JPH10336142A - Receiver for digital audio broadcast - Google Patents

Abstract

PROBLEM TO BE SOLVED: To smoothly perform automatic gain control in a receiver for digital audio broadcast. SOLUTION: A receiver which performs digital modulation of multi carrier and receives digital audio broadcast in each frame having no-modulation null symbol at its head is provided with; automatic gain control parts 2, 3 and 21 which control amplification gain of a received signal based on electric field strength information that is acquired after converting the received signal into a base data signal; and a receiving level fluctuation detecting part 10 which performs envelope-detection of a received signal in one frame section, detects fluctuations of an envelope-detected signal and reduces response time constant of amplification gain in the automatic gain control parts when detected fluctuations are large.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital audio broadcasting (DAB) receiving apparatus, and more particularly to a receiving apparatus capable of smoothly performing automatic gain control (AGC).

[0002]

2. Description of the Related Art Orthogonal frequency division modulation (OFDM) has been adopted as digital audio broadcasting. O
FDM is a multi-carrier modulation method, and a transmission signal is obtained by adding a large number of digitally modulated waves. As a modulation method of each digital modulation wave, for example, QPSK (Quad
rature Phase Shift Keying). Also,
OFDM reception modes include modes I, II, and III depending on the broadcast mode.

FIG. 12 is a diagram for explaining a data frame relating to digital audio broadcasting. As shown in the figure, the frame is a null symbol, a phase reference, and a FI.
C (Fast Information Channel) channel, MSC
(Main Service Channel). The null symbol is unmodulated, the phase reference is for reference data transmission, the FIC channel is mainly for control data transmission, and the MSC channel is for audio and data transmission.

[0004]

By the way, automatic gain control is adopted in a digital audio broadcast receiving apparatus. Since digital audio broadcasting employs a multi-carrier modulation method, the baseband signal has a form similar to a burst wave signal. AGC is applied to such a signal.
, There is a null symbol whose signal level is zero, so that the time constant must be reduced in order to speed up the AGC response. However, when the receiving apparatus is mounted on a vehicle, the signal level fluctuates drastically under the influence of multipath when the receiving apparatus enters a shadow of a building. At this time, if the response of the AGC is fast, there is a problem that the AGC cannot follow the fluctuation of the signal level due to the overshoot. Keyed as such a remedy
There is AGC. Keyed AGC is a television receiver A
It is used as a GC and is selected only during the period of the horizontal synchronizing signal so that the AGC voltage is applied. It is used in places where there is a lot of impact noise. However, in order to apply the keyed AGC to the present receiving apparatus, it is necessary to select only a signal excluding a null symbol.
There is a problem that no voltage is applied.

Accordingly, the present invention has been made in view of the above problems,
It is an object of the present invention to provide a digital audio receiving device capable of performing an AGC operation smoothly even under the influence of multipath.

[0006]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a receiving apparatus for performing digital modulation on a multicarrier and receiving digital audio broadcast for each frame having a non-modulated null symbol at the beginning. An automatic gain control unit that controls an amplification gain of the received signal based on electric field strength information obtained after converting the received signal into a base data signal; and an envelope detection unit that performs envelope detection on the reception signal of one frame period, and And a reception level fluctuation detecting section for reducing a response time constant of the amplification gain in the automatic gain control section when the fluctuation is detected and the detected fluctuation is large.

