JPH03181228A - A/d converter - Google Patents

Abstract

PURPOSE:To attain A/D conversion with high accuracy by outputting the output of a digital low pass filter having its improved quantizing frequency when an input signal is equal to a low frequency signal and then outputting the unfiltered A/D conversion result with the inclusion of high frequency signals larger than the prescribed number. CONSTITUTION:It is decided whether a high frequency component higher than a prescribed frequency is included in an input signal or not based on the difference between the output obtained through a digital los pass filter 2 and the direct output of an A/D converter 1 which is not obtained through the filter 2. When the high frequency component is lower than a prescribed level, the output of the filter 2 having its improved quantizing accuracy is selected. Meanwhile the unfiltered output of the converter 1 is selected and outputted when the high frequency component is higher than the prescribed level.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an apparatus for converting an analog signal into a digital signal.

(Summary of the Invention) The present invention solves the problem of generation of false contour noise when converting an analog signal to a digital signal, especially when the low frequency component of the analog signal has a low frequency component. This device is used to convert problematic video signals, etc., to a device that performs high-precision A-D conversion, and for low-frequency signals below a predetermined frequency, the A-D conversion output is passed through a digital filter to convert the equivalent signal. Taking advantage of the fact that the number of quantization bits can be improved,
A device that selects and outputs the output of the filter when the human analog signal does not contain a high frequency component higher than a predetermined frequency than a predetermined value, and selects and outputs the output that does not pass through the filter when the high frequency component contains a predetermined value or more. be.

(Prior Art) In recent years, many video devices are performing digital signal processing.

However, the human input signals in these video devices are usually analog signals, so when digitizing the circuit, A
-D converter is required, and this function may be problematic.

By the way, if the signal to be handled is a video signal, the required A-
The number of quantization bits of the D converter may be about 8 to 10 bits, which is less than that for audio signals (usually 16 bits are required), but the conversion speed is on the contrary, the number of quantization bits for audio signals is several hundred. You'll need twice as much.

Therefore, as shown in FIG. 3, two types of reference voltages, VCCIVE and
20 resistors 33 (n is the number of quantization bits) are connected between the input terminals 31 and 32 of the input terminal 31, and the voltage divided by the resistors 33 and the analog signal input from the input terminal 34 are connected by 2' comparators 35. A so-called flash type A-D converter is generally used, which compares the data, encodes the result using an encoder 36, and outputs the result as an n-bit digital signal to an output terminal 37.

By the way, with this flash type A-D converter,
As is clear from the above explanation, each time the required number of quantization bits n increases by 1, the number of required comparators doubles. It is necessary to keep it to

On the other hand, as mentioned above, in ordinary video equipment, it is said that a quantization bit number of 8 can provide a reasonably satisfactory image quality, and for this reason, video A-D converters, which are currently popular as general-purpose products, The standard is an 8-bit type.

However, in broadcasting equipment that requires the highest quality video, especially television cameras that require gamma correction, 8 bits often lacks quantization precision.

In such cases, an A-D converter with 9 bits or more is usually required, but using these A-D converters, which are not very general-purpose, may cause problems in terms of cost, size, power, etc. many. Similar cases also occur in fields other than video signals.

For this reason, a method such as that shown in Japanese Patent Laid-Open No. 127883/1983 can be considered as a method of using an A-D converter with a small number of bits to carve into low frequency components and achieve high bit accuracy. ing.

To briefly explain this method using FIG.
upF)n'), the extension is a low pass filter (LPF)
43, the small amplitude signal (noise component) of the output of HPF 42 is clipped by clipping circuit 44, and added to the output of LPF 43 whose bit precision has been improved by addition circuit 45 to output.

However, using this method has the disadvantage that high-frequency signal components with small amplitudes are uniformly attenuated by a certain level due to the influence of the clipping circuit 44, and when the signal to be processed is a video signal, the resolution of the high-frequency signal components is This results in the deterioration of the

(Problems to be Solved by the Invention) As described above, A-D conversion accuracy is mainly required for video signals, etc. when capturing an image of a subject with a gentle slope and changing light intensity. In many cases, this is true for human-powered signals that contain many low-frequency components.
In conventional technology, in order to increase the quantization accuracy for low-frequency signal components, the number of quantization bits in the A-D converter must be increased, or the quantization accuracy can be increased using the configuration shown in Figure 4 above. However, the problem of increasing costs and deteriorating the resolution of high-frequency signal components such as in the case of decoctions could not be solved.

The purpose of the present invention is to solve these drawbacks and realize an A-D converter that is low cost, low power consumption, and can obtain high quantization accuracy by quantizing low frequency signal components, especially below a predetermined frequency. That is.

(Means for Solving the Problem) In order to achieve the above object, the present invention provides an n-bit A-D
a first digital low-pass filter that smoothes the digital signal of the converter output, extends the bit length to n+1 or more, and outputs the digital signal; and the output of the A-D converter and the output of the first digital low-pass filter; The second low-pass filter has a wider passband than the first low-pass filter. Third. means for comparing the outputs of the digital low-pass filters with the outputs of the digital low-pass filters;
．． Third. The configuration includes means for selecting and outputting one of the outputs of the digital low-pass filters.

(Effect) No. 51! l, an analog signal waveform A whose value changes with a gentle slope (does not include high frequency components), and this signal A-
Waveform B shows a D-converted digital signal as an analog image, and waveform C shows this signal passed through a digital low-pass filter.The basic operation of a digital low-pass filter is to use n bits sent at regular intervals. The digital signal is weighted averaged over a certain period on the time axis, and the quantization error is also averaged, and the quantization precision of this output is usually equivalent to n+1 bits or more.

