JPH0369287A - Demodulator - Google Patents

Abstract

PURPOSE:To prevent the frequency characteristics of an SAP output signal from being degraded in a weak electric field and to suppress the increment of a noise component by controlling the band of a band variable low pass filter(LPF) connected to a SAP demodulating circuit by the DC output of a noise rectifying and smoothing circuit. CONSTITUTION:A noise component included in a composite signal is allowed to pass a noise BPF 14 and converted into a DC signal by the rectifying/ smoothing circuit 15. The frequency characteristics of the band variable LPF 13 are controlled by an DC output level. When the noise component is high, the interruption frequency of the LPF 13 is set up to a low level to prevent a high frequency noise component from being inputted to a dbx decoder 10. Thereby, the occurrence of malfunction in the variable deemphasis of the dbx decoder 10 can be prevented and the frequency characteristics of the SAP signal in the vicinity of 1kHz can be prevented from being degraded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a demodulator, and more particularly to a demodulator for TV audio multiplex signals.

[Conventional technology]

Conventionally, the audio multiplex signal system for TV broadcasting has been used for monaural broadcasting, stereo broadcasting, and
Among the bilingual broadcasts, there is a method in which stereo broadcasting and bilingual broadcasting are received simultaneously, and a receiving method, which is used in Japan and other countries, in which either bilingual broadcasting or stereo broadcasting is selected.

In particular, noise reduction becomes important in the demodulation of TV audio multiplexed signals, which is carried out in the United States, because two signals are received simultaneously.

In other words, the conventional American TV audio multiplex signal consists of an L (left channel), an R (right channel) signal, a by-ot signal fu (15,734 KHz), and an L-R signal.
The modulated 2 fH (31, 4, 68 KHz) signal and the second audio (S A P: 5econdAudio
5 fH (7
8,67KH2) signal.

Further, both the LR signal and the SAP signal are encoded by a noise reduction system consisting of a dbx decoder.

Hereinafter, a demodulator for US TV audio multiplexed signals will be described with reference to the drawings.

FIG. 3 is a block diagram of a conventional TV audio multiplex signal demodulator.

As shown in FIG. 3, the conventional US TV audio multiplex signal demodulator includes an SAP removal LPF 2 and a 5fB bandpass filter 11 connected to a composite input terminal (composite input) 1, and a 5A-P removal LPF 2. Connected L-R
Demodulation circuit 5, low-pass filters 3 and 6, SAP demodulation circuit 12 and LPF 16, and a changeover switch 9 that switches the output of LPF 16 through LPF 6 and the SAP signal.
and dbx that encodes the output of this changeover switch 9.
It has a decoder 10 and a matrix 4, and outputs demodulated audio multiplexed signals from a left channel (X) terminal 7 and a right channel (R) terminal 8.

In the conventional TV audio multiplexing demodulator described above, when the electric field strength of the received signal is weak, the frequency characteristics of the demodulated SAP signal are
There is a problem in that the problem worsens in the vicinity of Hz, and high-frequency noise increases, giving audible discomfort.

FIG. 4 is a frequency characteristic diagram of the SAP demodulated signal in the demodulator shown in FIG. 3.

As shown in Figure 4, when receiving a strong electric field signal, the characteristic A'
As shown in the figure, there is no deterioration of the characteristics at the SAP signal frequency IKHz, but when receiving a weak electric field signal, the IK
Characteristic deterioration occurs at Hz.

When the electric field strength of such a received signal is weak, the S/N ratio of the modulated signal input to the TV audio multiplexing demodulator is poor, and the demodulated SAP signal also contains many noise components.

Next, the dbx decoder used in this US TV audio multiplex broadcasting system will be explained.

FIG. 5 is a block diagram of the dbx decoder shown in FIG. 3.

As shown in FIG. 5, the signal supplied to the dbx decoder 10 is filtered through a wideband filter 17 that passes all bands connected in parallel to the input terminal, a spectrum chill filter 19 that passes only high frequency signals, and a first fixed filter. and de-emphasis 21, respectively. The signals that have passed through the wideband filter 17 and the spectral filter 19 are converted into DC by the effective value detection circuits 18 and 20, respectively, and the DC-converted control signals 25A, 25
B controls the gain and frequency characteristics of the variable gain amplifier 23 and variable de-emphasis 22. Further, the output of the variable gain amplifier 23 is outputted via a second fixed difference amplifier 24.

