JP2001223961A - Digital broadcasting receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a digital broadcasting receiver which prevents frequent muting operation and makes noise and distortion less obvious in acoustic feeling. SOLUTION: A code error in a non-fixed code string in a demodulated OFDM signal outputted from an OFDM demodulator 21 is detected to calculate the first code error ratio. An error in a fixed code string in a compressed digital information signal outputted from a channel decoder 23 is detected to calculate the second code error ratio. A mute judging means 25 outputs a mute control signal for stopping the output of an MPEG decoder 27 in accordance with the first code error ratio and the second code error ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital broadcast receiving apparatus, and more particularly to a DAB receiving apparatus which receives digital audio broadcasting (Digital Audio Broadcasting: hereinafter also abbreviated as DAB) practically used in Europe.

[0002]

2. Description of the Related Art In digital broadcasting, audio information signals, image information signals, character information signals, and the like are compressed in a predetermined format, and information signals of a plurality of channels are multiplexed and transmitted as data signals so as to support a plurality of services. Is done. As a system for transmitting and receiving the data signal of the predetermined format including digital voice, for example, a DAB system conforming to the European standard (Eureka 147) has been put to practical use. In the DAB system, one ensemble is composed of a plurality of services, and each service is also composed of a plurality of components. The details are described in the DAB standard, and the description is omitted.

FIG. 6 is a diagram showing a configuration of a transmission frame in a DAB system. The transmitted DAB signal is an orthogonal frequency division multiplex modulation system, that is, OFDM (Orthogonal
OFD modulated by Frequency Division Multiplex)
The transmission frame (Tran) shown in FIG.
(smission frame) is obtained from a decoded OFDM signal obtained by decoding the OFDM signal. As shown in FIG. 6, a transmission frame in a decoded OFDM signal includes a synchronization channel (Synchronization Channel) and an FIC (Fast Informa).
tion Channel: High-speed information channel, MSC (Main Servo)
vice Channel: main service channel). The FIC transmits information necessary for the receiver to select a program and auxiliary information for the program, and the MSC transmits audio and data sub-channels.
l) transmit.

The FIC for one frame is composed of three FIBs.
(Fast Information Block), and the MSC varies depending on the transmission mode, but is 1 to 4
And a data block called CIF (Common Interleaved Frame), which is time-interleaved in CIF units. A plurality of sub-channels are multiplexed on the CIF, and one sub-channel corresponds to one program. Therefore, when the user wants to receive one program, the user selects a specific sub-channel from the CIF and selects a channel. In the DAB system, when a certain ensemble is received, information on a plurality of services and components included in the ensemble can be obtained, and it is possible to instantly switch to a different service or component without changing the reception frequency. ing.

[0005] A synchronization channel block is provided at the head of the transmission frame. This synchronization channel block includes a null signal part for coarse synchronization and a reference phase symbol that bears a reference phase for differential QPSK demodulation in OFDM demodulation. Tuning in the DAB receiver is performed in ensemble units, and when another ensemble is received, the frequency is changed, that is, ensemble up / down is performed.

FIG. 4 is a block diagram showing an example of a conventional digital broadcast receiving apparatus. Hereinafter, a conventional digital broadcast receiving apparatus will be described with reference to FIG. 4 using a DAB receiver as an example. In this DAB, OFDM is used as a modulation method.
(Orthogonal Frequency Divi
The description is based on the assumption that the multiplexing is used. In FIG. 4, the antenna 1
The DAB carrier is received via 1 and signal processed.
The intermediate frequency signal (I
F signal) and converted to an analog / digital converter (A
/ D converter) 15. The A / D converter 15 converts the analog signal into a digital information signal,
16. I / Q16, FFT17 and DSP1
8 together form an OFDM demodulator 21.

The I / Q 16 divides the signal into an I signal and a Q signal so that the OFDM signal can be processed at a subsequent stage.
7 is supplied. In the FFT 17, a fast Fourier transform is performed by using a DSP (digital operation device) 18, and the result is supplied to a channel decoder 23. In the channel decoder 23, de-interleaving (de-interleaving) for returning the signal order to the original order, de-puncturing (de-pancturing) for inserting untransmitted code bits to return to the original data, an error, Code detection and correction are performed. The microprocessor (MPU) 31 calculates the code error rate together with the correction of the error code. When the code error rate is larger than a predetermined value,
A mute control signal for stopping the decoding operation and the transmission of the decoded signal obtained by the decoding is supplied to the D / A converter 29. The channel decoder 23 will be described later in detail.

The compressed audio information is supplied from the channel decoder 23 to the MPEG decoder 27,
In the EG decoder 27, the compressed audio information is decoded and decompressed, and converted into analog information by a digital / analog converter (D / A converter) 29 and output as an analog audio signal.

