JPH0481279B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to an analog-to-digital conversion device, and in particular to an analog-to-digital conversion device capable of reducing phase rotation added to a digital signal by a low-pass filter used in an analog system. It is related to.

BACKGROUND ART In recent years, with the rapid development of electronic technology, there is a tendency for analog signals to be digitally processed.
For example, a digital audio disk is one in which an audio signal is converted into a digital signal and then optically recorded on the surface of the disk, and a digital audio tape is one in which a digital audio signal is recorded on a magnetic tape. When the audio signals digitized in this manner are recorded on a disk or magnetic tape as a recording medium, an audio signal of high sound quality and high dynamic range can be obtained by the respective playback devices.

In this case, when digitally recording an audio signal on the recording medium described above, a microphone or the like is used to convert the acoustic signal into an electrical signal. When this analog audio signal is converted to a digital signal as shown by the envelope of the shaded area in the frequency spectrum diagram in Figure 2, it is within the audible band below f _s /2 with respect to the sampling frequency f _s . In order to prevent aliasing noise from being mixed in, a low-pass filter is used to remove frequency components of f _s /2 or higher. In this way, the analog audio signal from which frequency components of f _s /2 or more have been removed is sampled at the sampling frequency f _s and sequentially converted to a digital signal, and this converted signal is modulated and parity-processed in a signal processing circuit. After processing, etc., the data is digitally recorded onto a disk or magnetic tape as a recording medium.

Furthermore, reproduction of an audio signal digitally recorded on a recording medium into an analog signal is performed through a process reverse to the digital recording process described above.

In the series of digital recording and reproducing processes as described above, the original signal and the reproduced analog audio signal are subject to phase distortion due to the phase rotation of the low-pass filter provided in the recording system and the low-pass filter provided in the reproducing system. It is known that it is difficult to obtain faithful reproduced sound due to the influence of (group delay distortion).

In order to solve this problem, for example,
As shown in Publication No. 140822, the phase distortion of the low-pass filter provided in the recording system, that is, the analog-to-digital converter side, and the low-pass filter provided in the reproduction string, that is, the digital-to-analog converter side, is canceled out. In addition, a digital signal processing method has been considered in which each low-pass filter is configured to have complementary characteristics.

However, this method is effective if the various characteristics of the low-pass filters for the recording system and playback system can be determined and designed in advance in a complete signal processing system for analog-digital-digital-analog signals;
In the case of a signal processing system that uses a recording medium recorded by an unspecified number of digital audio recording devices and reproduced by an unspecified number of digital audio playback devices, various low-pass filters in each of the recording system and the playback system are used. It is extremely difficult to determine and design complementary characteristics in advance, so it is not effective.

Therefore, in a signal processing system that uses a recording medium,
Independently from the playback system, that is, the digital-to-analog converter side, a digital signal that is faithful to the original signal is generated by reducing the phase distortion of the low-pass filter provided in the recording system, that is, the analog-to-digital converter side, and is transferred to the recording medium. need to be recorded.

DISCLOSURE OF THE INVENTION Accordingly, an object of the present invention is to provide an analog-to-digital converter capable of generating a digital signal that is faithful to the original signal. Another object of the present invention is to provide an analog-to-digital converter capable of making the cutoff frequency characteristic of a low-pass filter for removing aliasing noise used in an analog system gentle.

In order to achieve such an object, an analog-to-digital converter according to the present invention includes a low-pass filter that inputs an analog signal, removes high-frequency components, and outputs it, and a sample-and-hold circuit that samples and holds the analog signal from the low-pass filter. , an analog-to-digital conversion circuit that converts the sampled and held analog signal into a digital signal; a digital filter that receives the digital signal and passes a desired frequency component; and a signal processor that processes the digital signal from the digital filter. In the analog-to-digital conversion device comprising a circuit, the digital filter has a phase rotation characteristic opposite to that of the low-pass filter.

Furthermore, the sampling frequency of the sample and hold circuit is made higher than the sampling frequency of the signal processing circuit, and the digital filter has the characteristics of the low-pass filter, thereby making the cut-off frequency characteristics required of the low-pass filter gentle. It is something.

In the analog-to-digital converter configured in this manner, the generated digital signal is not subject to phase rotation, and as a result, a digital signal faithful to the original signal can be generated. When the digital signal generated in this way is recorded on a recording medium and reproduced into an analog signal by a reproduction system, that is, a digital-to-analog converter, the digital signal on the recording medium is faithful to the original sound, so an unspecified number of digital This makes it possible to make the characteristics of the low-pass filter of the analog playback device uniform, which has the effect of making the design of the low-pass filter extremely simple.
In addition, by raising the sampling frequency during analog-to-digital conversion higher than the sampling frequency of the circuit that processes the converted digital signal, and giving the digital filter the characteristics of a low-pass filter, the frequency of the signal component that becomes aliasing noise becomes higher. Since the interval between 1/2 of the sampling frequency _fs is widened, there are various excellent effects such as making it possible to make the cut-off frequency characteristic of the low-pass filter gentle.

BEST MODE FOR CARRYING OUT THE INVENTION FIG. 1 is a block diagram showing an embodiment of an analog-to-digital converter according to the present invention, particularly when applied to a digital audio recording device.