6 is a frequency characteristic diagram of the spectral filter shown in FIG. 5, and FIG. 7 is a frequency characteristic diagram of the variable de-emphasis shown in FIG. 5.

As shown in FIG.
It has a function of passing only high-frequency signals of Hz or higher, and sends its output to the effective value detection circuit 20.

Further, as shown in FIG. 7, the variable de-emphasis 22 is controlled by the output value 25B of the effective value detection circuit 20 (
Changes like (), (b), (c). That is, the magnitude of the output of the effective value detection circuit 20.

The output of the variable de-emphasis 22 changes as shown in (A), (B), and (C) depending on whether it is medium or small.

Next, the circuit operation of the conventional demodulator described above will be explained in detail. In particular, as described above, when receiving a weak electric field signal, the demodulated SAP signal contains many noise components. Therefore, in the following, the frequency of the demodulated SAP signal is (1)
If it is sufficiently lower than I K Hz, (2) I K
A case near Hz and a case sufficiently higher than 30 KHz will be described.

(1) When the demodulated SAP signal is sufficiently lower than I KHz.

In this case, since the SAP signal cannot pass through the spectral filter 19 of the dbx decoder IO, only the high-frequency noise component passes through the spectral filter 19 and is converted to DC by the effective value detection circuit 20, so the variable de-emphasis 22 make it work.

However, when the SAP signal is sufficiently lower than IK Hz, the output characteristic of the variable de-emphasis 22 is the characteristic shown in (b) in FIG. , SA after decoding
P output is constant regardless of the presence or absence of noise.

(2) When the demodulated SAP signal is around 1KHz.

Similar to the above-mentioned item (1), since the SAP signal cannot pass through the spectral filter 19, only the high-frequency noise component passes through the spectral filter 19, is converted to DC by the effective value detection circuit 20, and is similarly variable. The de-emphasis 22 is operated. Such a SAP signal is I KHz
In the case where the output characteristics are close to 100, the output characteristics of the variable de-emphasis 22 are in a region that changes depending on the output value of the effective value detection circuit 20, so if there is no noise or there is very little noise, the state of the characteristic (c) in FIG. 7 is reached. On the other hand, when there is noise, the state is as shown in characteristic (b) in FIG. As a result, the decoded SAP signal has higher characteristics than its original characteristics.

(3) When the demodulated SAP signal is sufficiently higher than 1 KHz.

In this case, both the SAP signal and the noise can pass through the spectral filter 19, and since the noise has a lower level than the SAP signal, it is hidden in the signal, and the output of the effective value detection circuit 2o is determined by the level of the SAP signal.

Therefore, the gain of the variable de-emphasis 22 is determined by the level of the SAP signal regardless of the presence or absence of noise, resulting in the state of characteristic (a) in FIG. Therefore, SAP after decoding
The output is constant regardless of the presence or absence of noise.

[Problem to be solved by the invention]

The above-described conventional demodulator has the disadvantage that when the demodulated SAP signal contains a lot of noise, the frequency characteristics of the decoded SAP signal deteriorate around IKHz and the noise component increases.

An object of the present invention is to provide a demodulator that improves such frequency characteristics and prevents increase in noise components.

[Means to solve the problem]

The demodulator of the present invention is connected to a modulation signal input terminal and has a noise bandpass filter having a pass band in one region where no modulation signal exists and noise exists, and an output terminal of the noise bandpass filter. a rectifying and smoothing circuit that rectifies the DC output signal according to the output signal of the noise bandpass filter; a demodulation circuit that is connected to the modulation signal input terminal directly or via a filter and outputs the demodulated signal; and the demodulation circuit that outputs the demodulated signal. A variable band low-pass filter connected to the circuit and having the DC output of the rectifying and smoothing circuit as a control input, a switch for switching the output of the variable band low-pass filter and a stereo broadcast output, and a dbx decoder connected to the switch. and is configured to realize noise reduction in the dbx decoder by controlling the variable band low-pass filter.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of a demodulator showing one embodiment of the present invention.

As shown in FIG. 1, this embodiment is an example of a demodulator that demodulates a TV audio multiplexed signal, and moreover, is an example of a demodulator that simultaneously receives stereo broadcasts and bilingual broadcasts.

