JPH08265183A - Audio demodulation circuit in satellite broadcast receiver - Google Patents

Abstract

PURPOSE: To obtain an output audio signal with high image quality both in modes A, B by using a 1-bit D/A converter so as to improve the distortion factor when an analog audio signal in the mode A is outputted. CONSTITUTION: In the case of the reception in the mode A, each moving contact (c) of switches SW1, SW2 of an LPF changeover section 10a is thrown to the position of a fixed contact (a) to select an LPF 11a by an A/B mode switching signal Sw and an Lch output audio signal from which a high frequency noise component of nearly 15kHz or over generated in the reception of the mode A is eliminated is sent through the moving contact (c) of the switch SW 2. In the case of the reception in the mode B, the switches SW1, SW2 select an LPF 11b and an Lch output audio signal from which a high frequency noise component of nearly 20kHz or over is eliminated is sent through the moving contact (c) of the switch SW2. An LPF changeover section 10b is operated similarly and an Rch output audio signal from which a high frequency noise component of nearly 15kHz or 20kHz or over is eliminated is sent.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an audio demodulation circuit in a satellite broadcast receiver for reducing a distortion rate when a demodulated audio signal is obtained by using a 1-bit D / A converter.

[0002]

2. Description of the Related Art FIG. 5 is a block diagram showing a configuration of a voice demodulation circuit in a conventional satellite broadcast receiver. In FIG. 5, F from the satellite broadcasting (BS) receiving second converter
M demodulated signal is passed, for example, center frequency 5.75
MHz band pass filter (BPF) 1 and BPF 1
4 phase demodulation (QPSK) for the FM demodulated signal from
Performs digital signal processing such as error correction and interleaving peculiar to BS, and Lch (left channel) and Rc
QPS that outputs an analog audio signal of h (right channel)
K demodulator 2, PCM decoder 3 and digital filter
A D / A converter 4 is provided. Further, an analog low-pass filter (LPF) 5 is provided which sends out the Lch output audio signal and the Rch output audio signal which have been converted into analog by removing unnecessary frequency components.

Next, the operation of this conventional example will be described. In FIG. 5, an FM demodulated signal from a satellite broadcast (BS) receiving second converter or the like is sent to QP through BPF1.
Input to the SK demodulation unit 2. The QPSK demodulator 2, the PCM decoder 3, and the digital filter / D / A converter 4 are used to perform four-phase demodulation (Q) on the FM demodulated signal from the BPF 1.
PSK), BS-specific error correction, digital signal processing such as interleaving, and Lch and Rch analog audio signals are output. Furthermore, analog LPF
5, the quantization noise is shifted to the high frequency band and the high frequency noise component is removed, as will be described later in detail.
The ch output audio signal and the Rch output audio signal are transmitted.

In such an operation, in order to improve the audio distortion rate, a 1-bit D /
The A converter 4 is used. Since the dynamic range of the 1-bit D / A converter 4 is insufficient for the audio frequency band as it is, the modulation circuit (noise shaper) shifts the quantization noise in the audio frequency band to a high frequency band.

FIG. 6 is a diagram showing a state in which the quantization noise in the voice frequency band is shifted to a high band, and FIG. 7 is a diagram showing BS.
It is a figure which shows the noise characteristic at the time of A and B mode reception in a broadcast. In FIGS. 6 and 7, the quantization noise is shifted to a high frequency band, and the high frequency band noise component is removed by the analog LPF 5 to obtain a distortion-free Lch output audio signal and Rch output audio signal.

The noise shaping characteristic of such a high frequency noise component changes depending on the transmission mode of BS broadcasting. That is, BS broadcasting has A mode (sampling frequency 32 KHz), B mode (sampling frequency 48 KHz).
KHz), and the noise shaping characteristics are different between these two transmission modes. In the case of the A mode, as shown in FIG. 7A, the high frequency noise component increases from approximately 15 KHz or higher. Further, in the B mode, the high frequency noise component increases from around 20 KHz as shown in FIG. 7B.

