JP3822881B2 - Digital modulation signal evaluation device - Google Patents

Description

  The present invention relates to a technique for enabling an apparatus for evaluating a digital modulation signal used in terrestrial digital broadcasting or the like to easily grasp whether or not it is affected by an interference wave.

  Television terrestrial broadcasting has shifted from a conventional analog system to a digital system, and various apparatuses for evaluating digital modulation signals used in terrestrial digital broadcasting have been proposed.

  The quality of this kind of digital modulation signal is generally evaluated by the CN ratio, the bit error rate (BER), the modulation error ratio (MER), etc., which represent the ratio of carrier to noise. For example, when the CN ratio is small, the bit error rate is large and the modulation error ratio is small. Conversely, when the CN ratio is large, the bit error rate is small and the modulation error ratio is large. If there is a correlation and the system and modulation method are determined, the relationship between them is also uniquely determined.

  The modulation error ratio is obtained by calculating the distance between the constellation symbol on the I and Q coordinates obtained by demodulating the received digital broadcast signal and the theoretical constellation symbol as an error, and calculating the effective value. In the following Patent Document 1, the present applicant discloses a modulation error ratio measuring apparatus capable of measuring the modulation error ratio.

JP 2002-124931 A

  FIG. 6 shows a schematic configuration of a conventional digital modulation signal evaluation apparatus 10 (hereinafter referred to as evaluation apparatus 10) having a modulation error ratio measurement function.

  In this evaluation apparatus 10, for example, the digital modulation signal d (t) in the intermediate frequency band obtained by frequency-converting the digital broadcast signal to be measured modulated by the BST-OFDM modulation method adopted in the ISDB-T system is used. The A / D converter 11 converts the signal into a digital signal sequence D (k), the quadrature demodulator 12 demodulates the baseband signals I (k) and Q (k), and outputs them to the demodulator 13.

  The demodulation unit 13 includes a frequency error correction unit 14, a symbol timing detection unit 15, and a symbol demodulation unit 16, and performs frequency error correction processing and symbol timing detection processing on the input baseband signals I (k) and Q (k). And constellation symbol demodulation processing on the I and Q coordinates is performed based on the information obtained by these processes.

  The constellation symbol information obtained by the demodulator 13 is output to the modulation error ratio calculator 17, and a modulation error ratio (MER) indicating the quality of the input digital modulation signal d (t) is calculated. This calculation is performed based on the equation (1) in Patent Document 1.

  If the modulation error ratio of the digital modulation signal to be evaluated can be obtained as described above, the quality of the digital modulation signal can be grasped from the magnitude of the modulation error ratio.

  Currently, terrestrial broadcasting is in the transition period from analog broadcasting to digital broadcasting, and the channel frequency band of digital broadcasting and analog broadcasting is the same, so analog broadcasting exists as a jamming wave in digital broadcasting channels in some regions. The situation to be considered.

  When digital broadcasting and analog broadcasting are present at the same channel frequency in this way, signals in the audio band and video band of analog broadcasting are added as noise to the digital modulation signal, and the modulation error ratio is reduced.

  The presence of jamming waves can generally be observed with a spectrum analyzer or the like. However, in the case of a digitally modulated signal, the jamming wave is masked because there are many subcarriers in the entire channel band, and the spectrum analyzer. It is difficult to recognize the presence of interfering waves in the observation by.

  The present invention has been made in view of such circumstances, and provides a digital modulation signal evaluation apparatus capable of accurately and easily grasping whether a digital modulation signal is disturbed by analog broadcasting or the like. It is aimed.