More specifically, the reception level fluctuation detecting section provides a threshold for the envelope detection signal, and detects a fluctuation of the envelope detection signal according to the number of times the envelope detection signal exceeds the threshold; The fluctuation of the envelope detection signal is detected according to the period of the base data signal; the fluctuation of the envelope detection signal is detected in the symbol portion of the phase reference following the null symbol based on the information after digital demodulation of the base data signal, and the fluctuation is detected in the remaining frame portion. On the other hand, determine whether the time constant of the amplification gain in the automatic gain control unit should be reduced,
Envelope detection is performed on the received signal in the section of the null symbol of one frame, and the fluctuation of the envelope detection signal is detected. If the detected fluctuation is large, the response time constant of the amplification gain in the automatic gain control unit is reduced; Envelope detection of the received signal in the section is performed, and the envelope detection signal level of the null symbol and another symbol is obtained.If the ratio of the envelope signal level of the null symbol to other symbols is smaller than a predetermined value, the automatic gain control unit The response time constant of the amplification gain is reduced; the received signal in one frame period is envelope-detected, and the fluctuation of the envelope-detected signal is detected. If the detected fluctuation is large, the amplification gain in the automatic gain control unit is increased in the next frame. The response time constant of the received signal is reduced; the received signal in one frame period is envelope-detected according to the reception mode, and the envelope is detected. If the fluctuation of the detection signal is detected and the detected fluctuation is large, the response time constant of the amplification gain in the automatic gain control section is reduced; demodulation after frequency analysis of the base data signal and error correction processing When an error signal is input and the demodulated error signal is larger than a predetermined level and has a large number of errors, the response time constant of the amplification gain in the automatic gain control unit is reduced by assuming that the variation of the received signal is large; The frequency analysis is performed, and the current frame and the previous frame are compared for each level of the multicarrier of one frame. If the current frame is small, it is determined that the fluctuation of the received signal is large, and the automatic gain control unit responds with the amplification gain. Decrease the constant. By this means, it is possible to accurately detect the fluctuation of the reception state due to the multipath in consideration of the surrounding conditions and the reception mode, and furthermore, to control the AGC response time constant by controlling the response time constant of the AGC, thereby enabling a smooth AGC operation.

Further, a band-pass filter for limiting the band of the base data signal converted from the received signal is provided, and the reception level fluctuation detecting section detects the reception signal of the null symbol section of one frame. Envelope detection and detection of fluctuation of the envelope detection signal, and when the detection fluctuation is large, narrows the band of the band-pass filter; and further provides an antenna for receiving digital audio broadcast with an attenuator for attenuating the input level; The level fluctuation detecting section is the reception level fluctuation detecting section.
The received signal in the section of the null symbol of the frame is envelope-detected, and the fluctuation of the envelope-detected signal is detected. If the detected fluctuation is large, the attenuation of the attenuator is increased. By this means, the interference wave removal performance is improved.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an example of a digital audio receiving device according to the present invention. As shown in the figure, a front end (F / F) is provided to an antenna 1 for receiving a digital audio broadcast in a digital audio receiving apparatus.
E) 2 is connected. The front end 2 amplifies the received signal at a high frequency, and varies the gain of the high frequency amplification by the AGC voltage. Further, the front end 2 is provided with a response changing unit 21 composed of an RC circuit capable of changing a time constant. The AGC voltage is used for gain control of high frequency amplification via the response changing unit 21. Further, the front end 2 has an attenuator 2 located between the antenna and its terminal.
2 are provided. The attenuator 22 excludes the interfering wave when it is mixed.

A quadrature demodulator 3 quadrature demodulates the high-frequency signal amplified by the front end 2 and converts it into a baseband signal. Then, the quadrature demodulation unit 3 extracts the DC component of the converted signal to obtain electric field strength information, forms an AGC voltage based on the information, and outputs it to the front end 2. The bandpass filter (BPF) 4 extracts a baseband signal having a frequency bandwidth of 1.5 MHz.
It can be made variable in the range of Hz. The transmission path decoding circuit 5 performs fast Fourier transform (FFT) on the baseband signal that has passed through the bandpass filter 4, decodes the data, and performs processing such as error correction. Audio decoding unit 6
Performs further audio compression decoding of the output data of the transmission path decoding circuit 5, performs digital-to-analog conversion, and outputs broadcast sound via the power amplifier 7 and the speaker 8. The display unit 9 displays output data from the transmission path decoding circuit 5.