This high frequency noise component is removed through the analog LPF 5 shown in FIG. 5, but the frequency characteristic of the analog LPF 5 in this case corresponds to the B mode. That is, the cutoff frequency is 20 KHz. For this reason, the high frequency noise component of about 15 to 20 KHz that occurs during A mode reception cannot be removed. Such an analog LPF5
The high frequency noise component from is measured with a distortion meter.

FIG. 8 is a block diagram showing the measurement state of the high frequency noise component from the analog LPF 5. In FIG. 8, this example shows that the Lch output audio signal and the Rch output audio signal from the analog LPF 5 have a cutoff frequency of 20 KH.
It is input to the distortion rate meter 7 through the LPF 6 of z, and the total harmonic distortion (THD) and the high frequency noise component are added and measured. The high frequency noise component in this case is mainly the distortion of the 1-bit digital filter / D / A converter 4 (quantization noise shifted to the high frequency range), and in particular, the high frequency noise component shifted by the noise shaper is The distortion rate measured by the distortion meter 7 is determined depending on whether or not the analog LPF 5 can remove the distortion.

As a result, in the conventional satellite broadcasting receiver, a high frequency noise component of about 15 to 20 KHz remains in the Lch output audio signal and the Rch output audio signal from the analog LPF 5 when receiving the A mode of the BS broadcast. It will be.

[0010]

As described above, in the audio demodulation circuit in the satellite broadcast receiver of the above-mentioned conventional example, the 1-bit D / A converter is used and the analog LPF having the cutoff frequency corresponding to the B mode is used. To obtain output (demodulated) audio signal through 15 to 20 KHz when receiving in A mode
However, the noise cannot be removed, and the distortion rate of the output audio signal deteriorates.

The present invention solves the above-mentioned drawbacks in the prior art, and the distortion factor when outputting an A mode output (demodulated) audio signal using a 1-bit D / A converter is described. It is an object of the present invention to provide an audio demodulation circuit in a satellite broadcast receiver which can reduce the output audio signal of high quality in both A mode and B mode.

[0012]

In order to achieve the above object, the invention according to claim 1 eliminates quantization noise when D / A conversion is performed on 1-bit audio data in a plurality of reception modes in satellite broadcasting. In a voice demodulation circuit in a satellite broadcasting receiver that shifts to a high frequency and demodulates, a high frequency noise component is removed from a D / A converted analog demodulated voice signal with frequency characteristics corresponding to each of a plurality of reception modes. This is a configuration that includes a filter that allows the light to pass through.

According to a second aspect of the present invention, there is provided an audio demodulation circuit for a satellite broadcast receiver, wherein a plurality of cut-off frequency filters for removing high frequency noise components in demodulated audio signals in a plurality of reception modes are provided, and a plurality of the filters are provided. The configuration is such that selection means for selecting one of the reception modes is provided, and the high frequency noise component for each reception mode is removed by the filter selected by the selection means.

According to a third aspect of the present invention, there is provided an audio demodulation circuit for a satellite broadcast receiver, wherein a plurality of filters having a cutoff frequency for removing high frequency noise components in demodulated audio signals of a plurality of reception modes and a plurality of the filters are provided. Selective means for selecting one or filters to be connected in series is provided, and the high frequency noise component for each reception mode is removed by one or the filters connected in series selected by the selecting means.

The audio demodulation circuit in the satellite broadcasting receiver according to claim 4 is configured to use an analog switch or a semiconductor switch circuit as the selecting means.

According to a fifth aspect of the present invention, there is provided an audio demodulation circuit for a satellite broadcasting receiver, wherein the semiconductor switch circuit is based on a first switching element for setting an input / output terminal of a filter to conductive or non-conductive, and a plurality of receiving modes. And a second switching element that sets the switching element to conductive or non-conductive.

[0017]

According to the audio demodulation circuit of the satellite broadcast receiver of the present invention having the above-mentioned structure, the high-frequency noise component (mainly the high-frequency-shifted 1 One of a plurality of filters having a cutoff frequency for removing (quantization noise due to bit D / A conversion) is selected by an analog switch or a semiconductor switch circuit in accordance with the reception mode. Then, the demodulated audio signal from which the high frequency noise component has been removed is output through the selected filter.

