JP4100365B2 - AD converter - Google Patents

Description

  The present invention relates to an AD converter that converts an analog signal into a digital signal, and more particularly, to an AD converter that can reduce a high-frequency noise component contained in an analog signal.

Conventionally, as a means for converting an analog signal into a digital signal, an AD converter having the configuration shown in FIG. 6 is well known.
In this figure, an analog input signal 1 is first filtered by an LPF 2 which is a low-pass filter, after an unnecessary high-frequency band is cut off in advance so as not to generate aliasing interference components by sampling at the next stage. Thus, digital conversion is performed by sampling and quantization, and a digital output signal 5 is obtained.

In the case of this configuration, if the analog input signal 1 includes high frequency noise (hereinafter referred to as high frequency noise), there is a problem that the analog input signal 1 is directly converted into a digital signal including high frequency noise.
Therefore, when the analog input signal 1 has a lot of high-frequency noise, there is a means for lowering the cutoff frequency of the LPF 2 and removing the high-frequency noise, and then performing AD conversion. Since the signal component in the high frequency band of the signal 1 is also removed at the same time, there is a drawback that the digital signal has a degraded frequency characteristic.

On the other hand, when converting an analog signal into a digital signal, it is well known that the frequency band of an analog signal that can be transmitted is limited by the sampling frequency. For example, in the case of a so-called compact disc (CD), processing is performed at a sampling frequency of 44.1 kHz, so that the frequency band of the analog signal is limited to 22.05 kHz.
In recent years, many improvement techniques for generating a frequency component of an analog signal of 22.05 kHz or higher by digital signal processing have been disclosed (for example, for example), such as a CD frequency band digitized at the above sampling frequency. Patent Document 1).
This improvement technique improves frequency characteristics from a signal that has been digitized at a sampling frequency determined by the standard, such as the CD, but is included in an analog signal by applying this frequency characteristic improvement technique. No attempt has been made to improve high-frequency noise.
Japanese Patent Laid-Open No. 11-126097

When converting an analog signal containing a lot of high-frequency noise into a digital signal, the analog signal is conventionally converted into a digital signal as it is, or the high-frequency noise is blocked by a low-pass filter and then converted into a digital signal. It was.
In the former case, the high frequency noise is transmitted as it is, and in the latter case, there is a drawback that the frequency characteristics of the analog signal deteriorate.

  The present invention has been made in view of the above-described conventional problems, and its purpose is that when an analog signal containing a lot of high-frequency noise is converted into a digital signal, the frequency characteristic of the analog signal is good. The present invention also provides an AD conversion device that enables conversion into a digital signal that does not contain a high frequency noise component.

The present invention has the following configuration as means for solving the above-mentioned problems.
Ie
In an AD converter that converts an input analog signal supplied as one input signal into a digital signal and outputs the digital signal,
Low-pass filtering means for blocking the high-frequency band of the input analog signal;
AD conversion means for converting the input analog signal whose high frequency band is cut off by the low-pass filtering means into a first digital signal;
High frequency component adding means for generating a second digital signal generated by adding the signal component of the blocked band based on the first digital signal;
Output means for outputting the second digital signal;
With
The low-pass filtering unit detects a spectrum of high-frequency noise included in the input analog signal, and varies a cutoff characteristic of the high-frequency band of the low-pass filtering unit by a control signal corresponding to the spectrum. An AD converter characterized by that.

  A digital signal with good high-frequency characteristics and no high-frequency noise components can be obtained, so if the analog signal is an audio signal, there is no unpleasant noise and AD conversion has good clarity. Is possible. In addition, if the analog signal is a video signal, there is an effect that AD conversion can be performed in which an image with good resolution is obtained without a fine noise on the screen.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of an AD conversion apparatus according to the present invention will be described below with reference to the drawings using preferred examples.
FIG. 1 is a block diagram showing a first embodiment of an AD conversion apparatus according to the present invention, and FIGS. 2, 3, and 4 are block diagrams showing second, third, and fourth embodiments, respectively. . FIG. 5 is a diagram showing a frequency spectrum for explaining the state of signal processing in the AD conversion apparatus of the present invention.

In FIG. 1, an analog input signal 1 containing a lot of high-frequency noise is input to the LPF 2 of the low-pass filter. This LPF 2 is different from the pre-LPF for preventing sampling aliasing commonly used for AD conversion. In addition to the role of the pre-LPF, the LPF 2 blocks the transmission of the frequency band occupied by the high-frequency noise and reduces the high-frequency noise. To be removed.
The analog signal from which the high frequency noise has been removed is converted into a digital signal by the next AD converter 3. At this time, the sampling frequency of the AD conversion unit 3 is set to at least twice the upper limit frequency of the analog signal band to be finally obtained.
The digital signal created by the AD conversion unit 3 is then input to the high frequency component adding unit 4, where analog signal components in a band blocked by the LPF 2 are generated and added by means described later.
In this way, the high frequency component adding unit 4 can remove the high frequency noise component and obtain the digital output signal 5 having a good analog signal band.

