, the output of the orthogonal transformation circuit 29 is (k / 2) · sin2π · f2 · (t− td), and the output of the delay circuit 21 is (k / 2) .cos2.pi.f2. (t-td).

The outputs of the orthogonal transform circuit 29 and the output of the delay circuit 21 are multiplied by the carrier components sin2π · fc · t and cos2π · fc · t by the multiplication circuits 22 and 23, respectively. The output of the circuit 22 is (k / 2) · sin2π · fc · t · sin2π · f2 · (t−td), and the output of the multiplication circuit 23 is (k / 2) · cos2π · fc · t · cos2π · f2 · (t−td). The outputs of the multiplying circuits 22 and 23 are added by the adding circuit 24, so that the output S2 (t) of the adding circuit 24 becomes

[0019]

(Equation 2)

As shown in FIG.
An SSB modulated signal having a frequency spectrum as shown in (e) is obtained.

On the other hand, the other output cos2π · f1 · t of the distribution circuit 14 is given the same delay time td as the processing time by the orthogonal transformation circuit 20 by the delay circuit 25, and
Is multiplied by the modulation index k shown in FIG.
Is multiplied by cos2π · fc · t in the multiplication circuit 28, whereby the output S 1 of the multiplication circuit 28 is obtained.
(T)

[0022]

(Equation 3)

As shown in FIG.
The DSB modulated signal shown in (d) is obtained.

The addition circuit 29 causes
By adding the DSB modulated signal and the SSB modulated signal shown in (d) and (e), the output S
(T)

[0025]

(Equation 4)

Thus, a VSB modulated signal having a frequency spectrum as shown in FIG. Thereafter, the VSB modulated signal output from the addition circuit 29 is converted into an analog signal by a D / A (digital / analog) conversion circuit 30 and taken out via an output terminal 31 to be subjected to power amplification processing for transmission. Is done.

Therefore, according to the configuration as in the above embodiment, the low-frequency D
Since the VSB modulation signal is obtained by generating the SB modulation signal and the high-band SSB modulation signal and combining them, each circuit portion is simply replaced with a digital circuit having the same function as in the related art. In particular, the configuration can be made very simple and downsizing can be promoted. Also, the same
DSB modulation signal generated from the baseband signal and SSB modulation
Since the control signal is synthesized with the key signal,
The frequency spectrum of the eyes can be smoothed.

FIG. 4 shows a modification of the embodiment shown in FIG. That is, when the same parts as those in FIG. 2 are denoted by the same reference numerals, a multiplication circuit 1 for multiplying each output of the subtraction circuit 15 and the delay circuit 25 by a modulation index k.
7 and 26 are deleted, the outputs of the subtraction circuit 15 and the delay circuit 25 are supplied directly to the distribution circuit 19 and the addition circuit 27, and the baseband signal supplied to the input terminal 11 is multiplied by the multiplication circuit 32 with the modulation index k. Is multiplied to the distribution circuit 12, so that the same operation and effect as those of the embodiment shown in FIG. 2 can be obtained.

Here, the embodiment shown in FIG. 2 and FIG.
Although the case where fs> 2fc is satisfied has been described, FIG. 5 shows a modified example where this condition is not satisfied. In FIG. 5, the same parts as those in FIG. 4 are denoted by the same reference numerals, and the description will be made as follows: between the adding circuit 27 and the multiplying circuit 28, between the orthogonal transforming circuit 20 and the multiplying circuit 22, Between the interpolators 33 and 3 respectively
4, 35 are interposed. This interpolator 3
3, 34 and 35 have a function of converting the sampling frequency while keeping the spectrum of the input signal, and set the output sampling frequency fs1 of the interpolators 33, 34 and 35 so that fs1> 2fc. Then, the same operation as the embodiment shown in FIG. 2 can be performed.

FIG. 6 shows still another modification of the modification shown in FIG. 5, in which the outputs of the adder circuit 27, the orthogonal transformation circuit 20 and the delay circuit 21 are respectively connected to a D / A conversion circuit. The signal is converted into an analog signal via 36, 37, 38. Therefore, the D / A conversion circuit 3
The multiplication circuits 28, 22, 23 and the addition circuits 24, 29 after 6, 37, 38 are all constituted by analog circuits, and the D / A conversion circuit 30 at the subsequent stage of the addition circuit 29 is omitted. Also in this case, the multiplying circuits 28, 22, and 23 and the adding circuits 24 and 29 have the same function as in the case of the digital processing. A signal can be obtained.