For example, in A mode (sampling frequency 32 KHz) in BS broadcasting, in the case of 1-bit D / A conversion, the dynamic range for the audio frequency band is insufficient, so that the quantization noise is increased to a high frequency band by the modulation circuit (noise shaper). Have shifted to. This shift increases the high frequency noise component of approximately 15 KHz or more,
An LPF having a cutoff frequency of about 15 KHz is used and selected to output a demodulated audio signal from which a high frequency noise component of about 15 KHz or higher is removed.

In addition, B mode (sampling frequency 48
KHz), the high frequency noise component of about 20 KHz or more increases due to the shift, so that the cutoff frequency of 20 KHz is L
A demodulated audio signal from which a high frequency noise component of approximately 20 KHz or higher has been removed is output by using and selecting the PF.

As a result, the high-frequency noise component at the time of A-mode reception was removed by the LPF having a cut-off frequency of 20 KHz corresponding to the B-mode, while using the 1-bit D / A converter. The LPF with a cut-off frequency of 15 KHz dedicated to this mode
Since it is removed in step 1, the distortion rate of the output audio signal (demodulated audio signal) is reduced, and good audio reception is possible.

The audio demodulation circuit in the satellite broadcast receiver according to any one of claims 1, 3, 4 and 5 is one of a plurality of filters having a cutoff frequency for removing a high frequency noise component in each of a plurality of reception modes. The filter is selected by an analog switch or a semiconductor switch circuit so as to be connected in series. Then, the demodulated audio signal from which the high frequency noise component has been removed is output through the selected filter.

For example, a high frequency noise component at the time of B mode reception is removed by an LPF having a cutoff frequency of 20 KHz. Also,
High-frequency noise component of A-mode reception has a cut-off frequency of 15 kHz
And the LPF having a cutoff frequency of 20 KHz are connected in series and removed, and the high frequency noise component thereof is further reduced.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of an audio demodulation circuit in a satellite broadcast receiver of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing a main configuration of a first embodiment of an audio demodulation circuit in a satellite broadcast receiver according to the present invention. In FIG. 1, this first embodiment corresponds to the analog low-pass filter (LPF) 5 shown in FIG. Less than,
The BPF 1, the QPSK demodulation unit 2, the PCM decoder 3, and the digital filter / D / A converter 4 shown in FIG.

In FIG. 1, to the analog LPF 10, the L audio signals of the Lch and the Rch from the digital filter / D / A converter 4 are input.
It has PF switching sections 10a and 10b. In addition, LPF
The configuration of the switching unit 10b is omitted in the figure, but L
It has the same configuration as the PF switching unit 10a.

The LPF switching unit 10a (10b) is a switch SW1 in which the Lch (Rch) analog audio signal from the digital filter / D / A converter 4 is supplied to the movable contact c.
Is connected to one fixed contact a of the switch SW1,
When receiving A mode in S broadcasting, the high frequency noise component (mainly 1-bit digital filter
An LPF 11a having a cutoff frequency of 15 KHz for removing the quantization noise generated by the D / A converter 4 is provided.

Further, it is connected to one fixed contact b of the switch SW1 and, when receiving B mode in BS broadcasting, its high frequency noise component (mainly a 1-bit digital filter / D / A converter 4 shifted to high frequency). Quantization noise)
LPF 11b with a cut-off frequency of 20 KHz for eliminating
Lch or R from either PF11a or 11b
A switch SW2 is provided for selecting the fixed contacts a and b to which the ch output audio signal (demodulated audio signal) is supplied by the movable contact c and sending out one of the signals. In addition,
The movable contacts c of the switches SW1 and SW2 are simultaneously switched by the A / B mode switching signal Sw.

FIG. 2 is a circuit diagram showing in detail the configuration shown in FIG. In FIG. 2, this example shows the switch S in FIG.
Semiconductor switches are used as W1 and SW2. LPF switching unit 10a (10b) of this analog LPF 10
Is an LPF 11a with a cut-off frequency of 15 kHz for A mode.
FETs Q1a and Q1b (semiconductor switching elements) for turning on / off (conducting / non-conducting) the input / output terminals of the source and drain are provided.

Further, the cutoff frequency for the B mode is 20 KH.
FETs Q2a and Q2b for turning on / off (conducting / non-conducting) the input / output terminal of the LPF 11b of z between the source and the drain,
When the A mode is received, the low (L) level A / B mode switching signal Sw is input to the base and turned off (non-conducting), and the high (H) level A / B mode switching signal Sw is received when the B mode is received. And a transistor Q3 that is turned on (conducts) by being input to.

The two gates of the FETs Q2a and Q2b are connected to each other through a resistor R, and A /
It is connected to the supply line of the B mode switching signal Sw.
Further, the gates of the FETs Q1a and Q1b are also connected to each other by the resistor R and are also connected to the collector of the transistor Q3. This collector is connected to the power supply line (Vcc) through the resistor R.

Next, the operation of the first embodiment will be described. In FIG. 1, the LPF switching units 10a and 10b of the analog LPF 10 are supplied with the respective analog audio signals of Lch and Rch from the digital filter / D / A converter 4. The operation up to the digital filter / D / A converter 4 is the same as that described above with reference to FIG.

When the A mode is received, the switch SW1 of the LPF switching section 10a is switched by the A / B mode switching signal Sw.
The movable contact c of SW2 selects the fixed contact a and switches to select the LPF 11a. The LPF 11a has a cutoff frequency of 15 KHz, and sends out the Lch output audio signal from which the high frequency noise component of about 15 KHz or more shown in FIG.

When the B mode is received, the switch SW1 of the LPF switching section 10a is switched by the A / B mode switching signal Sw.
The movable contact c of SW2 switches to the fixed contact b, and the LPF
Select 11b. This LPF 11b has a cutoff frequency of 20.
It is KHz, and it occurs when receiving B mode.
The Lch output audio signal from which the high frequency noise component of approximately 20 KHz or more shown in (b) is removed is set as the movable contact c of the switch SW2.
Sent from.

The LPF switching section 10b operates in the same manner to remove a high frequency noise component of approximately 15 KHz or more from the Rch analog audio signal from the digital filter / D / A converter 4 at the time of A mode reception, and the B mode. 2 when receiving
The Rch output audio signal from which the high frequency noise component of 0 KHz or higher is removed is transmitted.

In FIG. 2, this configuration also operates in the same manner as in FIG. 1, but at the time of A mode reception, a low (L) level A /
The B-mode switching signal Sw is input. At this low level, the transistor Q3 is turned off (non-conductive). The collector of the transistor Q3 becomes the voltage of the power supply line (Vcc), and this high voltage is applied to the gates of the FETs Q1a and Q1b through the resistor R, and the FET Q1
a and Q1b are turned on (conduction between the source and drain).
The gates of the FETs Q2a and Q2b are low level and are off (non-conducting).

As a result, the LPF 11a is selected, and Lc from the digital filter / D / A converter 4 is selected.
h The analog voice signal is LPF11 through FETQ1a.
Input to a. The LPF 11a removes the high-frequency noise component of about 15 KHz or more shown in FIG. 7A, which is generated when the A mode is received, and outputs the Lch output audio signal through the FET Q1b.

At the time of receiving the B mode, the high level A / B mode switching signal Sw is input. At this high level, the transistor Q3 turns on (conducts), and the FETs Q1a and Q1
Each gate of 1b is grounded through the resistor R and turned off (non-conductive). Further, the FETs Q2a and Q2b are turned on (conduction between the source and drain).

As a result, the LPF 11b is selected, and Lc from the digital filter / D / A converter 4 is selected.
h The analog voice signal is passed through the FET Q2a to LPF11.
Input to b. The LPF 11b removes the high-frequency noise component of about 20 KHz or more shown in FIG. 7 (b), which occurs when the B mode is received, and outputs the Lch output audio signal through the FET Q2b.