Next, the operation of the AD converter will be supplementarily described with reference to the frequency spectrum diagram shown in FIG. Reference numerals in FIG. 1 are referred to.
In FIG. 5, (1) shows the frequency spectrum of the analog input signal 1, and the high frequency band above the dotted line contains a lot of noise.
(2) shows the passband of LPF2. By passing the analog input signal of (1) through the LPF 2 of (2), the signal component indicated by the solid line in (3) passes, and the band of the portion containing a lot of noise indicated by the dotted line is cut off.
(4) is a spectrum in a state in which the signal component of (3) is converted into a digital signal by the AD converter 3 at the sampling frequency fs. A portion indicated by a dotted line is a folded component by digitalization.
(5) shows the spectrum of the digital signal in the state where the high frequency band of the analog signal component is expanded by the operation of the high frequency component adding unit 4 of the digital signal of (4).
In this manner, the AD conversion apparatus of the present invention can obtain a digital output signal in which a high frequency band is well secured while blocking high frequency noise of an analog input signal.

Next, the high frequency component adding unit 4 will be described. The operation of this part is to shape the waveform of the analog signal component band-limited by the LPF 2 on the digital signal and emphasize or add harmonics to widen the band.
This specific means has already been actively researched. For example, Japanese Patent Laid-Open No. 5-127672 discloses a technique for obtaining a non-linear waveform using a conversion table, and Japanese Patent Laid-Open No. 7-175478 discloses. Further, a technique for forming a complex nonlinear waveform by adding a differential operation is disclosed.
The inventor of the present invention discloses a novel technique that has a smaller circuit scale and costs less than the conventional technique in Patent Document 1 regarding the technique for the same purpose.
Thus, any of these existing technologies can be used as a specific means of the high-frequency component addition unit 4. For the sake of simplification, detailed operation description of this part is omitted.

FIG. 2 is a block diagram showing a second embodiment of the present invention. In this embodiment, the low-pass filter is switched according to the frequency component of the high-frequency noise included in the analog input signal.
In FIG. 2, LPF (1) 2-1 to LPF (n) 2-n are low-pass filters having different cutoff frequencies, and are selected by the switch 6. A switch control signal is supplied to the switch 6 from the outside.
In this embodiment, for example, when the analog input signal 1 is an audio signal and is a so-called hiss noise when it is reproduced as a high-frequency noise by a tape player, and the situation of the hiss noise varies depending on the player and the tape speed. It can be optimized when used for the above.

FIG. 3 is a block diagram showing a third embodiment of the present invention. In this embodiment, the low-pass filter is automatically switched according to the frequency component of the high-frequency noise included in the analog input signal.
In FIG. 3, the cutoff characteristic variable LPF 7 can continuously change the cutoff frequency of the LPF by a control signal supplied from the outside. As this control signal, the spectrum of the high-frequency noise included in the analog input signal 1 is detected, and the lowest high-frequency noise frequency is set as the cutoff frequency of the cutoff characteristic variable LPF 7.

FIG. 4 is a block diagram showing a fourth embodiment of the present invention. In the present embodiment, unlike the configurations of the first to third embodiments, the low-pass filter is not placed in front of the AD conversion unit, and a low-pass filtering operation is performed on the digital signal after AD conversion.
In FIG. 4, the analog input signal 1 is sampled by the oversampling AD converter 8 at a frequency higher than the normal sampling frequency to become a digital signal.
Next, this digital signal is converted to a predetermined sampling frequency by the decimation filter + LPF unit 9 and at the same time, the high frequency noise component is cut off. The subsequent configuration is the same as in the previous embodiments.
In this embodiment, since the function of the low-pass filter can be configured on the digital signal, there is an advantage that a highly accurate cutoff characteristic can be obtained.
As in the second and third embodiments, the cutoff frequency of the low-pass filter of the decimation filter + LPF unit 9 may be varied so as to correspond to various high-frequency noise distributions.

1 is a block diagram showing a first embodiment of an AD conversion apparatus according to the present invention. It is a block diagram which shows the 2nd Example of the AD converter device which concerns on this invention. It is a block diagram which shows the 3rd Example of the AD converter device which concerns on this invention. It is a block diagram which shows the 4th Example of the AD converter concerning this invention. It is a frequency spectrum diagram of each part signal in the AD converter of the present invention. It is a block diagram which shows the conventional AD converter.

Explanation of symbols

1: Analog input signal 2: LPF (low-pass filter)
2-1 to 2-n: LPF (1) to LPF (n)
3: AD conversion unit 4: High frequency component addition unit 5: Digital output signal 6: Switch 7: Cutoff characteristic variable LPF
8: Oversampling AD conversion unit 9: Decimation filter + LPF unit

Claims (1)

In an AD converter that converts an input analog signal supplied as one input signal into a digital signal and outputs the digital signal,
Low-pass filtering means for blocking the high-frequency band of the input analog signal;
AD conversion means for converting the input analog signal whose high frequency band is cut off by the low-pass filtering means into a first digital signal;
High frequency component adding means for generating a second digital signal generated by adding the signal component of the blocked band based on the first digital signal;
Output means for outputting the second digital signal;
With
The low-pass filtering unit detects a spectrum of high-frequency noise included in the input analog signal, and varies a cutoff characteristic of the high-frequency band of the low-pass filtering unit by a control signal corresponding to the spectrum. An AD converter characterized by that.

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