In the figure, 1 is a microphone,
It converts the sound emitted from the sound source into an electrical signal and outputs it. 2 is a line amplifier that amplifies the analog level audio signal output from microphone 1; 3 is a line amplifier that amplifies the analog level audio signal output from microphone 1; and 3 is a line amplifier that amplifies the analog level audio signal outputted from microphone _{1 ;} /
Prevent aliasing noise from occurring in frequency bands below 2.
4 is a sample hold circuit that samples the analog audio signal supplied from the low pass filter 3 _, and its sampling frequency is the same as that of the signal recorded on the recording medium. The sampling frequency f _s is set twice (2f _s ). Reference numeral 5 denotes an analog-to-digital conversion circuit which converts the output signal of the sample hold circuit 4 into a digital signal and outputs the digital signal. Reference numeral 6 denotes a digital filter, which has filter characteristics that allow frequency components of f _s /2 or less to pass, and the low-pass filter 3.
It has a phase rotation characteristic opposite to that of , and operates with a 1/f _s period. A signal processing circuit 7 performs signal processing such as modulation and parity processing on the output signal of the digital filter 6, and then records the digital audio signal A on a recording medium (not shown).

In the digital audio recording device configured in this way, when the microphone 1 detects the sound emitted from a sound source (not shown) and converts it into an analog audio signal, the line amplifier 2 amplifies this signal to a predetermined level. It is supplied to the low pass filter 3. And this low pass filter 3 is
The input signal is supplied to the sample and hold circuit 4 after removing components having a frequency of 3f _s /2 or more. The sample and hold circuit 4 sequentially samples and holds the signal supplied from the low-pass filter 3 at a frequency of 1/ _2fs , and supplies the sampled signal to the analog-to-digital conversion circuit 5. The analog-digital conversion circuit 5 converts the signal supplied from the sample hold circuit 4 at a period of 1/ _2fs into a digital value, and then supplies the digital value to the digital filter 6. Here, the digital filter 6 has a period of 1/
Since the digital signal is operated by f _s , the digital signal of 1/2 f _s period is returned to 1/f _s period, and
A characteristic that allows frequency components of f _s /2 or less to pass,
Since the phase rotation characteristic is set to be opposite to the characteristic of the low-pass filter 3, the phase rotation generated by the low-pass filter 3 is corrected by being canceled in the digital filter 6. become. The output signal of the digital filter 6, which has been corrected for this phase rotation, is subjected to modulation and parity processing in the signal processing circuit 7, and then outputted as a digital audio signal A and recorded on a recording medium. Become. Therefore, the digital audio signal recorded on this recording medium is passed through the low pass filter 3.
This results in a signal that is not subjected to any phase rotation due to this, and as a result, faithful recording of the original sound can be performed.

Furthermore, when the sampling frequency of the sample and hold circuit 4 is _fs , the frequency spectrum is as shown in Fig. 2. In order to eliminate the influence of aliasing noise, only signals of _fs /2 or less are supplied to the sample and hold circuit 4. If this is attempted, the cutoff frequency characteristics of the low-pass filter 3 must be made steep, as shown by the envelope in the shaded area in FIG. 2, and it becomes difficult to realize a filter with stable characteristics. On the other hand, as described with reference to FIGS. 1 and 3, if the sampling frequency of the sample-and-hold circuit 4 is set to 2f _s , the frequency characteristics will become as shown in FIG. 3, ranging from f _s /2 to The period between 3/2 f _s is a no-signal portion. Therefore, since the characteristics of the low-pass filter 3 can be made to have gentle characteristics as shown by the dotted line in FIG. 3, its manufacture becomes extremely easy compared to the conventional method. This means that
The higher the sampling frequency of the sample-and-hold circuit 4, the more noticeable this becomes. For example, when the sampling frequency is set to _4fs , it becomes as shown in the frequency spectrum diagram of FIG. 4.

As explained above, the analog-to-digital converter according to the present invention includes a digital filter on the output side of the analog-to-digital converter circuit, and this digital filter has a phase rotation characteristic that is opposite to the phase rotation characteristic of the low-pass filter used to remove aliasing noise. Since the converted digital signal is given a phase rotation characteristic, the converted digital signal becomes a signal that is faithful to the original signal without phase rotation. When the digital signal generated in this way is recorded on a recording medium and reproduced, that is, reproduced into an analog signal by a digital-to-analog converter, the digital signal on the recording medium is faithful to the original sound, so an unspecified number of digital and analog This makes it possible to make the characteristics of the low-pass filter of the reproducing device uniform, and has the effect that the design of the low-pass filter becomes extremely simple. Additionally, the sampling frequency of the sample-and-hold circuit used to digitize the analog signal is higher than the sampling period of the circuit that uses the converted digital signal, and the digital filter has a low-pass characteristic for eliminating aliasing noise. To,
The cutoff frequency characteristic of the low-pass filter characteristic can be made gentle.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an analog-to-digital converter according to the present invention, and FIGS.
The figure is a frequency spectrum diagram when the sampling frequency is changed. 1... Microphone, 2... Line amplifier,
3...Low pass filter, 4...Sample hold circuit, 5...Analog-digital conversion circuit, 6
...Digital filter, 7...Signal processing circuit.

Claims (1)

[Claims] 1. A low-pass filter that receives an analog signal and removes high-frequency components, a sample-and-hold circuit that samples and holds the analog signal from the low-pass filter, and converts the sample-and-held analog signal into a digital signal. An analog-to-digital conversion device comprising: an analog-to-digital conversion circuit; a digital filter for inputting the digital signal and passing a desired frequency component; and a signal processing circuit for processing the digital signal from the digital filter. An analog-to-digital conversion device characterized in that the low-pass filter has a phase rotation characteristic opposite to that of the low-pass filter. 2. The sampling frequency of the sample-and-hold circuit is made higher than the sampling frequency of the signal circuit, and the digital filter has the characteristics of the low-pass filter, thereby making the cut-off frequency characteristics required of the low-pass filter gentle. An analog-to-digital converter according to claim 1, characterized in that: