JPH06296262A - Digital filter to provide transmission line characteristic - Google Patents

Abstract

PURPOSE:To provide transmission line characteristic to perform transmission with no distortion without using an analog filter of highorder/high performance. CONSTITUTION:A simplified LPF 2 with sufficient attenuation quantity in a frequency >= resampling frequency fs is provided at the front stage of an ADC 3, and over-sampling in a frequency >=2fs is performed by the ADC 3. A digital signal without distortion can be obtained by using a digital filter 4, transmission line characteristic detecting means 5 and a characteristic control means 6 for a digital signal. Therefore, since the digital signal passing the ADC can be generated as the one without distortion by preventing a fold from occurring when the over-sampling is performed, an analog filter of low-order/low price can be used as the one at the front stage of the ADC.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter which realizes a desired transmission line characteristic required for analog transmission of sampled values, and more particularly to a filter which is required to realize the desired transmission line characteristic. The present invention relates to a digital filter that applies a filtered filter characteristic to a resampled digital signal.

[0002]

Hi-vision transmission is given as an example of analog transmission of sample values. High-definition broadcasting system 1
As one, the MUSE method was developed by NHK.
This MUSE system is described in “MUSE-High-Definition Transmission System” by Ninomiya, Institute of Electronics, Information and Communication Engineers, pp. 22-27,4
6 to 53, 184 to 192, sample value transmission is performed, and in order to perform distortion-free transmission, a 10% cosine roll-off characteristic with a cutoff frequency fc = 8.15 MHz is used as a transmission path characteristic. Routes are distributed on the transmitting and receiving sides. Therefore, the input part of the receiver is 8.15M
A high-order, high-performance low-pass filter having a 10% cosine roll-off characteristic of 3 dB down at Hz is provided.

FIG. 6 shows a conventional example of the input section of this high-definition receiver, and the outline of the operation will be described below. Input terminal 1
The analog baseband MUSE signal input from the above passes through the above low-pass filter (LPF) 10, is a signal that satisfies the condition of distortion-free transmission, and is guided to the analog-digital converter (ADC) 11. The ADC 11 resamples at a predetermined timing (sampling frequency fs) to reproduce a distortion-free digital value. This is an ideal case, and in actual broadcasting, the transmission line characteristic does not have the theoretical cosine roll-off characteristic, and therefore the digital signal resampled by the ADC 11 contains the transmission line distortion. Therefore, in the conventional example, an equalizer 12 for equalizing the transmission line distortion is provided after the ADC 11, and a transmission line distortion detector 13 for detecting the transmission line distortion from the signal after the equalizing (waveform equalization) and a detected transmission line distortion Waveform equalization control means 14 for controlling the equalizer 12 so as to correct the transmission line distortion is used to equalize the transmission path distortion, and the digital MUSE signal in which the transmission path distortion is suppressed is guided to the MUSE decoding signal processing means (signal processing) 5. . In signal processing 5, MUS
The signal compressed in the E format is decoded, restored to the original broadband HDTV signal and output.

[0004]

As described above, in the conventional example, the 10% cosine roll-off characteristic of the cut-off frequency of 8.15 MHz, which is routed and provided to the receiver side for realizing the distortion-free transmission line characteristic, is obtained. Low pass filter 2
Is an analog low-pass filter (analog LPF) in front of the ADC 11. Here, in order to realize the characteristics that are route-distributed by the analog LPF, a high-order and high-performance filter configuration is required. Therefore, the use of such an LPF requires a technique for manufacturing and is expensive.

[0005]

In order to solve the above-mentioned problems, an ADC that performs oversampling at twice or more a resampling frequency fs as an ADC and a cutoff frequency fc as an analog low-pass filter before the ADC is used.
At (fc = 1 / 2fs), a low-order low-cost analog low-pass filter having an attenuation characteristic with which a sufficient amount of attenuation can be obtained at the resampling frequency fs is used, and a digital filter is provided after the ADC. This is achieved by subjecting the signal to a filtering process that realizes a desired transmission line characteristic.

[0006]

The signal transmitted by the sample value is filtered by a simple low-pass filter having a low order of the cutoff frequency fc, and the resampling frequency fs (fs = fs
At 2 fc) or more, the signal is sufficiently attenuated. The signal from which the high frequency component has been removed is converted into a digital signal by the ADC that oversamples at twice the resampling frequency fs. In the digital signal oversampled in this way, there is no folding back of harmonic components to low frequencies due to resampling. The digital signal after the ADC is subjected to a filtering process on this digital signal so that it is not distorted. As a result, it is possible to obtain a distortion-free digital signal from the analog signal transmitted as the sample value, so that the signal processing in the subsequent stage can be favorably performed.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention is shown in FIG. 1 and its operation will be described below. The present embodiment is different from the conventional example shown in FIG. 6 in that instead of a high-order analog low-pass filter (LPF) 10 that gives a desired transmission line characteristic, frequency components of a resampling frequency fs or higher are sufficiently attenuated. A simple analog low-pass filter (simple LPF) 2 having a low order that allows the output is oversampled by an analog-to-digital converter (ADC) 3 at 2fs, which is twice the resampling frequency fs. Based on the characteristic detected by the transmission line characteristic detector 5 from the digital signal guided by the digital filter 4 in the subsequent stage, the characteristic control means 6 controls the digital filter 4
The other points are the same as those in the conventional example in that the signal is controlled to obtain a distortion-free digital signal.