In order to achieve the above object, a digital modulation signal evaluation apparatus according to claim 1 of the present invention comprises:
A demodulator (30) that receives a digital modulation signal of a desired channel and demodulates all subcarriers included in the digital modulation signal;
A modulation error ratio calculation unit (35) for calculating a modulation error ratio for a demodulated signal for each subcarrier demodulated by the demodulation unit;
A threshold setting unit (37) for setting a threshold for determining the presence or absence of an interfering wave with respect to the modulation error ratio for each subcarrier calculated by the modulation error ratio calculating unit;
A determination unit (38) that compares the threshold set by the threshold setting unit with the modulation error ratio for each subcarrier to determine whether there is an interfering wave;
An indicator (42);
A digital modulation signal evaluation apparatus including a display control unit (41) for displaying the determination result of the determination unit on the display ;
The threshold setting unit modulates subcarriers that are not included in an interference band that is expected to receive interference of interference waves among the modulation error ratios for all subcarriers obtained by the modulation error ratio calculation unit. Based on the error ratio, configured to calculate a threshold for determining the presence or absence of jamming waves,
The determination unit compares the threshold calculated and set by the threshold setting unit with a modulation error ratio of subcarriers included in the interference band to determine whether or not there is an interference wave in the interference band. It is characterized by being comprised so that it may determine.

A digital modulation signal evaluation apparatus according to claim 2 of the present invention is the digital modulation signal evaluation apparatus according to claim 1 ,
The threshold setting unit obtains an average value and a standard deviation of a modulation error ratio of subcarriers not included in the interference band, and calculates a threshold value determined by the average value and the standard deviation. Yes.

The digital modulation signal evaluation apparatus according to claim 3 of the present invention, in the digital modulation signal evaluation apparatus according to claim 1 or claim 2 Symbol placement,
The display control unit displays the frequency characteristic of the modulation error ratio for each subcarrier calculated by the modulation error ratio calculation unit together with the determination result of the determination unit on the display.

The digital modulation signal evaluation apparatus according to claim 4 of the present invention is the digital modulation signal evaluation apparatus according to claim 3 ,
The display control unit displays the frequency characteristic of the modulation error ratio obtained every predetermined time on three-dimensional coordinates including a time axis.

  As described above, in the digital modulation signal evaluation apparatus of the present invention, the modulation error ratio is obtained for each demodulated signal demodulated for each subcarrier by the demodulator and is compared with a threshold value to determine whether there is an interference wave. It is determined whether or not, and the determination result is displayed.

  For this reason, it can be grasped correctly and easily whether it is influenced by interference waves, such as an analog broadcast wave.

  The threshold setting unit is configured to calculate a threshold based on a modulation error ratio of subcarriers not included in an interference band that is expected to receive interference of an interfering wave. The threshold value and the modulation error ratio of the subcarriers included in the interference band are compared to determine whether there is an interference wave in the interference band. Since the threshold value is automatically calculated and set, it is possible to easily know the presence or absence of an interference wave.

  In addition, the threshold value is calculated based on the average value and standard deviation of the modulation error ratios of subcarriers not included in the interference band. Therefore, it is possible to accurately determine the presence of an interference wave.

  Further, if the frequency characteristic of the modulation error ratio for each subcarrier is displayed together with the determination result, the magnitude of the influence of the interference wave can be intuitively grasped.

  In addition, when the frequency characteristic of the modulation error ratio obtained every predetermined time is displayed on the three-dimensional coordinates including the time axis, the change with time of the influence of the interference wave can be grasped on one screen.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a configuration of a digital modulation signal evaluation apparatus 20 (hereinafter referred to as an evaluation apparatus 20) to which the present invention is applied.

  Similar to the modulation error ratio measuring apparatus of Patent Document 1, the evaluation apparatus 20 calculates a modulation error ratio (MER) for a digital modulation signal output from various devices including a receiver used for terrestrial digital broadcasting. The quality is evaluated using this as an evaluation target value.

  As shown in FIG. 1, the digital modulation signal d (t) to be evaluated is converted into a digital signal sequence D (k) by the A / D converter 21 and input to the quadrature demodulation unit 22. Here, the digital modulation signal d (t) to be evaluated is directly sampled by the A / D converter 21, but the A / D converter is the same as the modulation error ratio measuring apparatus of Patent Document 1. A frequency conversion unit may be provided in front of 21 so that a digital modulation signal (digital broadcast wave) having a higher frequency can be directly input.