FIG. 5 is a flowchart showing a procedure for generating a mute control signal in a conventional example. In step S11 in FIG. 5, the decoded OFDM signal output from the OFDM demodulator 21 in FIG. 4 is processed. First, deinterleaving is performed in step S11 to cancel and restore the frequency interleaving performed to randomize an error on the frequency axis, and the transmission frame illustrated in FIG. 6 is obtained.

Next, the FIC and the MSC determine in step S
13 are separated. In step S15, Viterbi decoding is performed on the FIC data, and simultaneously, in step S17, the code error rate is calculated. Steps S15 and S
17 is actually performed almost simultaneously by the MPU 31,
When the bit error rate is larger than a predetermined value, MP
A mute control signal for stopping the decoding of the EG audio signal and the transmission of the decoded signal is output from the D / A converter 29.
Given to. The calculation of the code error rate in the step S17 is performed by comparing the data corrected by the Viterbi decoding in the step S15 with the data before the error correction. Next, in step S19, the scramble is released (descramble), and
At 21, a CRC check is performed and supplied to a microprocessor (MPU) 32 as a control signal.

In step S25, the data of the sub-channel corresponding to the program selected by the user among the plurality of sub-channels is separated. Next, in step S27, time de-interleaving for canceling time interleaving for randomizing errors on the time axis is performed. Incidentally, time deinterleaving is not performed on the FIC. Then, in step S29, Viterbi decoding is performed, and in step S31, descrambling is performed, and the DAB audio frame (DAB Audio)
frame) is output to the MPEG decoder 27 in FIG. In step S29, the detection and correction of a code error with respect to the convolutionally encoded data are performed by Viterbi decoding as in the case of FIC.

OFDM, which is a DAB modulation method, is a type of multi-carrier modulation method, and is said to be capable of performing stable reception that is less susceptible to multipath and fading in mobile communication. Nevertheless, actually received data will have some code errors due to various causes. Therefore, as described above, the FIC or MSC data is subjected to error correction by Vidabi decoding, the error-corrected data is re-encoded, and the error-corrected data is compared with the data before error correction to calculate a code error rate. . When the bit error rate is larger than a predetermined value, D /
The output from the A converter 29 is stopped.

[0013]

However, the bit error rate using the Viterbi decoding does not always match the magnitude of noise or distortion on the actual audibility, and this is why
Despite the code error rate being smaller than a predetermined threshold,
Noise and distortion may be conspicuous in an acoustic signal output from a speaker. If the code error rate (the threshold value E1) for generating the mute control signal is set small in order to avoid such a phenomenon, the mute operation is performed frequently, and the output of the audio signal is frequently interrupted. Further, it is conceivable to provide a plurality of complicated error detections for severely detecting a code error and combine them to generate a mute control signal. However, in that case, there is a problem that the load on the MPU 31 becomes heavy.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to provide a digital broadcast receiving apparatus which prevents frequent mute operations and makes noise and distortion less audible. .

[0015]

Means for Solving the Theme The present invention has the following structure to achieve the above object. First
The present invention provides an OFDM demodulator for demodulating an OFDM signal in a digital broadcast receiving apparatus for receiving a digital broadcast in which a plurality of programs are multiplexed by a digital information signal compressed in a predetermined compression format and transmitted. A channel decoder for detecting a code error of a non-fixed code sequence in the demodulated OFDM signal output from the demodulator, calculating a first code error rate, and extracting a specific program, and a compressed digital signal output from the channel decoder A decoder for decoding the information signal; and mute judging means for outputting a mute control signal for stopping the output of the decoder. The mute judging means detects an error of a fixed code string in the compressed digital information signal. To calculate a second bit error rate, wherein the first bit error rate is larger than a first predetermined value. Or when the second bit error rate is a digital broadcast receiving apparatus that outputs the mute control signal when a large made of the second predetermined value.

According to the first invention, the mute control is performed when the first bit error rate is larger than a first predetermined value or when the second bit error rate is larger than a second predetermined value. Outputting a signal, stopping output of the demodulated signal, wherein the first bit error rate is smaller than a first predetermined value and the second
When the code error rate is smaller than a second predetermined value, the mute control signal is not output and a demodulated signal is output. Since the method of calculating the first code error rate and the method of calculating the second code error rate are different methods, even when the first code error rate is calculated to be small, There is a case where the second code error rate is calculated to be large and the mute control signal is output. Therefore, the first
The mute control signal is output when the bit error rate of the second bit error rate is larger than the threshold value E1, and the second code error rate threshold value E2
By appropriately setting, noise and distortion in the audibility can be suppressed as a whole. Since the threshold value E1 of the first bit error rate can be set to a relatively large value, it is possible to prevent the analog audio output from being interrupted frequently. Further, since the calculation of the second bit error rate is based on error detection of a predetermined fixed code string, the load increase of the microprocessor (MPU) due to the calculation of the second bit error rate is negligible. It is.

According to a second aspect, in the digital broadcast receiving apparatus according to the first aspect, the fixed code sequence is a predetermined code sequence included in a header of a compressed audio frame in the compressed digital information signal. is there.

According to the second aspect, the following effects are obtained. That is, the extraction of the fixed code string can be easily realized by extracting the header arranged at the head of the compressed audio frame output from the channel decoder and reading the data.

According to a third aspect, in the digital broadcast receiving apparatus according to the first or second aspect, the fixed code sequence is such that all bits are 1 or all bits are 0. Device.

According to the third invention, the following effects are obtained. That is, by counting the number of bits having a value of 1 or the number of bits having a value of 0, the second bit error rate can be easily calculated. The increase in the load on the MPU due to the calculation can be made small.

According to a fourth aspect, in the digital broadcast receiving apparatus according to the third aspect, the mute judging means counts the number of 0s or 1s in the fixed code string, and determines the count value and a predetermined threshold value. Is a digital broadcast receiving apparatus that compares the magnitudes of the above and outputs the mute signal in accordance with the comparison result.

According to the fourth aspect, the following effects are obtained. That is, the second bit error rate can be easily calculated, and the mute control signal is transmitted according to whether the second bit error rate is larger or smaller than a predetermined value. M involved in the judgment means
The load on the PU can be slightly increased. Further, the operation of the above-described mute determination means can be realized by simple hardware without using the MPU.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A digital broadcast receiving apparatus according to the present invention detects a code error of a non-fixed code sequence in a demodulated OFDM signal output from an OFDM demodulator, calculates a first code error rate, and calculates a channel decoder. Detects an error in a fixed code string in the compressed digital information signal output from the CDMA, calculates a second code error rate, and calculates the second code error rate according to the first code error rate and the second code error rate. A digital broadcast receiving apparatus for outputting a mute control signal for stopping output of a digital information signal from a decoder.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the digital broadcast receiving apparatus according to the present invention. In FIG. 1, FIG.
Elements having the same functions as those described above are denoted by the same reference numerals, and description thereof may be omitted in the following description. In FIG. 1, the digital broadcast coming from the antenna 11 is assumed to be the same as that of the conventional example described with reference to FIG. 4, and the audio information signal, the image information signal, the character information, etc. are compressed and transmitted in a predetermined format and transmitted. It is a thing. In the digital broadcast receiving apparatus according to the embodiment of the present invention described below, DAB (Digital
Audio Broadcasting) A receiver will be described as an example. In the DAB, description will be made assuming that OFDM (orthogonal frequency division multiplexing) is adopted as a modulation method.

FIG. 1 differs from FIG. 4 in that a mute judging means 25 is provided and that the OFDM demodulator 2
1, a channel decoder 23, a mute judging means 25 and an MPEG decoder 27 are a microprocessor (MPU).
32 is controlled comprehensively. The mute judging means 25 can also achieve its function by a microprocessor (MPU) 32 without having independent hardware. In the channel decoder 23, a code error of a non-fixed code sequence in a demodulated OFDM signal output from the OFDM demodulator is detected using Vidabi decoding, and the first The rate is calculated. In the mute determining means 25, an error of the fixed code string in the compressed digital information signal output from the channel decoder 23 is output, and a second code error rate is calculated. The MPU 32 determines whether the first code error rate is greater than a first predetermined value (threshold value) E1 or the second code error rate is a second predetermined value (threshold value). By providing a mute signal to the MPEG decoder 27 when the threshold value is larger than E2, the decoding operation of the MPEG decoder 27 and the transmission of the signal to the D / A converter 29 are stopped.

FIG. 2 is a flowchart showing a procedure for generating a mute control signal according to the present invention. In FIG. 2, the same steps as those in FIG. 5 are denoted by the same reference numerals, and description thereof may be omitted. The difference between FIG. 2 and FIG. 5 is that FIG. 2 includes a mute determination step (step 40) composed of steps S33, S35 and S37, and the mute control signal is obtained in step S17. It is generated according to the first bit error rate and the second bit error rate obtained in step S37. The first code error rate obtained in step S17 is a code error rate in a non-fixed code string whose code string value is not fixed, as described in the conventional example, and is actually separated in step S13. Vidabi decoding is performed on the data of the FIC that has been corrected, the data that has been error-corrected by the Vidavi decoding is re-encoded, error detection is performed by comparing the data with the data before error correction, and the first code error rate is calculated. Is done. The calculation of the bit error rate is actually performed by the MPU 32.