The operation of the MUSE transmission system will be described as an example. The analog baseband MUSE signal input from the input terminal 1 is guided to the simple LPF 2. FIG. 2 shows an example of pass band characteristics of the simple LPF 2.
In this example, the Butterworth low pass filter characteristic of order 3 is shown, and the simple LPF 2 has a cutoff frequency fc =
At 8.15MHz, the slope of the attenuation region is 18dB / oct
Becomes As a result, the resampling frequency fs = 1
Most frequency components above 6.2 MHz (fs = 2fc) are removed. The analog signal output from the simple LPF 2 is guided to the ADC 3 and the resampling frequency f
It is oversampled at a frequency 2fs = 32.4 MHz, which is twice the frequency s. The frequency component of the digital signal converted by resampling by the ADC3 is shown in FIG.
As shown in, the aliasing from the harmonic components hardly occurs. This digital signal is guided to the digital filter 4. The digital filter 4 is the ADC 3
The digital signal converted by is filtered and output. The digital signal after this filter processing is guided to the transmission path characteristic detector 5. The transmission line characteristic detector 5 detects the transmission line characteristic from the guided digital signal,
The detection result is output to the characteristic control means 6. The characteristic control means 6 controls the digital filter 4 based on the derived detection result so that it becomes a distortion-free digital signal having a desired characteristic.
To control. The frequency characteristic of the digital filter 4 is expressed by the following equation in order to correct the distortion due to the Butterworth filter characteristic and obtain a 10% cosine roll-off characteristic at 8.15 MHz, when a Butterworth filter of order 3 is used as the simple LPF 2, for example. It becomes a characteristic.

[0009]

[Equation 1]

In this equation, the following equation is a distortion correction term due to the third-order Butterworth filter characteristic.

[0011]

[Equation 2]

The relationship between these filter characteristics is shown in FIG. FIG. 4A shows the Butterworth filter characteristic of the third order, which is filtered by the digital filter having the filter characteristic of FIG.
The distortion-free characteristic of (c) is obtained. In this way, the characteristic control means 6
By controlling the digital filter 4 by the digital filter 4, the digital signal output from the digital filter 4 becomes distortion-free. In this example, there is no distortion in the transmission line,
When there is a transmission line distortion X (f), the characteristic control means 6 also gives the digital filter 4 a characteristic 1 / X (f) for correcting the distortion. Therefore, the digital signal which is not distorted by the digital filter after the ADC becomes MU.
Since it is guided to the SE decoding processing circuit 7, the MUS is favorably performed.
E decoding processing is performed.

As described above, according to the present invention, MUSE
Cutoff frequency required by transmission method 8.15
Digital MUSE without distortion even if a simple low-order low-pass filter is used without using a high-order analog low-pass filter having a 10% cosine roll-off characteristic of MHz.
Since the signal can be obtained, the analog processing unit can be simplified and the cost can be reduced. Although the MUSE transmission method has been described in the description of this embodiment, the present invention is not limited to this.
The present invention is applicable not only to the E transmission method but also to a method of performing sample value transmission. Also, the case of a Butterworth filter of order 3 is taken as an example of the analog low-pass filter, but the present embodiment is not limited to this, and AD for oversampling is used.
A low-pass filter having an attenuation characteristic that does not cause aliasing of harmonic components at C may be used.

FIG. 5 shows another embodiment of the present invention. This embodiment differs from the embodiment shown in FIG. 1 in that a digital low-pass filter 9 is used instead of the digital filter 4, and the transmission line characteristic detector 5 and characteristic control means 6 are omitted. Same as the example. The different points will be described below by taking the case of the MUSE transmission method as an example.

The baseband analog MUSE signal input from the input terminal 1 is filtered by the simple LPF 2 and then converted into a digital signal by the ADC 3 which oversamples at a frequency of 32.4 MHz, as in the embodiment of FIG. . This digital signal is guided to the digital low pass filter 9. The digital low-pass filter 9 has a frequency characteristic in which a digital signal having a distortion due to the characteristics of the simple LPF 2 becomes distortion-free. If, for example, a Butterworth filter of order 3 is used as the simple LPF 2, the characteristic represented by the following equation with the order n = 3 may be the characteristic of the digital low-pass filter 9.