The LPF switching section 10b shown in FIG. 2 operates in the same manner, and the digital filter D
15 from the Rch analog audio signal from the A / A converter 4
Rc that removes high-frequency noise components above KHz and removes high-frequency noise components above approximately 20 KHz during B mode reception
h Output audio signal.

As described above, in the first embodiment,
While the 1-bit D / A converter was used to remove the high-frequency noise component in the A-mode analog audio signal through the analog LPF with a cut-off frequency of 20 KHz corresponding to the B-mode, when receiving the A-mode. Since the high-frequency noise component (approximately 15 to 20 KHz) of the analog audio signal of is removed by the LPF 11a with a cut-off frequency of 15 KHz dedicated to this mode, the distortion rate of the output audio signal (demodulated audio signal) is reduced and is good. It is possible to receive various voices.

FIG. 3 is a block diagram showing a main configuration of a second embodiment of the audio demodulation circuit in the satellite broadcast receiver of the present invention. In FIG. 3, this second embodiment also corresponds to the analog low-pass filter (LPF) 5 shown in FIG. In FIG. 1, the analog LPF 10 has
Lch and R from the digital filter / D / A converter 4
LPF to which each analog audio signal of ch is input
It has switching parts 10a and 10b. The LPF switching unit 10b has the same configuration as the LPF switching unit 10a, although its configuration is omitted in the figure.

The LPF switching unit 10a (10b) is a switch S for supplying the Lch (Rch) analog audio signal from the digital filter / D / A converter 4 to the movable contact c.
An LPF having a cut-off frequency of 15 KHz, which is connected to W11 and one fixed contact b of the switch SW11 and removes a high frequency noise component at the time of A mode reception in BS broadcasting.
21a are provided.

Further, the output signal of the LPF 21a (Lch or Rch output audio signal) is supplied to the fixed contact b, and the fixed contact a of the switch SW11 and the switch SW12 for connecting the fixed contact a of the switch to each other. An LPF 21b having a cut-off frequency of 20 KHz, which is connected to the movable contact c of the switch SW12 and removes the high frequency noise component when the B mode is received in BS broadcasting, is provided.
The movable contact c of the switches SW11 and SW12 is A
The signals are switched simultaneously by the / B mode switching signal Sw.

FIG. 4 is a circuit diagram showing in detail the configuration shown in FIG. In FIG. 4, this example is the switch S in FIG.
Semiconductor switches are used as W11 and SW12.
The LPF switching unit 10a (10 of this analog LPF 10
b) LPF2 with a cut-off frequency of 15 KHz for A mode
FETs Q5a and Q5b for turning on / off (conducting / non-conducting) the input / output terminal of the buffer 1a and the buffer 1a between the source and the drain.
And LPF 21b with a cut-off frequency of 20 KHz for B mode
FET Q4 for turning on / off (conducting / non-conducting) the input end of the source and drain between the source and drain, and turning off (non-conducting) when a low (L) level A / B mode switching signal Sw is input to the base. It has a transistor Q3 which conducts and turns on (conducts) when the high (H) level A / B mode switching signal Sw at the time of receiving the B mode is input to the base.

The FET Q4 receives the A / B mode switching signal Sw through the resistor R. In addition, FET Q5a,
The respective gates of Q5b are connected by a resistor R and are also connected to the collector of the transistor Q3. This collector is connected to the power supply line (Vcc) through the resistor R.

Next, the operation of the second embodiment will be described. In FIG. 3, the LPF switching units 10 a and 10 b of the analog LPF 10 are supplied with the Lch and Rch analog audio signals from the digital filter / D / A converter 4. Also in this case, digital filter D /
The operation up to the A converter 4 is the same as that described with reference to FIG.

When receiving the A mode, the switch SW1 of the LPF switching section 10a is changed by the A / B mode switching signal Sw.
1, the movable contact c of SW12 is switched to the fixed contact b to select the LPF 21a. The LPF 21a has a cutoff frequency of 15 KHz, and sends out the Lch output audio signal from which the high frequency noise component of about 15 KHz or more shown in FIG. The Lch output audio signal from the movable contact c is input to the LPF 21b, and from there, about 20 KHz
The Lch output audio signal from which the above high frequency noise components have been removed is output.

That is, during A mode reception, the Lch analog audio signal from the digital filter / D / A converter 4 passes through two LPFs 21a and 21b and is output.

When receiving the B mode, the switch SW1 of the LPF switching section 10a is changed by the A / B mode switching signal Sw.
1, the movable contact c of SW12 selects the fixed contact a. Therefore, L from the digital filter / D / A converter 4
The ch analog audio signal is directly input to the LPF 21b. The LPF 21b has a cut-off frequency of 20 KHz, and outputs the Lch output audio signal from which the high frequency noise component of about 20 KHz or more shown in FIG.

The LPF switching section 10b operates in the same manner to remove a high frequency noise component of approximately 15 KHz or more from the Rch analog audio signal from the digital filter / D / A converter 4 when receiving the A mode, and the B mode. 2 when receiving
The Rch output audio signal from which the high frequency noise component of 0 KHz or higher is removed is transmitted.

In FIG. 4, this configuration also operates in the same manner as in FIG. 3, but at the time of A mode reception, A / A of low (L) level is used.
The B-mode switching signal Sw is input. At this low level, the transistor Q3 is turned off (non-conductive). The collector of the transistor Q3 becomes the voltage of the power supply line (Vcc), and this high voltage is applied to the gates of the FETs Q5a and Q5b through the resistor R, and the FET Q5
a and Q5b are turned on (conduction between the source and drain).
The FET Q4 has its gate at a low level and is off (non-conducting).

As a result, the LPF 21a is selected, and Lc from the digital filter / D / A converter 4 is selected.
h The analog voice signal is passed through the FET Q5a to LPF21.
Input to a. The LPF 21a removes the high-frequency noise component of about 15 KHz or more shown in FIG. 7A, which is generated at the time of A mode reception, and outputs the Lch output audio signal to the LPF 21b through the buffer 7 and the FET Q5b. L from which high-frequency noise components of approximately 20 KHz or more are removed
ch Output audio signal.

That is, when receiving in the A mode, the Lch analog audio signal from the digital filter / D / A converter 4 passes through two LPFs 21a and 21b and is output.

When receiving the B mode, the high level A / B mode switching signal Sw is input. At this high level, the transistor Q3 is turned on (conduction), and the FETs Q5a, Q
The gate of 5b is grounded through the resistor R and turned off (non-conductive). Further, the FET Q4 is turned on (conduction between the source and drain). As a result, the digital filter D /
The Lch analog audio signal from the A converter 4 is FETQ4.
Is input to the LPF 21b through. In the LPF 21b, about 2 as shown in FIG.
The Lch output audio signal from which the high frequency noise component of 0 KHz or higher is removed is output.

The LPF switching unit 10b in FIG. 4 operates in the same manner, and the digital filter D
15 from the Rch analog audio signal from the A / A converter 4
The high frequency noise component of KHz or higher is removed, and when receiving the B mode, the Rch output audio signal from which the high frequency noise component from around 20 KHz is removed is transmitted.

As described above, in the second embodiment, similarly to the first embodiment, the high frequency noise component at the time of receiving the B mode is removed by the LPF 21b having the cutoff frequency of 20 KHz. further,
High-frequency noise component of A-mode reception has a cut-off frequency of 15 kHz
LPF21a and the LPF21 with a cutoff frequency of 20 KHz
b), and the high frequency noise component is further reduced.