  The quadrature demodulator 22 calculates a product of a sine wave local oscillation signal (actually a numerical sequence of its amplitude) having a frequency corresponding to the digital modulation signal d (t) and a digital signal sequence D (k) as a baseband in-phase component. The signal I (k) is output as a product of a signal (actually a numerical sequence of the amplitude) having a phase difference of 90 ° with respect to the local oscillation signal and the digital signal sequence D (k), and the baseband quadrature component signal Q Output as (k).

  Baseband signals I (k) and Q (k) output from the quadrature demodulator 22 are input to the demodulator 30.

  As shown in FIG. 2, the demodulating unit 30 includes a symbol timing detecting unit 31, a frequency error correcting unit 32, a symbol / data demodulating unit 33, and the like, and inputs the baseband signals I (k) and Q (k). On the other hand, symbol timing detection processing, frequency error correction processing, constellation symbol demodulation processing, and data demodulation (decoding) processing for all subcarriers included in the digital modulation signal are performed according to the modulation scheme of the subcarrier. Each is performed, and demodulated signals u (1) to u (N) including constellation symbol information and modulation scheme information for each subcarrier are input to modulation error ratio calculation section 35.

  The modulation error ratio calculation unit 35 receives the demodulated signals u (1) to u (N) for each subcarrier demodulated by the demodulation unit 30, and calculates the modulation error ratios MER (1) to (N) for each subcarrier. It calculates according to the modulation system. This calculation is performed based on Equation (1) in Patent Document 1.

  The modulation error ratios MER (1) to (N) for each subcarrier obtained by the modulation error ratio calculation unit 35 are stored in the memory 36.

  The threshold value setting unit 37 is for setting a threshold value for determining the presence or absence of an interfering wave with respect to the modulation error ratio for each subcarrier calculated by the modulation error ratio calculation unit. However, the automatic setting will be described here.

  That is, the threshold value setting unit 37 includes sub-carriers that are not included in the interference band that is expected to receive interference from the interference wave among the modulation error ratios MER (1) to (N) for each subcarrier stored in the memory. Based on the modulation error ratio of the carrier, a threshold value R for determining the presence / absence of an interfering wave is calculated and set in the determination unit 38 described later.

  Here, if the interfering wave is NTSC analog broadcasting adopted in Japan or the United States, an audio carrier Ca exists from −1.25 MHz with reference to the video carrier Cv, as shown in FIG. It has a channel band of about 6 MHz width up to 5 MHz.

  The video signal employs amplitude modulation with respect to the video carrier Cv, and the audio signal employs FM modulation with respect to the audio carrier Ca. The level of the video carrier Cv and the audio carrier Ca is much higher than that of the video signal component in the entire channel band of this method. It is expected that the influence of the video signal component excluding the carrier is small.

  Therefore, in this case, a band B1 having a predetermined width centered on a frequency higher by 1.25 MHz than the lower limit of the channel band and a band B2 having a predetermined width centered on a frequency higher by 4.5 MHz from the center of the band A, The interference band in which the digitally modulated signal is expected to be disturbed is set as a non-interference band. The widths of the interference bands B1 and B2 are arbitrary, but the band B2 on the voice carrier side to be FM-modulated needs to be set wider than the frequency modulation width.

  Further, the threshold value R is arbitrary as long as the magnitude of the influence of the interference wave can be identified. Here, as an example, it is assumed that the modulation error ratio expected to have a small influence of the interference wave shows a normal distribution, so that it is possible to identify whether the modulation error ratio of the interference bands B1 and B2 is out of this distribution. A threshold value R is determined.

  That is, the average value A and the standard deviation σ of the modulation error ratio for the subcarriers in the non-interference band are obtained, A−kσ (k is an arbitrary coefficient, for example, 3 or 4) is calculated, and this is used as a threshold value. R.

  As described above, the threshold value R is calculated based on the average value A and the standard deviation σ of the modulation error ratio of the subcarriers in the non-interference band, that is, the information that determines the distribution of the modulation error ratio in the non-interference band. Therefore, it is possible to obtain an appropriate threshold value R corresponding to the degree of variation in the modulation error ratio that is not affected by the interference wave, and to accurately determine the presence of the interference wave.

