JPS6229219A - Sampling circuit for analog signal - Google Patents

Abstract

PURPOSE:To improve the group delay distortion in a low pass filter by providing an A/D conversion circuit applying sampling at a specific sampling frequency, a digital filter of specific linear phase and a thinning circuit. CONSTITUTION:The sampling frequency in the A/D conversion circuit 2 is set to a frequency 2fs, being n-time, e.g., twice of a sampling frequency fs desired to be obtained finally and its output is fed to a digital filter 3. The filter 3 has no group delay distortion, acts like a low pass filter function only and has a steep cut-off characteristic at fs/2 at a band of frequencies 0-fs. Thus, the output has a component below the frequency fs/2 and a foldoner component at + or -fs/2 of a frequency being an integral number of times of the frequency 2fs. The digital signal is fed to a thinning circuit 4 and switched by the frequency fs, then the signal is thinned by n-1=1 every n+2 and the foldoner component is given at locations being an integral number of multiple of the frequency fs. Thus, the group delay distortion is improved remarkably.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an analog signal sampling circuit for sampling acoustic signals and the like.

[Prior Art] Conventionally, when sampling audio signals in digital audio such as compact discs, in order to prevent aliasing distortion caused by sampling, a cutoff of less than half of the sampling frequency fs (generally 44.1 KHz) has been used. By passing through an analog low-pass filter 6 (shown in Figure 7) having a frequency f
High frequency components of s/2 or higher are removed. Then, it is sampled at the sampling frequency fs by the synchronization signal of frequency fs from the synchronization signal generation circuit 8, and this sample value is transferred to the A/D.
A conversion circuit 7 converts it into a digital signal.

8A shows the frequency spectrum of the acoustic signal supplied to the input terminal p, FIG. 8B shows the attenuation characteristic of the analog low-pass filter 6, and FIG. 8C shows the output signal of the analog low-pass filter 6. The frequency spectrum is shown, and the eighth diagram shows the A/D
The frequency spectrum of the digital signal from the conversion circuit 7 is shown.

Problems to be Solved by the Invention C In general, the band of the input analog signal is treated as 20 KHz or less, so the sampling frequency is set to 44.1 KHz or 48 KHz so as to satisfy the sampling theorem. Therefore, the cutoff frequency of the analog low-pass filter is around 20 KHz, and in order to completely remove aliasing noise, a high-order filter with steep attenuation characteristics (for example, a 9th to 13th order active filter) is required. .

However, such high-order analog filters have the disadvantage that the phase characteristics of the passband near the cutoff frequency are nonlinear, resulting in so-called group delay distortion.

That is, in the case of an analog low-pass filter having a steep attenuation characteristic as shown in FIG. 10, its cutoff frequency f s / 2
In the vicinity, the group delay time suddenly increases as shown in FIG. Due to this distortion, the digital signal after A/D conversion is recorded on the recording medium while being distorted, and this distortion is also reproduced as it is during playback, causing deterioration in sound quality.

The present invention aims to significantly improve group delay distortion in a low-pass filter.

[Means for Solving the Problems] The present invention provides a sampling circuit that obtains a digital signal with a sampling frequency fs. Using an A/D conversion circuit that converts the output signal into an input signal, a linear phase digital filter with a sampling frequency of n-fs and a cutoff frequency of fs/2 or less is provided, and the output data of this digital filter is divided into n pieces. A decimation circuit is provided to decimate (n-1) numbers for each time.

[Embodiment] In FIG. 1, reference numeral 1 denotes an analog low-pass filter, and its input terminal p is supplied with an analog signal having a relatively wide frequency spectrum as shown in FIG. 2A.

Further, the cutoff frequency of the analog low-pass filter 1 is lower than the sampling frequency fs, and its attenuation characteristic is extremely gentle as shown in FIG. 2B.

Since this attenuation characteristic is gentle, group delay distortion can be improved, and the reason why the attenuation characteristic is made so gentle will become clear from the following explanation.

2 is an A/D converter circuit with a sampling frequency of 2 fs, and 3 is a low-pass linear phase digital filter with a sampling frequency of 2 fs and a cutoff frequency of f s / 2, the attenuation characteristics of which are shown in FIG. 2E. Reference numeral 4 denotes a thinning circuit that thins out every other piece of output data from the digital filter 3. 5 is a synchronization signal generation circuit that generates clock pulses of frequencies fs and 2fS.

FIG. 3 shows an example of a specific configuration of the digital filter 3 and the thinning circuit 4. In the same figure, T-
T is a delay element of time T (T-1/2fs), a t −
a N is a multiplier with filter coefficient a, (i −1 to N), and A is an adder. The decimation circuit 4 consists of a switch S that is opened and closed by a clock pulse of frequency fs.

Next, the operation will be explained. In this example, the sampling frequency in the A/D conversion circuit 2 is set to a frequency 2fs, which is twice the sampling frequency fs that is desired to be finally obtained, and this point is a feature of the present invention. That is, it can be seen that the band of the input signal to the A/D conversion circuit 2 should be limited to the frequency fs or less based on the sampling theorem. Moreover,
Since the analog signal we ultimately want to retain is less than fs/2, the cutoff characteristic of analog low-pass filter 1 is to attenuate gently in a wide band between frequencies fs/2 and fs, as shown in Figure 2B. This eliminates the need for steep cutoff characteristics as in the past. Therefore, group delay distortion near f s /2 can be improved.