[0056]

As is apparent from the above description, according to the audio demodulation circuit in the satellite broadcast receiver of the first, second, fourth, and fifth aspects, the high-frequency noise component of each of the plurality of reception modes is detected. Since one of a plurality of filters having a cutoff frequency to be removed is selected according to the reception mode and a demodulated audio signal from which a high frequency noise component has been removed is output through the selected filter, conventionally, a 1-bit D / D While the high frequency noise component at the time of A mode reception was removed through the LPF of only the cutoff frequency corresponding to the B mode using the A converter, the high frequency noise component at the time of A mode reception was dedicated to this mode. Since it is removed by the LPF having the cut-off frequency, there is an effect that the distortion rate of the output audio signal is lowered, and good audio reception becomes possible.

According to the audio demodulation circuit in the satellite broadcast receiver of the first, third, fourth and fifth aspects, one of a plurality of filters having a cutoff frequency for removing a high frequency noise component in each of a plurality of reception modes is provided. Filters are selected so that they are connected in series or in series, and the demodulated audio signal from which the high-frequency noise component has been removed is output through this selected filter. Therefore, the high-frequency noise component during B-mode reception is the LPF of this cutoff frequency.
And the high frequency noise component of A mode reception is
The LPF having the cutoff frequency and the LPF having the cutoff frequency for the B mode are connected in series to be removed, and the high frequency noise component can be further reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing a main configuration of a first embodiment of an audio demodulation circuit in a satellite broadcast receiver of the present invention.

FIG. 2 is a circuit diagram showing in detail the configuration shown in FIG.

FIG. 3 is a block diagram showing a main configuration of a second embodiment of an audio demodulation circuit in a satellite broadcast receiver of the present invention.

FIG. 4 is a circuit diagram showing in detail the configuration shown in FIG.

FIG. 5 is a block diagram showing a configuration of an audio demodulation circuit in a conventional satellite broadcast receiver.

FIG. 6 is a diagram showing a state in which a quantized high band noise component in a voice frequency band is shifted to a high band in a conventional example.

FIG. 7A is a characteristic diagram of a high frequency noise component during A mode reception in BS broadcasting. (B) is a characteristic diagram of a high frequency noise component at the time of B mode reception in BS broadcasting.

FIG. 8 is a block diagram showing a measurement state of a high frequency noise component from an analog LPF in a conventional example.

[Description of Reference Signs] 4 Digital filter / D / A converter 10 Analog LPF 10a, 10b LPF switching unit 11a, 11b, 21a, 21b LPF FET Q1a, Q1b, Q2a, Q2b, Q4, Q5
a, Q5b Q3 transistor SW1, SW2, SW11, SW12 switch

Claims (5)

[Claims] 1. A voice demodulation circuit in a satellite broadcast receiver for shifting to a high frequency band and demodulating quantization noise when D / A converting voice data of a plurality of reception modes in satellite broadcast by 1 bit, A voice in a satellite broadcast receiver, comprising a filter for removing the high-frequency noise component and passing the D / A-converted analog demodulated voice signal with frequency characteristics corresponding to each of the plurality of reception modes. Demodulation circuit. 2. A plurality of filters having a cut-off frequency for removing a high frequency noise component in each demodulated voice signal of a plurality of reception modes, and a selection means for selecting one of the plurality of filters in accordance with the reception mode. 3. The audio demodulation circuit in a satellite broadcast receiver according to claim 1, wherein the high-frequency noise component for each reception mode is removed by the filter selected by the selection means. 3. A plurality of filters having a cutoff frequency for removing a high frequency noise component in each demodulated audio signal in a plurality of reception modes, and one of the plurality of filters or the filters are selected to be connected in series. 2. The audio demodulation circuit in a satellite broadcast receiver according to claim 1, wherein selection means is provided, and the high frequency noise component for each reception mode is removed by one selected by the selection means or a filter connected in series. 4. The audio demodulation circuit in a satellite broadcast receiver according to claim 2, wherein an analog switch or a semiconductor switch circuit is used as the selection means. 5. The semiconductor switch circuit includes a first switching element that sets an input / output end of the filter to be conductive or non-conductive, and a first switching element that sets the switching element to be conductive or non-conductive based on a plurality of reception modes. An audio demodulation circuit in a satellite broadcast receiver according to claim 4, further comprising: two switching elements.