  The determination unit 38 compares the threshold value R calculated and set by the threshold value setting unit 37 with the modulation error ratios of the subcarriers included in the interference bands B1 and B2, and the modulation error ratio is smaller than the threshold value R. Is present, it is determined that there is an interfering wave, and when there is no modulation error ratio smaller than the threshold value R, it is determined that there is no interfering wave, and the determination result is output to the display control unit 41 described later.

  Further, the interference band information calculation unit 39 obtains the average values A1 and A2 of the modulation error ratios of the subcarriers included in the interference bands B1 and B2, and the minimum values MER (L1) and MER (L2).

  The information setting unit 40 provides the threshold setting unit 37, the determination unit 38, and the interference band information calculation unit 39 with information and thresholds necessary for determining an interference band that is expected to be affected by the interference wave. This is for setting information such as the coefficient k necessary for determining the value R. The operation unit (not shown) is used to specify, for example, either the NTSC system or the PAL system, and the coefficient k is set to a value such as 3 or 4. Let it be specified.

  The display control unit 41 uses the frequency characteristics of the modulation error ratios MER (1) to (N) for each subcarrier calculated by the modulation error ratio calculation unit 35, the threshold value obtained by the threshold setting unit 37, The average value A and standard deviation σ of the interference band, the determination result of the determination unit 38, the calculation result of the interference band information calculation unit 39, and the like are displayed on the display 42 in a predetermined layout.

  FIG. 4 shows an example of the display screen. In the center and right range of the screen, the specified interference wave type (in this case, NTSC), determination result (in this case, there is interference), and frequency are on the horizontal axis. Graph of frequency characteristics of modulation error ratio MER, average value A and standard deviation σ of modulation error ratio in non-interference band, average values A1 and A2 and minimum value MER (L1) of modulation error ratios in interference bands B1 and B2, MER (L2), threshold value R (3σ in this case), etc. are displayed on one screen. Further, in the graph of the frequency characteristic of the modulation error ratio MER, the boundary lines of the interference bands B1 and B2, the threshold value R line, and the like are also displayed.

  On the left side of the screen, the measurement conditions set by the information setting unit 40, the measurement channel, the interference wave type, the threshold value (coefficient k), and the like are displayed. Further, various demodulation parameters (hierarchy values and the like) of the demodulation unit 30 may be displayed as measurement conditions.

  In this frequency characteristic graph, a characteristic part having a modulation error ratio smaller than the threshold value R may be displayed so that it can be distinguished from other parts. For example, the color of the portion smaller than the threshold value R is displayed with a different color, or is displayed with a thick line or another line type. In this way, the presence / absence of the interference wave can be grasped more intuitively.

  As described above, the digital modulation signal evaluation apparatus 20 according to the embodiment obtains the modulation error ratio for each demodulated signal demodulated for each subcarrier by the demodulator 30, and the interference error interference is included in the modulation error ratio. Based on the modulation error ratio of subcarriers not included in the interference band expected to be received, a threshold value R for determining the presence or absence of the jamming wave is obtained, and the threshold value R and the subband included in the interference band are determined. The carrier modulation error ratio is compared to determine whether there is an interference wave, and the determination result is displayed.

  For this reason, it can be grasped correctly and easily whether it is influenced by interference waves, such as an analog broadcast wave.

  Further, since the frequency characteristic of the modulation error ratio is displayed in a graph, the degree of the influence of the interference wave can be grasped on the graph.

  The display format of the display device 42 is arbitrary and is not limited to the example of FIG. In addition to the single measurement described above, a modulation error ratio is obtained every predetermined time and is stored in the memory 36 in sequence, and a threshold for the modulation error ratio at each time is determined according to the operation of the operation unit or the like. The calculation process and the determination process may be performed, and the measurement results for each time may be sequentially displayed on the display 42 as described above.