The reception level fluctuation detecting section 10 receives the output signal of the front end 2, detects a fluctuation in the level of the received signal, and detects a response change section (RC circuit) 21 of the front end 2.
Is changed as follows. When no fluctuation in the level of the received signal is detected, the time constant is small and the AGC has a fast response. FIG. 2 is a diagram illustrating a configuration example of the reception level fluctuation detection unit 10. As shown in the figure, a resistor 10 is provided on the input side of the output of the front end 2.
1 and an envelope detection unit including a capacitor 102 are provided. The envelope-detected signal is half-wave rectified via a diode 103. The comparison unit 104 outputs a high level when the level of the half-wave rectified signal is higher than a predetermined level V1. The predetermined level V1 is a criterion for determining whether the reception level is normal. Counter 10
5 is counted up at the rising edge of the comparator 104, and is cleared by the transmission path decoding circuit 5 upon detection of the final data of the MIS channel described later, and counts the number of times C1 at which the signal level increases in one frame. The comparator 106 receives the counter value C1 of the counter 105, and when it becomes larger than the predetermined count number C0, it goes to a high level. With this output, the response changing unit 21 of the front end 2 changes the time constant to be larger. You. This slows down the AGC response. The predetermined count number C0 is a criterion for changing the time constant, and is determined in consideration of the influence of multipath, that is, a reception state such as a valley of a building. Hereinafter, the operation of the reception level fluctuation detection unit 10 will be described.

FIG. 3 is a diagram for explaining the envelope detection and the like of the reception level fluctuation detecting section 10. As shown in FIG. 7A, if there is no change in the received signal, the signal obtained by the envelope detection becomes a rectangular wave. As shown in FIG.
If the received signal fluctuates, the signal obtained by the envelope detection fluctuates. As shown in FIG. 4C, the level of the fluctuation is detected by detecting the level of the received signal subjected to the envelope detection and the half-wave rectification. Note that the AGC signal has a reverse waveform to the half-wave rectification waveform.

Therefore, according to the present invention, it is possible to detect an envelope of one frame section, detect the frequency of occurrence of multipath from the detected waveform, and change the response time constant of AGC to obtain an optimum reception situation. become. FIG. 4 is a diagram showing a modification of the reception level fluctuation detecting unit 10 of FIG. 2, and FIG. 5 is a diagram for explaining the operation of the configuration of FIG. As shown in FIG. 4, the difference from the configuration of FIG. 2 is that the counter 205 clears and starts the count at the rising edge of the output of the comparator 104 and counts the clock signal.
And a comparison unit 206 that compares the count number C3 at the time of clearing with the reference count number C2 (see FIG. 5). The count number C3 indicates the cycle of the change in the signal level due to the multicarrier. If the cycle is short, the response changing unit 21 of the front end 2 changes the time constant from small to large, that is, from a fast response to a slow response. To be changed.

FIG. 6 is a diagram showing a modification of the reception level fluctuation detecting section 10 of FIG. 2, and FIG. 7 is a diagram for explaining the operation of the configuration of FIG. As shown in FIG. 6, what differs from the configuration of FIG. 2 is that a comparison unit 304 that determines whether the output of the diode 103 is a null symbol or a symbol signal input, and a resistor 3
A comparison unit 306 that makes the response changing unit 21 of the front end 2 increase the time constant when the noise level of the null symbol is large and determines the noise level of the null symbol via When it is determined that a symbol has been input, the switch is turned off and the diode 10 is turned off.
3 and a switch 307 for connecting the comparison unit 306 to the comparison unit 306. As shown in FIG. 7, the reference voltage V2 is a reference for determining a null symbol from a signal level based on the signal level, and the reference voltage V3 is a reference indicating that the noise level of the null symbol is large.

As described above, when the noise level of the null symbol is high, the response of the AGC is delayed. This makes it possible to eliminate the effects of multipath. When the noise level of the null symbol is high, the bandwidth of the band-pass filter 4 is narrowed. Thereby, it is possible to remove the interference wave. When the noise level of the null symbol is high, the attenuation of the attenuator 22 of the front end 2 is increased. This makes it possible to remove the interference wave.

Further, the reception level fluctuation detecting section 10 inputs a demodulation error signal indicating the degree of demodulation error in the error correction processing of the transmission path decoding circuit 5, and when the demodulation error signal level is large, the influence of multipath is reduced. Since it is large, the time constant may be increased in the response changing unit 21 of the front end 2 to delay the AGC response. FIG. 8 is a diagram showing a modification of the reception level fluctuation detection unit 10 of FIG. As shown in the figure, the difference from the configuration of FIG. 2 is that the output of the diode 103 is connected to an A / D converter 404 (Analog to Digital Conv.
erter) and a carrier / noise determining unit (C / C / C) that determines whether to change the time constant based on the level ratio between carrier and noise using the data signal obtained by the A / D converter 404.
N) 405 and a D / A converter 406 (Digital to Analog Converte) connected to the carrier / noise determination unit 405.
r) is provided.