[0016]

[Equation 3]

The digital MUSE signal which has become distortion-free by the digital low-pass filter 9 is guided to the MUSE decoding processing circuit 7 and subjected to MUSE decoding processing. Therefore, since the distortion-free digital MUSE signal is to be decoded, the decoding can be satisfactorily performed.

As described above, the MU is also used in this embodiment.
Cutoff frequency required by SE transmission system 8.
It is possible to obtain a distortion-free digital signal by a digital low-pass filter without using a high-order high-performance analog low-pass filter having a 10% cosine roll-off characteristic of 15 MHz and using a simple low-order low-pass filter. Since MUSE decoding can be performed on
A low-order low-pass filter can be used as the analog filter, and the analog processing unit can be simplified and the cost can be reduced. Further, according to the present embodiment, the distortion-free MUSE digital signal can be obtained only by the digital low-pass filter without using the transmission line characteristic detecting means and the characteristic controlling means, so that the digital signal processing section can be simplified. Becomes Although the MUSE transmission method has been described as an example in the description of the present embodiment, the present invention is not limited to the MUSE transmission method, and the present invention can be applied to a method for transmitting sample values. Also, the case of a Butterworth filter of order 3 has been taken as an example of the analog low-pass filter, but the present embodiment is not limited to this, and has an attenuation characteristic such that the oversampling ADC does not cause aliasing of harmonic components. Any low pass filter will do.

Further, although not shown, in the embodiment of FIG. 5, the digital low pass filter 9 and the signal processing circuit 7 are provided.
An equalizer 12, a transmission line distortion detector 13, and a waveform equalization control means 14 may be provided between the input and output lines to equalize the transmission line distortion. In this case, the transmission line characteristic can be corrected.

[0020]

According to the present invention, the transmission line characteristic for obtaining the distortion-free digital signal required by the transmission system is set to the frequency required as the resampling frequency of 2
Since it can be realized by a digital filter in the latter stage of the ADC that performs oversampling by more than double,
As the analog low-pass filter required in the preceding stage of the DC, it is possible to use a simple and low-cost analog low-pass filter having a sufficient attenuation amount at a sampling frequency fs or higher.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing an example of characteristics of a simple low-pass filter.

FIG. 3 is a diagram showing an example of frequency characteristics of an oversampled digital signal.

FIG. 4 is a characteristic diagram in which an example of filter characteristics is combined.

FIG. 5 is a diagram showing another embodiment of the present invention.

FIG. 6 is a diagram showing a conventional example.

[Explanation of symbols]

1 ... Input terminal, 2 ... Simple analog low-pass filter, 3
... oversampling A / D converter, 4 ... digital filter, 5 ... transmission line characteristic detector, 6 ... characteristic control means,
7 ... Signal processing means, 8 ... Output terminal, 9 ... Digital low-pass filter, 10 ... High-order high-performance analog low-pass filter, 11 ... A / D converter, 12 ... Equalizer, 13 ...
Transmission line distortion detector, 14 ... Waveform equalization control means.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kentaro Teranishi Kentaro Teranishi 292 Yoshida-cho, Totsuka-ku, Yokohama, Kanagawa Stock Company, Hitachi Media Visual Media Laboratory (72) Inventor Yasutaka Tsuru 292 Yoshida-cho, Totsuka-ku, Yokohama, Kanagawa Inside Hitachi Media Media Research Laboratories

Claims (2)

[Claims] 1. An analog filter capable of obtaining sufficient attenuation at a predetermined resampling frequency or higher by filtering an analog signal transmitted as a sample value, and oversampling the filter output at twice or more the predetermined resampling frequency. Analog-digital converter, a variable digital filter that adaptively filters the digital signal converted by the analog-digital converter according to a control signal, and the transmission from the digital signal output from the variable digital filter. A transmission line characteristic detecting unit for detecting a line characteristic and a characteristic control unit for controlling a filter characteristic of the variable digital filter from the characteristic detected by the transmission line characteristic detecting unit so as to obtain a desired transmission line characteristic. However, the characteristics of the digital filter are Digital filter frequency characteristics of the digital signal output from the digital filter realizes the channel characteristics, characterized in that it is controlled to be a desired characteristic. 2. An analog filter capable of obtaining sufficient attenuation at a predetermined resampling frequency or higher by filtering an analog signal transmitted as a sample value, and oversampling the filter output at twice or more the predetermined resampling frequency. And a digital filter for filtering the digital signal converted by the analog-to-digital converter. The characteristics of the digital filter are desired to be the frequency characteristics of the digital signal output from the filter. A digital filter that realizes a transmission line characteristic characterized by having the characteristics of.