Now, the output from the A/D conversion circuit 2 is converted into a digital signal of sampling frequency fs as follows. First, the frequency spectrum of the output from the analog low-pass filter 1 is as shown in Figure 2C, and the frequency spectrum of the output from the A/D conversion circuit 2 is at an integer multiple of the frequency 2fs, as shown in Figure 2. It has a shape that includes folded components.

The digital signal with a period of 1/2 fs from this A/D conversion circuit 2 is supplied to the digital filter 3, and all the digital signals are calculated by the configuration shown in FIG. Therefore, it is filtered. Since this digital filter 3 is of a linear phase finite impulse response type, it has no group delay distortion at all and only functions as a low-pass filter. As shown in FIG. 2E, the digital filter 3 has a sharp cutoff characteristic at fs/2 in the frequency band from 0 to fs. Therefore, as shown in FIG. 2F, the output of the digital filter 3 contains components with frequencies below fs/2 and frequencies fs/2 before and after frequencies that are integral multiples of the frequency 2fs.
It becomes a digital signal with a aliasing component at 2.

This digital signal is supplied to the decimation circuit 4, and since the decimation circuit 4 is switched at the frequency fs as shown in FIG. A digital signal is generated. In other words, a digital signal sampled at the desired frequency fs is finally obtained. Figure 2G shows the frequency spectrum of this digital signal.
It is. As we will see, this digital signal has aliasing components at integral multiples of the frequency fs, which indicates that it was sampled at a frequency of 1ifs without aliasing distortion. ing.

As described above, since a low-pass filter having a slow cutoff characteristic can be used, group delay distortion can be significantly improved. In other words, instead of the conventional one with a steep cutoff characteristic as shown in FIG. 10, one with a gentle cutoff characteristic as shown in FIG. It becomes flat at f s / 2 or less, and group delay distortion is improved.

In the above embodiment, after all the digital signals with a frequency of 2 fs from the A/D conversion circuit are calculated in the digital filter, they are thinned out every other signal.
Actually, there is no need to perform calculations on the digital signals to be thinned out. Therefore, FIG. 4 shows an example in which the thinning operation and calculation are performed simultaneously, and unnecessary calculation operations are omitted.

In the figure, the digital filter has a switching circuit S-8, which is opened and closed every 1/2 fs. Therefore, the digital signal of each delay element T is multiplied by 1 every time 1/fs, and the addition result is output. The result of this addition is the same as the output of the thinning circuit in the previous embodiment.

In this example, the number of multiplications is half that of the previous example.

Note that in each of the two examples, the sampling frequency in the A/D conversion circuit and digital filter was set to twice the final required sampling frequency, but this is an integer multiple of twice or more. If so, you can set it to any number of times. When set to n times, the digital signals from the digital filter are thinned out every n (n-1).

[Effects of the Invention] According to the present invention, since the analog low-pass filter before the A/D conversion circuit is not required to have a steep cutoff characteristic, it is possible to use one with a slow cutoff characteristic, and the group delay distortion is significantly improved. be able to. Furthermore, since the order of the analog low-pass filter can be reduced, for example, in an active filter, the number of stages of operational amplifiers can be reduced, so that the configuration can be simplified.

Furthermore, since the finally determined band-limiting characteristics depend on the digital filter, the characteristics do not change due to the temperature characteristics of the element, aging, etc.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing an embodiment of the present invention, Figure 2 is an explanatory diagram showing the characteristics and frequency spectrum of each part in Figure 1, and Figure 3 is a specific example of the main parts in Figure 1. 4 is a circuit diagram showing the main parts of another example, FIG. 5 is a characteristic diagram showing the group delay characteristics according to the present invention, and FIG. 6 is a diagram showing the cutoff characteristics of the analog low-pass filter according to the present invention. A characteristic diagram showing an example, Fig. 7 is a block diagram showing an example of a conventional configuration, Fig. 8 is an explanatory diagram showing the characteristics and frequency spectrum of each part of Fig. 7, and Fig. 9 is a group delay in the conventional configuration. FIG. 10 is a characteristic diagram showing the cutoff characteristics of a conventional analog low-pass filter. 1... Analog low-pass filter 2... A/D conversion circuit 3... Digital filter 4... Thinning circuit T-T... Delay elements a1 to aN... Multiplier A... Addition Device S...Switching circuit Figure 1 Φ 3rd FI! J Fig. 2 fs/2 fs t5fs 2f
s 2.5fszus/2fs1.5fs2fs2
．． 5fsfs/2 fs 1.5fs
2fs 2.5fs Fig. 4 Fig. 7

Claims (1)

[Claims] In a sampling circuit that samples an analog signal into a digital signal with a sampling frequency fs, there is an analog low-pass filter whose cut-off frequency is less than or equal to the sampling frequency fs, and a sampling frequency n that receives the output signal of this analog low-pass filter as an input.・f
s (n; integer) and a low-pass linear phase digital signal that uses the output signal of this A/D conversion circuit as input and has a sampling frequency of n fs and a cutoff frequency of fs/2 or less. An analog signal sampling circuit comprising a filter and a thinning circuit that thins out (n-1) output data of the digital filter every n pieces.