First, in receiving stereo broadcasting, an SAP removal LPF 2 connected to a conduit input terminal l into which a modulated signal is input, a left-right channel (LR) demodulation circuit 5 connected to this LPF 2, LPF3 and 6
and a matrix 4 that mixes the signals from the LPF 3 and the dbx decoder 10, and the output thereof is sent to the L output terminal 7.
and R blade terminal 8.

On the other hand, when receiving bilingual broadcasting, the modulated signal is input through the composite input terminal l of the 5fHBPF.
11 and noise BPF14 and their respective BPFII
, 14, and a variable band low-pass filter 13 in which the frequency characteristics of the SAPA signal from the SAPA adjustment circuit 12 are controlled by the DC output of the rectification and smoothing circuit 15. ing.

The signal output of the variable band low-pass filter 13 and the output of the LPF 6 are switched by the LR/SAP switching circuit 9 and sent to the dbx decoder 10 to control the matrix 4.

FIG. 2 is a frequency characteristic diagram of the variable band LPF shown in FIG. 1.

As shown in Fig. 2, this variable band LPF 13 exhibits characteristic A when receiving a strong electric field signal, and characteristic B when receiving a weak electric field signal, and although it changes in the high frequency SAPA signal, the signal remains constant around s I K Hz. It remains constant regardless of the strength of the

Next, the operation of this embodiment will be explained with reference to FIGS. 1 and 2.

First, the noise component included in the composite signal passes through the noise BPF 14 and is converted into DC by the rectification and smoothing circuit 15. However, the frequency characteristics of the variable band low-pass filter 13 are controlled by the DC output level of the rectifier and smoothing circuit 15.

Here, when the noise component is large, the cutoff frequency of the variable band low-pass filter 13 is set low to prevent the high-frequency noise component from being input to the dbx decoder 110. Therefore, the malfunction of the variable de-emphasis (22) of the dbx decoder/P10 is prevented, and the IK of the SAPA
Deterioration of frequency characteristics near Hz can be prevented.

Furthermore, since high-frequency noise components are attenuated, auditory discomfort is also reduced.

On the other hand, when the noise component is small, the cutoff frequency of the variable band low-pass filter 13 is set high, and the frequency characteristics can be extended to a high frequency range, as shown in FIG.

〔Effect of the invention〕

As explained above, in the demodulator of the present invention, a variable band LPF is connected after the SAPA modulation circuit, and a noise bandpass filter and a noise rectification and smoothing circuit are connected in series to the input terminal, while the noise rectification By controlling the band of the variable band low-pass filter connected to the SAPA adjustment circuit using the DC output of the smoothing circuit, SAPA in weak electric fields can be improved.
This has the effect of preventing the deterioration of the frequency characteristics of the force signal and the increase in noise components.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a demodulator showing an embodiment of the present invention;
Figure 2 is a frequency characteristic diagram of the variable band LPF shown in Figure 1;
Fig. 3 is a block diagram of a TV audio multiplex signal demodulator showing an example of the conventional technology, and Fig. 4 shows the SAP in the demodulator shown in Fig. 3.
Decoded signal frequency characteristic diagram Figure 5 is a block diagram of the dbx decoder l shown in Figure 3, Figure 6 is a frequency characteristic diagram of the spectral filter shown in Figure 5, and Figure 7 is a diagram of the variable de-emphasis shown in Figure 5. FIG. 1... Composite input (modulation input), 2...
...SAP removal LPF, 3,6...LPF
, 4... Matrix, 5... L-R demodulation circuit, 7... L output, 8... R output, 9... L-R/SAP selector switch, 10.
...dbx-i" coder, J, 11...
5f. BPF, 12... SAPA adjustment circuit, 13...
... Bandwidth variable LPF, 14... Noise BPF
, 15... Rectifier smoothing circuit.

Claims (1)

[Claims] a noise bandpass filter connected to the modulation signal input terminal and having a passband in a band where no modulation signal is present and where noise is present; a rectifying and smoothing circuit that rectifies a DC output according to an output signal; a demodulating circuit that is connected to the modulation signal input terminal directly or via a filter and outputs a demodulated signal; and a rectifying and smoothing circuit that is connected to the demodulating circuit and that a variable band low-pass filter that uses the DC output of the circuit as a control input; a switch that switches between the output of the variable band low-pass filter and a stereo broadcast output; and a dbx connected to the switch.
A demodulator comprising: a decoder, the demodulator realizing noise reduction in the dbx decoder by controlling the variable band low-pass filter.