FIG. 9 shows the carrier / noise judging section 40 shown in FIG.
9 is a flowchart illustrating an operation example of FIG. In step S1, it is determined whether or not the output level of the diode 103 is higher than V2. In step S2, V2
Since the signal is larger than the symbol signal, the level is stored as VC. In step S3, since the signal is a null symbol signal smaller than V2, the level is stored as VN. In step S4, VC / VN is determined as the ratio between the carrier and noise levels. If VC / VN is greater than K in step S5, it is determined that there is no multipath and the time constant is determined to be small. If VC / VN is smaller than K in step S6, it is determined that the multipath exists and the time constant is increased. Here, K is a reference value for judging the presence or absence of the influence of multipath.

FIG. 10 shows the reception level fluctuation detecting section 10 shown in FIG.
FIG. 4 is a diagram for explaining the time of setting a time constant by using FIG. As shown in the figure, the time constant is set for the next frame based on the determination of VC / VN obtained during one frame period. By clarifying the setting time, stable control can be performed. Further, the reception level fluctuation detection unit 10 is a result of the fast Fourier transform (FFT) of the transmission path decoding circuit 5.
The respective levels Li of the previous multicarrier and the respective levels Li 'of the present multicarrier are input.

FIG. 11 is a diagram showing an example of a multicarrier level which changes in a long cycle. As shown in the figure, regarding the change of the level of the multicarrier between the previous time and the current time obtained by the fast Fourier transform, when the level of the multicarrier satisfies the following equation (1), the reception level fluctuation detecting unit 10 sets the time constant to Judging that it should be large, the following equation (2)
If it satisfies, it is determined that the time constant should be reduced.

[0020]

(Equation 1)

In this way, the result of each frame can be stabilized against a long-term fluctuation. Although various characteristics such as a frame length and a null symbol length are different between the operation modes I, II, and III of the OFDM, the reception level fluctuation detecting unit 10 can be applied to the modes I, II, and III regardless of these differences. .

[0022]

As described above, according to the present invention, since the response time constant of the AGC is controlled in accordance with the variation of the received signal due to multipath, stable reception becomes possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a digital audio receiving device according to the present invention.

FIG. 2 is a diagram illustrating a configuration example of a reception level fluctuation detection unit 10.

FIG. 3 is a diagram illustrating envelope detection and the like of a reception level fluctuation detection unit 10;

FIG. 4 is a diagram illustrating a modification of the reception level fluctuation detection unit 10 of FIG. 2;

FIG. 5 is a diagram illustrating the operation of the configuration of FIG. 4;

FIG. 6 is a diagram illustrating a modification of the reception level fluctuation detection unit 10 of FIG. 2;

FIG. 7 is a diagram for explaining the operation of the configuration of FIG. 6;

FIG. 8 is a diagram showing a modification of the reception level fluctuation detection unit 10 of FIG.

FIG. 9 is a flowchart illustrating an operation example of a carrier / noise determination unit 405 in FIG. 8;

FIG. 10 is a diagram illustrating a setting time of a time constant by the reception level fluctuation detecting unit 10 of FIG. 8;

FIG. 11 is a diagram illustrating an example of a multicarrier level that changes in a long cycle.

FIG. 12 is a diagram illustrating frames of data related to digital audio broadcasting.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Antenna 2 ... Front end 3 ... Quadrature demodulation part 4 ... Bandpass filter 5 ... Transmission line decoding circuit 10 ... Reception level fluctuation detection part 21 ... Response change part 22 ... Attenuator

Claims (12)

[Claims] 1. A receiving apparatus for performing digital modulation on a multi-carrier and receiving digital audio broadcast for each frame having a non-modulated null symbol at the head, wherein electric field intensity information obtained after converting a received signal into a base data signal And an automatic gain control unit that controls the amplification gain of the received signal based on the received signal. Envelope detection is performed on the received signal in one frame period to detect fluctuations in the envelope detection signal. A digital audio receiving apparatus comprising: a reception level fluctuation detecting unit for reducing a response time constant of the amplification gain of the digital audio signal. 2. The reception level fluctuation detection section according to claim 1, wherein a threshold is provided for the envelope detection signal, and the fluctuation of the envelope detection signal is detected based on the number of times the envelope detection signal exceeds the threshold. A digital audio receiving device according to claim 1. 3. The digital audio receiving apparatus according to claim 1, wherein the reception level fluctuation detecting section detects fluctuation of the envelope detection signal based on a cycle generated in the envelope detection signal. 4. The reception level fluctuation detecting section detects fluctuation of an envelope detection signal in a phase reference symbol portion following a null symbol based on information after digital demodulation of a base data signal, and detects the fluctuation in the remaining frame portion. 2. The digital audio receiving device according to claim 1, wherein it is determined whether the time constant of the amplification gain in the automatic gain control unit should be reduced. 5. The reception level fluctuation detection section envelope-detects a reception signal in a section of a null symbol of one frame, detects fluctuation of the envelope detection signal, and amplifies the signal by an automatic gain control section when the detection fluctuation is large. 2. The digital audio receiving apparatus according to claim 1, further comprising: a reception level fluctuation detecting unit configured to reduce a gain response time constant. 6. A band-pass filter for limiting a band of a base data signal converted from a received signal,
The reception level fluctuation detection unit detects the fluctuation of the envelope detection signal by performing envelope detection on the reception signal in a null symbol section of one frame, and detects the fluctuation of the envelope detection signal. The digital audio receiving device according to claim 1, wherein a band is narrowed. 7. An antenna for receiving digital audio broadcasting, further comprising an attenuator for attenuating an input level, wherein the reception level fluctuation detection section envelopes a reception signal in a null symbol section of one frame. 2. The digital audio receiving apparatus according to claim 1, wherein the detection is performed to detect a change in the envelope detection signal, and when the detected change is large, the attenuation of the attenuator is increased. 8. The reception level fluctuation detecting section detects an envelope detection signal level of a null symbol and another symbol by performing envelope detection on a reception signal of one frame period, and calculates an envelope detection signal level of a null symbol with respect to other symbols. 2. The digital audio receiving device according to claim 1, wherein when the ratio is smaller than a predetermined value, the response time constant of the amplification gain in the automatic gain control unit is reduced. 9. The reception level fluctuation detecting section detects an envelope of a received signal in one frame period, detects a fluctuation of the envelope detection signal, and detects a fluctuation of the envelope detection signal. A digital audio receiving apparatus comprising: a reception level fluctuation detection unit that reduces a response time constant of an amplification gain. 10. The reception level fluctuation detecting section detects an envelope of a received signal in one frame section according to a reception mode, detects a fluctuation of the envelope detection signal, and performs automatic gain control when the detected fluctuation is large. 2. The digital audio receiving device according to claim 1, further comprising: a reception level fluctuation detection unit that reduces a response time constant of an amplification gain in the unit. 11. A receiving apparatus for performing digital modulation on a multicarrier and receiving digital audio broadcast for each frame, wherein an amplification gain of the received signal is adjusted based on electric field strength information obtained after converting the received signal into a base data signal. An automatic gain control unit for controlling the frequency of the base data signal, inputting a demodulation error signal after performing error correction processing, receiving the demodulation error signal when the demodulation error signal is larger than a predetermined level and there are many errors. A digital audio receiving apparatus, comprising: a reception level fluctuation detection unit that reduces a response time constant of an amplification gain in an automatic gain control unit when a signal fluctuation is large. 12. A receiving apparatus for performing digital modulation on a multicarrier and receiving digital audio broadcast for each frame, wherein an amplification gain of the received signal is adjusted based on electric field strength information obtained after converting the received signal into a base data signal. An automatic gain control unit for controlling the frequency of the base data signal, comparing the current frame with the previous frame for each level of one frame of multicarrier, and if the current frame is small, the received signal fluctuates. A digital audio receiving apparatus characterized in that a response time constant of an amplification gain in an automatic gain control unit is reduced as being large.