JP3531802B2 - Digital broadcast receiver - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital broadcast receiving apparatus, and more particularly to a DAB receiving apparatus for receiving a digital audio broadcasting (hereinafter abbreviated as DAB) which has been put into practical use in Europe.

[0002]

2. Description of the Related Art In digital broadcasting, audio information signals, image information signals, character information signals, etc. 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. To be done. As a system for transmitting and receiving a data signal of the predetermined format including digital voice, for example, a DAB system conforming to the European standard (Eureka 147) has been put into 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. Since the details are described in the DAB standard, the description is omitted.

FIG. 6 is a diagram showing the structure of a transmission frame in the 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 the decoded OFDM signal obtained by decoding the OFDM signal. As shown in FIG. 6, the transmission frame in the decoded OFDM signal includes a synchronization channel (Synchronization Channel) and an FIC (Fast Informa).
tion Channel: High-speed information channel), MSC (Main Ser
vice Channel: Main service channel). The FIC transmits information necessary for the receiver to select a program, auxiliary information for the program, and the MSC is a subchannel for voice and data.
l) is transmitted.

The FIC for one frame has three FIBs.
It consists of a data block called (Fast Information Block), and the MSC is 1 to 4 depending on the transmission mode.
CIF (Common Interleaved Frame), and is time-interleaved in CIF units. A plurality of sub-channels are multiplexed in the CIF, and one sub-channel corresponds to one program. Therefore, when the user wants to receive one program, he or she selects a specific sub-channel from the CIF and selects it. When a certain ensemble is received, the DAB system can obtain information about a plurality of services and components included in the ensemble, and can instantaneously switch to different services and components without changing the reception frequency. ing.

A synchronization channel block is provided at the beginning of the transmission frame. This synchronization channel block is composed of a null signal portion for coarse synchronization and a reference phase symbol that carries a reference phase for differential QPSK demodulation in OFDM demodulation. Channel selection in the DAB receiver is performed in units of ensembles, and when other ensembles are received, frequency change, 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 receiver will be described with reference to FIG. 4 by taking a DAB receiver as an example. In this DAB, the modulation method is OFDM
(Orthogonal Frequency Divi
sion Multiplex) is adopted as the explanation. In FIG. 4, the front end 13 has an antenna 1
The DAB carrier is received via 1 and signal processed.
Then, an intermediate frequency signal (I
F signal) and converted into an analog / digital converter (A
/ D converter) 15. In the A / D converter 15, the analog signal is converted into a digital information signal, and the I / Q
16 are supplied. I / Q16, FFT17 and DSP1
The OFDM demodulator 21 is composed of 8 and.

In the I / Q 16, the FFT 1 is divided into an I signal and a Q signal so that the OFDM signal can be processed in the subsequent stage.
7 is supplied. In the FFT 17, a fast Fourier transform is performed using a DSP (digital arithmetic unit) 18 and supplied to the channel decoder 23. In the channel decoder 23, de-interleaving (de-interleaving) for returning the order of signals to the original order, de-pancturing (de-pancturing) for inserting untransmitted code bits to return to the original data, and error Code detection and correction are performed. The microprocessor (MPU) 31 corrects the error code and calculates the code error rate, and 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 applied to the D / A converter 29. The channel decoder 23 will be described in detail later.

The compressed audio information is supplied from the channel decoder 23 to the MPEG decoder 27, and MP
The EG decoder 27 decodes the compressed audio information and decompresses it, and the digital / analog converter (D / A converter) 29 converts it into analog information and outputs it as an analog audio signal.

FIG. 5 is a flowchart showing a procedure for generating a mute control signal in the conventional example. In step S11 of FIG. 5, the decoded OFDM signal output from the OFDM demodulator 21 of FIG. 4 is processed. First, in step S11, de-interleaving is performed in which the frequency interleaving performed to randomize the error on the frequency axis is canceled and returned to the original state, and the transmission frame shown in FIG. 6 is obtained.