On the other hand, a specific sub-channel is separated in step S25 from the MSC data separated in step S13 in order to receive a desired program, and the separated data is subjected to time deinterleaving, Decoding and descrambling are performed, and a DAB audio frame (DAB Audio frame) is output. Note that the first bit error rate is calculated in step S
The bit error rate of the MSC obtained in 29 may be used. FIG. 3 shows DAB Audio fr in DAB.
It is a figure showing composition of ame. In the DAB system, data is transmitted by continuously transmitting a DAB Audio frame as shown in FIG. DAB
DAB Audio is at the beginning of the Audio frame.
o frame Header, followed by CRC (Cyclic Redundancy Check Word) for error detection, various parameters required for decoding audio data,
Audio data and the like are arranged.

According to the CRC immediately after the header,
The latter 16 bits of the 2-bit header and Bit All
Error correction up to the location and SCFSI is performed, and the X-PAD and the F-PAD are corrected by the SCF CRC disposed therebetween. The DAB
Sy is at the beginning of the Audio frame Header
A 12-bit fixed code string called ncword is arranged. The Syncword is a fixed code string in which the value of the code string is fixed. In this case, all 12 bits are set to 1.

In step S33 of FIG. 2, DAB Aud
The Syncword is extracted from the io frame, and in step S35, 0 of the extracted fixed code string is extracted.
Or the number of ones is counted. In this case, the value of each bit is added by the adder, and the addition result becomes a decimal number 12 if the bit error rate is zero. The adder is M
It may be composed of the PU 32 or may be composed of independent hardware. In this way, the second bit error rate is calculated. For example, when the addition result is 9 or less, the mute control signal is transmitted. The mute determination in step 37 may actually be performed using the MPU 32, and when the first bit error rate is larger than a first predetermined value E1, or when the second bit error rate is
Is larger than the predetermined value E2, a mute control signal is sent to the D / A converter 29, and the D / A converter 29
Is stopped. That is, the reason why the signal is transmitted from the D / A converter 29 without generating the mute control signal is that the first bit error rate is equal to the first predetermined value E1.
It is limited only to the case where the second bit error rate is smaller than the second predetermined value E2.

Since the method of calculating the first bit error rate and the method of calculating the second bit error rate are separate methods, the second bit error rate is calculated even when the first bit error rate is small. There is a case where the mute control signal is output due to a large bit error rate. Therefore, even if the first code error rate threshold value E1 is increased to some extent, by appropriately setting the second code error rate threshold value E2, it is possible to suppress noise and distortion on hearing as a whole. Can be. And it can prevent that audio output is interrupted frequently. Note that the present invention is not limited to the DAB system. For example, a synchronization signal in DPSK in BS audio broadcasting has a 16-bit subcode of 0001001101011110 in which the value of a code string is fixed for each frame. In addition, the second code error rate in the present invention can be calculated using the fixed code string.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an embodiment of a digital broadcast receiving apparatus according to the present invention.

FIG. 2 is a flowchart showing a procedure for generating a mute control signal according to the present invention.

FIG. 3 is a DAB Audio frame in DAB.
FIG. 14 is a diagram showing a configuration of e.

FIG. 4 is a block diagram showing an example of a conventional digital broadcast receiving device.

FIG. 5 is a flowchart showing a procedure for generating a mute control signal in a conventional example.

FIG. 6 is a diagram illustrating a configuration of a transmission frame in the DAB system.

[Description of Signs] 21 OFDM demodulator 23 Channel decoder 25 Mute determination means 27 MPEG decoder 29 D / A converter 32 Microprocessor (MPU)

Claims (4)

[Claims] A digital broadcast receiving apparatus for receiving a digital broadcast in which a plurality of programs are multiplexed and transmitted by a digital information signal compressed in a predetermined compression format, comprising: an OFDM demodulator for demodulating an OFDM signal; OFDM
A channel decoder for detecting a code error of a non-fixed code sequence in the demodulated OFDM signal output from the demodulator, calculating a first code error rate, and extracting a specific program, and a compressed digital signal output from the channel decoder A decoder for decoding the information signal; and mute judging means for outputting a mute control signal for stopping the output of the decoder. The mute judging means detects an error of a fixed code string in the compressed digital information signal. To calculate a second bit error rate, when the first bit error rate is greater than a first predetermined value or when the second code error rate is greater than a second predetermined value A digital broadcast receiving device for outputting a mute control signal. 2. A digital broadcast receiving apparatus according to claim 1, wherein said fixed code string is a predetermined code string included in a header of a compressed audio frame in said compressed digital information signal. . 3. The digital broadcast receiving apparatus according to claim 1, wherein all bits of said fixed code string are 1 or all bits are 0. 4. The digital broadcast receiving apparatus according to claim 3, wherein said mute judging means comprises a function for determining whether the fixed code string is 0 or not.
A digital broadcast receiving apparatus which counts the number of 1s or 1s, compares the counted value with a predetermined threshold, and outputs the mute signal according to the comparison result.