The analog multiplication circuits 28, 22, 2
3 can be realized by a balanced modulation circuit, and the analog addition circuits 24 and 29 can be easily realized by a resistance synthesizing circuit or a transformer.

For this reason, when the carrier frequency fc is high, it can be realized with a simple configuration by adopting the circuit configuration shown in FIGS.

In each of the above-described embodiments, the high-frequency component is cut off from the original baseband signal to extract the low-frequency component, and the extracted low-frequency component is subtracted from the original baseband signal. Thus, a high-band component is obtained. However, in order to divide a baseband signal into a low band and a high band, the baseband signal may be passed through a low-pass filter and a high-pass filter, respectively.

It should be noted that the present invention is not limited to the above-described embodiment, and can be variously modified and implemented without departing from the scope of the invention.

[0035]

As described in detail above, according to the present invention,
V from the baseband signal digitized with a simple configuration
Very good digital VS capable of generating SB modulated signal
A B modulation device can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a block diagram showing an embodiment of the present invention.

FIG. 3 is a view for explaining the operation of the embodiment.

FIG. 4 is a block diagram showing a modification of the embodiment.

FIG. 5 is a block diagram showing a modification of the embodiment.

FIG. 6 is a block diagram showing a modification of the embodiment.

[Explanation of symbols]

11 input terminal, 12 distribution circuit, 13 LPF, 14
... Distribution circuit, 15 ... Subtraction circuit, 16 ... Delay circuit, 17 ...
Multiplication circuit, 18 ... SSB modulation circuit, 19 ... Distribution circuit, 2
0: orthogonal transformation circuit, 21: delay circuit, 22, 23: multiplication circuit, 24: addition circuit, 25: delay circuit, 26: multiplication circuit, 27: addition circuit, 28: multiplication circuit, 29: addition circuit, 30 ... D / A conversion circuit, 31 output terminal, 32 multiplication circuit, 33 to 35 interpolator, 36 to 38
D / A conversion circuit, 39 ... multiplication circuit.

──────────────────────────────────────────────────続 き Continued on the front page (72) Nobuyuki Miki 1 inventor, Komukai Toshiba-cho, Saiwai-ku, Kawasaki-shi, Kanagawa Prefecture (56) References JP-A-1-212108 (JP, A) JP-A-49-84712 (JP, A) JP-A-63-181509 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H03C 1/00-1/62

Claims (3)

(57) [Claims] 1. A separating means for separating a digitized baseband signal into a low-frequency component and a high-frequency component, and a low-frequency component of the baseband signal separated by the separating means is multiplied by a carrier signal to perform DSB modulation. a DSB modulation means for obtaining a signal, the high-frequency component of the baseband signal separated by said separating means
Orthogonal transformation processing that distributes two and shifts the phase by 90 degrees to one side
And a delay corresponding to the orthogonal transformation processing time
In the processed state, both signals are 90 ° out of phase with each other
Multiply each carrier signal having a difference, and the multiplication result
And SSB modulation means to obtain a SSB modulated signal by adding, characterized by comprising comprises a synthesizing means for obtaining synthesized by VSB modulated signal and an output of the output and the SSB modulation means of the DSB modulation means Digital VSB modulator. 2. A multiplication circuit for multiplying said carrier signal in said DSB modulation means and said SSB modulation means,
An interpolator for converting the sampling frequency of the digitized baseband signal is installed,
2. The digital VSB modulation apparatus according to claim 1, wherein the VSB modulation signal is generated even when the sampling frequency is lower than twice the carrier frequency. 3. A multiplication circuit for multiplying the carrier signal in the DSB modulation means and the SSB modulation means,
2. A digital VSB modulation apparatus according to claim 1, wherein D / A conversion means are provided, respectively, and said multiplication circuit and said synthesis means are constituted by analog circuits.