Next, the FIC and MSC perform step S.
Separated at 13. Viterbi decoding is performed on the FIC data in step S15, and at the same time, the code error rate is calculated in step S17. Steps S15 and S
17 is actually done almost simultaneously by the MPU 31,
When the code error rate exceeds a predetermined value, MP
The mute control signal for stopping the decoding of the EG audio signal and the sending of the decoded signal is the D / A converter 29.
Given to. The calculation of the code error rate in step S17 is performed by comparing the data error-corrected by the Viterbi decoding in step S15 with the data before the error correction. Next, in step S19, the scramble is descrambled (descramble), and step S
At 21, a CRC check is performed and the control signal is supplied to the microprocessor (MPU) 32.

In step S25, the subchannel data corresponding to the program selected by the user is separated from the plurality of subchannels. Next, in step S27, time de-interleaving for canceling time interleaving for randomizing errors on the time axis is performed. By the way, FIC is not time deinterleaved. Then, in step S29, the Viterbi decoding is performed, and in step S31, the descrambling is performed, and the DAB audio frame (DAB Audio) is displayed.
frame) is output to the MPEG decoder 27 of FIG. In step S29, code error detection and correction for convolutionally encoded data is performed by Viterbi decoding as in the case of FIC.

OFDM, which is a DAB modulation system, is a type of multi-carrier modulation system and is said to be capable of stable reception in mobile communications, which is unlikely to suffer from multipath and fading. However, actually, the received data will have some degree of code error due to various causes. Therefore, as described above, the FIC or MSC data is error-corrected by Viterbi decoding, the error-corrected data is re-encoded, and the code error rate is calculated by comparing with the data before the error correction. . When the bit error rate is higher than a predetermined value, D /
The output from the A converter 29 is stopped.

[0013]

However, the code error rate using Viterbi decoding does not always match the magnitude of noise and distortion on the actual hearing, which is why
Despite the bit error rate being less than a predetermined threshold,
Noise and distortion may be noticeable in the acoustic signal output from the speaker. If the code error rate (the threshold value E1) for generating the mute control signal is set to be small in order to avoid such a phenomenon, the mute operation is frequently performed and the output of the audio signal is frequently interrupted. It is also conceivable that a plurality of complicated error detections are provided to detect a code error severely and they are combined to generate a mute control signal, but in that case, there is a problem that the load of the MPU 31 becomes heavy.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide a digital broadcast receiving apparatus that prevents frequent mute operation and makes noise and distortion inaudible. .

[0015]

[Means for Solving the Subject] The present invention has the following constitution in order to achieve the above object. First
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, an OFDM demodulator for demodulating an OFDM signal and an OFDM demodulator A channel decoder for detecting a code error of a non-fixed code sequence in a demodulated OFDM signal output from a demodulator to calculate 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 a mute judging means for outputting a mute control signal for stopping the output of the decoder are provided, and the mute judging means detects the error of the fixed code string in the compressed digital information signal. And a second code error rate is calculated, and the first code error rate is larger than the 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 code error rate is larger than a first predetermined value or when the second code error rate is larger than a second predetermined value. Outputting a signal, stopping the output of the demodulated signal, the first code error rate being smaller than a first predetermined value, and the second
When the code error rate is less than the second predetermined value, the mute control signal is not output and the 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, the above-mentioned method is used even when it is calculated that the first code error rate is small. The mute control signal may be output when the second code error rate is calculated to be high. Therefore, the first
If the mute control signal is output when the bit error rate is larger than the threshold value E1, the second code error rate threshold value E2 is set even if the threshold value E1 is set to be relatively large.
By appropriately setting, it is possible to suppress the noise and distortion on the hearing as a whole. Since the first code error rate threshold value E1 can be set to a relatively large value, frequent interruption of analog audio output can be prevented. Further, since the calculation of the second code error rate is based on the error detection of a predetermined fixed code sequence, the increase in the load of the microprocessor (MPU) due to the calculation of the second code error rate is slight. Is.

A second invention is the digital broadcast receiving apparatus according to the first invention, wherein the fixed code string is a predetermined code string included in a header of a compressed audio frame in the compressed digital information signal. is there.

According to the second invention, the following effects can be obtained. That is, the extraction of the fixed code sequence 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.

A third aspect of the invention is the digital broadcast receiving apparatus according to the first or second aspect of the invention, wherein the fixed code string has all bits of 1 or all bits of 0. It is a device.

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

According to a fourth aspect of the present invention, in the digital broadcast receiving apparatus according to the third aspect, the mute determining means counts the number of 0's or the number of 1's in the fixed code sequence, and the counted value and a predetermined threshold value are set. It is a digital broadcast receiving apparatus that compares the magnitudes of the above and outputs the mute signal according to the comparison result.

According to the fourth invention, the following effects can be obtained. That is, the second code error rate can be easily calculated, and the mute control signal is transmitted according to whether the second code error rate is larger or smaller than a predetermined value, thereby muting. M involved in judgment means
The increase in the load on the PU can be negligible. Further, the operation of the mute determining means described above can be realized by simple hardware without using the MPU.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION A digital broadcast receiving apparatus of the present invention detects a code error of a non-fixed code sequence in a demodulated OFDM signal output from an OFDM demodulator to calculate a first code error rate, and a channel decoder. Error detection of the fixed code sequence in the compressed digital information signal output from the second code error rate is calculated, and the second code error rate is calculated according to the first code error rate and the second code error rate. It is a digital broadcast receiving apparatus that outputs a mute control signal for stopping the output of a decoder of a digital information signal.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a digital broadcast receiving apparatus of the present invention. In FIG.
Elements having the same functions as those of the above are denoted by the same reference numerals, and the 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 in FIG. 4, and audio information signals, image information signals, character information, etc. are compressed 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. Further, in this DAB, it is assumed that OFDM (Orthogonal Frequency Division Multiplexing Method) is adopted as a modulation method.

1 is different from that of FIG. 4 in that a mute judging means 25 is provided and that the OFDM demodulator 2 is provided.
1, the channel decoder 23, the mute determination means 25, and the MPEG decoder 27 are microprocessors (MPU).
This is a point that is comprehensively controlled by 32. The mute judging means 25 can also achieve its function by the microprocessor (MPU) 32 without having independent hardware. In the channel decoder 23, the code error of the non-fixed code sequence in the demodulated OFDM signal output from the OFDM demodulator is detected by using the Viterbi decoding as in the case of the conventional example, and the MPU 32 detects the first code error. The rate is calculated. In the mute judging means 25, an error of the fixed code sequence in the compressed digital information signal output from the channel decoder 23 is issued and the second code error rate is calculated. In the MPU 32, the first code error rate is larger than a predetermined first predetermined value (threshold value) E1, or the second code error rate is a second predetermined value (predetermined value). When it is larger than the threshold value E2, a mute signal is given to the MPEG decoder 27, so that the decoding operation in the MPEG decoder 27 and the sending of the signal to the D / A converter 29 are stopped.

FIG. 2 is a flow chart showing the procedure for generating the mute control signal in the present invention. 2, the same steps as those in FIG. 5 are designated by the same reference numerals, and the description thereof may be omitted. 2 is different from FIG. 5 in that a mute determination step (step 40) including steps S33, S35, and S37 is provided in FIG. 2, and the mute control signal is obtained in step S17. It is generated according to the first code error rate and the second code error rate obtained in step S37. The first code error rate obtained in step S17 is the code error rate in the non-fixed code string in which the value of the code string is not fixed, as described in the conventional example, and is actually separated in step S13. The FIC data is subjected to Vidabi decoding, the data corrected in error by the Vidabi decoding is re-encoded, error detection is performed in comparison with the data before error correction, and the first code error rate is calculated. To be done. The calculation of the code error rate is actually performed by the MPU 32.

On the other hand, from the MSC data separated in step S13, a specific subchannel is separated in step S25 in order to receive a desired program, and time de-interleave and Vidabi are added to the separated data. Decoding and descrambling are performed, and a DAB audio frame (DAB Audio frame) is output. In addition, as the first code error rate,
The MSC code error rate obtained at 29 may be used. Figure 3 shows DAB Audio fr in DAB
It is a figure which shows the structure of ame. In the DAB system, data is transmitted by continuously transmitting DAB Audio frame as shown in FIG. DAB
DAB Audio at the beginning of the audio frame
o frame Header, followed by CRC (Cyclic Redundancy Check Word) for error detection, various parameters necessary for decoding audio data,
Audio data etc. are arranged.

According to the CRC immediately after the header, the
16 bits in the latter half of the 2-bit header and Bit All
error correction up to the position and SCFSI is performed, and the X-PAD and the F-PAD are error-corrected by the SCF CRC arranged between them. 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, and in this case, all 12 bits are set to 1.

In step S33 of FIG. 2, DAB Aud is selected.
The Syncword is extracted from the io frame, and in step S35, 0 of the extracted fixed code string is extracted.
, Or 1 is counted. In this case, the value of each bit is added by the adder, and the addition result is a decimal number 12 when the code error rate is 0. The adder is M
It may be configured by the PU 32 or may be configured by independent hardware. In this way, the second code error rate is calculated, and the mute control signal is transmitted when the addition result is 9 or less, for example. The mute determination in step 37 may actually be performed by using the MPU 32, and when the first code error rate is larger than the first predetermined value E1, or the second code error rate is the second.
If it is larger than the predetermined value E2 of the D / A converter 29, a mute control signal is sent to the D / A converter 29.
The transmission of the signal from is stopped. That is, the reason why the signal is transmitted from the D / A converter 29 without the mute control signal is that the first code error rate is the first predetermined value E1.
It is limited to the case where it is smaller and the second code error rate is smaller than the second predetermined value E2.

Since the method of calculating the first code error rate and the method of calculating the second code error rate are different methods, the second method is used even when the first code error rate is small. The mute control signal may be output due to a large code error rate. Therefore, even if the first code error rate threshold value E1 is increased to some extent, the noise and distortion on the auditory sense are suppressed as a whole by appropriately setting the second code error rate threshold value E2. You can Then, it is possible to prevent the audio output from being frequently interrupted. 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 00010011010111110 in which the value of the code string is fixed for each frame. Therefore, the second code error rate in the present invention can also be calculated using this fixed code sequence.

[Brief description of drawings]

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

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

FIG. 3 DAB Audio frame in DAB
It is a figure which shows the structure of e.

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

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

FIG. 6 is a diagram showing a structure of a transmission frame in the DAB system.

[Explanation of symbols]

21 OFDM demodulator 23 channel decoder 25 Mute judging means 27 MPEG decoder 29 D / A converter 32 microprocessors (MPU)

Claims (4)

(57) [Claims] 1. A 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, an OFDM demodulator for demodulating an OFDM signal, and the OFDM demodulator.
A channel decoder for detecting a code error of a non-fixed code sequence in a demodulated OFDM signal output from a demodulator to calculate 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 a mute judging means for outputting a mute control signal for stopping the output of the decoder are provided, wherein the mute judging means is an error in the fixed code string in the compressed digital information signal. Code or correct code
When a second code error rate is acquired as a value obtained by counting, and the first code error rate is larger than a first predetermined value or the second code error rate is larger than a second predetermined value. A digital broadcast receiving apparatus, wherein the mute control signal is output to the. 2. The digital broadcast receiving apparatus according to claim 1, wherein the fixed code string is a predetermined code string included in a header of a compressed audio frame in the compressed digital information signal. . 3. The digital broadcast receiving apparatus according to claim 1 or 2, wherein all bits of the fixed code string are 1 or all bits are 0. 4. The digital broadcast receiving apparatus according to claim 3, wherein the mute determining means is 0 of the fixed code string.
Or the number of 1 is counted, the magnitude of this count value and a predetermined threshold value is compared, and the mute signal is output according to the comparison result.