  In addition, as described above, the frequency characteristics of the modulation error ratio for each time may be displayed three-dimensionally on the three-dimensional coordinates including the modulation error ratio axis, the frequency axis, and the time axis as shown in FIG. .

  In the above description, the threshold value setting unit 37 automatically calculates the threshold value R based on the modulation error ratio in the non-interference band. However, a value arbitrarily input by an operation of an operation unit (not shown). May be set as a threshold value in the determination unit 38, the automatic setting mode and the manual setting mode described above may be selected, or the threshold value R obtained in the automatic setting mode may be selected. You may comprise so that it can variably adjust manually.

  Further, when manually setting the threshold value R in this way, in addition to the method of comparing the modulation error ratio for the interference band as described above, the analog broadcast is compared with the modulation error ratio for all subcarriers. The presence of interference waves other than waves may also be grasped.

  In the evaluation device 20, the analog digital modulation signal d (t) is input as an evaluation target. However, when the digitally converted digital modulation signal sequence D (k) is received as the evaluation target digital modulation signal. In the evaluation apparatus 20, the A / D converter 21 can be omitted, and when the baseband signals I (k) and Q (k) are received as the digital modulation signals to be evaluated, the A / D converter 21 The D converter 21 and the orthogonal demodulator 22 can be omitted.

The figure which shows the structure of embodiment of this invention Configuration diagram of the main part of the embodiment NTSC analog broadcast wave transmission band characteristics The figure which shows an example of the display screen of embodiment The figure which shows another example of the display screen of embodiment Diagram showing the configuration of a conventional device

Explanation of symbols

  DESCRIPTION OF SYMBOLS 20 ... Digital modulation signal evaluation apparatus, 21 ... A / D converter, 22 ... Orthogonal demodulation part, 30 ... Demodulation part, 31 ... Symbol timing detection part, 32 ... Frequency error correction part, 33 ... Symbol / data demodulation unit, 35... Modulation error ratio calculation unit, 36... Memory, 37... Threshold setting unit, 38 .. determination unit, 39 .. interference band information calculation unit, 40. 41 …… Display control unit, 42 …… Display

Claims (4)

A demodulator (30) that receives a digital modulation signal of a desired channel and demodulates all subcarriers included in the digital modulation signal;
A modulation error ratio calculation unit (35) for calculating a modulation error ratio for a demodulated signal for each subcarrier demodulated by the demodulation unit;
A threshold setting unit (37) for setting a threshold for determining the presence or absence of an interfering wave with respect to the modulation error ratio for each subcarrier calculated by the modulation error ratio calculating unit;
A determination unit (38) that compares the threshold set by the threshold setting unit with the modulation error ratio for each subcarrier to determine whether there is an interfering wave;
An indicator (42);
A digital modulation signal evaluation apparatus including a display control unit (41) for displaying the determination result of the determination unit on the display ;
The threshold setting unit modulates subcarriers that are not included in an interference band that is expected to receive interference of interference waves among the modulation error ratios for all subcarriers obtained by the modulation error ratio calculation unit. Based on the error ratio, configured to calculate a threshold for determining the presence or absence of jamming waves,
The determination unit compares the threshold calculated and set by the threshold setting unit with a modulation error ratio of subcarriers included in the interference band to determine whether or not there is an interference wave in the interference band. A digital modulation signal evaluation apparatus characterized by comprising: The threshold value setting unit calculates an average value and a standard deviation of a modulation error ratio of subcarriers not included in the interference band, and calculates a threshold value determined by the average value and the standard deviation. The digital modulation signal evaluation apparatus according to claim 1. The display control unit displays a frequency characteristic of a modulation error ratio for each subcarrier calculated by the modulation error ratio calculation unit together with a determination result of the determination unit on the display. Item 3. The digital modulation signal evaluation apparatus according to Item 2. 4. The digital modulation signal evaluation apparatus according to claim 3 , wherein the display control unit displays the frequency characteristic of the modulation error ratio obtained every predetermined time on three-dimensional coordinates including a time axis .

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