For example, the A-D conversion sampling frequency in commonly used digital video equipment is often about 14 to 18 MHz, but the band of this A-D conversion output is reduced to 1/8 of the sampling frequency using a digital filter. When the frequency is limited to about 2 MHz, which corresponds to 1/32, the equivalent quantization precision of the output of this filter increases to approximately n+1 bits, and when it is limited to about 0.5 MHz, which corresponds to 1/32, it improves to n+2 bits.

In other words, for low frequency signal components, by filtering the A-D conversion result with a digital filter, an n-bit A-D converter is used to convert the A-D conversion result to n+1 bits or more.
It becomes possible to obtain a digital signal output with quantization precision equivalent to that obtained using a converter.

The present invention determines whether a human signal contains a high frequency component higher than a predetermined frequency based on the difference between the output passed through a digital filter and the direct output of an A-D converter that does not pass through the digital filter, and determines whether the human signal contains a high frequency component higher than a predetermined frequency. In the following cases, a digital filter output with improved quantization accuracy is selected, and when the high frequency component is above a predetermined value, filtering is not performed.
By selecting and outputting the output of the converter, there is no resolution degradation and high quantization accuracy for low frequency signals.
It became possible to obtain D conversion results.

(Embodiment) FIG. 1 shows a first embodiment of the present invention, in which the analog human input signal is converted into a digital signal by an AD converter l. This output is divided into two parts, one of which is sent to a digital low pass filter (LPF) 2.

An example of this LPF 2 is shown in FIG. 6, and its operation will be briefly explained below.

The digital human input signal is sequentially delayed by unit time delay elements 60-63. The output of each delay element 60-63 and the digital input signal are sent to multipliers 64-68, each multiplied by a coefficient of 0 to 4, and added by an adder 69 to provide an output. Normally, the coefficient KO- is set to 4, which becomes 1 when added together, and as a result, the output of each multiplier 64 to 68 has a value below the decimal point.

Therefore, if we add this without rounding down, this LPF2
The output will have decimal places. That is,
If the input signal is n bits, this output signal is equivalent to becoming a digital signal of n+1 bits or more. Also,
As mentioned above, this is the result of di-weighted averaging, and the quantization errors are also averaged, so the data below the decimal point also has meaning.

As described above, the output of the A-D converter 1 is passed through the LPF 2 and output as data whose bit length has been expanded by one or more bits.

On the other hand, the output of the A-D converter 1 is also sent to a delay circuit (DL) 3, and the time is adjusted according to the amount of delay of the LPF 2. This output is then compared with the output of the LPF 2 by the comparison circuit 4, and the difference between the two is sent to the determination circuit 5. here,
The difference between the output of LPF2 and the output of DL3 is nothing but the amount of high frequency components contained in the input signal.

Therefore, in the present invention, when the determination circuit 5 determines that the difference between the two is less than a predetermined level, that is, the high frequency component of the human signal having a predetermined frequency or more is less than a predetermined value, the electronic switch 6 is changed to two LPFs (as shown by the solid line). Switching, data whose bit length has been expanded by LPF2 is output, and conversely, when it is determined that the high frequency component is greater than a predetermined value, switching to DL3 example (as shown by the dotted line), A-D converter 1 that does not pass through LPF2 The delayed output signal of is output.

Through the above-described operation, the A-D converter of the present invention:
When the input signal is a low frequency signal, a digital low-pass filter output with improved quantization accuracy is output, and when the high frequency signal is distorted by a predetermined value or more, the quantization accuracy remains the same as before, but there is no deterioration of high frequency components.A -D conversion results can be output as is.

Next, FIG. 2 shows a second embodiment of the present invention.

In this example, the output of the A-D converter 1 is passed through a first digital low-pass filter (LPF) 7 with a narrow passband, that is, a low cutoff frequency, a second LPF 8 with a higher cutoff frequency, and a second LPF 8 with a higher cutoff frequency. Send to delay circuit (DL) 3,
These three outputs are compared by a comparison circuit 9. As mentioned above, since the digital filter takes a weighted average of the input signal on the time axis, generally the cutoff frequency is the same as that of the output of DL3 in the comparator circuit 9 and the first .
The difference between the second LPF 7.8 and the second LPF 7.8 is taken and sent to the determination circuit 10. When the difference between the output of DL3 and the output of the first LPF 7 is below a predetermined level, the electronic switch 11
When this difference exceeds a predetermined level and the difference between the output of DL3 and the output of the second LPF 8 is less than a predetermined level, select the output of the second LPF 8. Otherwise, select the output of the second LPF 8. This allows the output of DL3 to be selected. In this explanation, the condition for selecting the output of the first LPF 7 is when the difference between the output of the DL3 and the output of the first LPF 7 is below a predetermined level. 1. In some cases, it may be better to set the value when the differences between the outputs of the second LPFs 7 and 8 are both below a predetermined level.

In this way, it is possible to realize an apparatus that outputs a highly accurate A-D conversion result in accordance with the high frequency components contained in the human input signal with even more stripes than in the first embodiment.

It is also possible to increase the total number of digital low-pass filters in the same way, and even increase the number of digital low-pass filters. The effects of the present invention can also be obtained by using a digital low-pass filter whose cutoff frequency is set to the upper limit of the frequency band of the required signal in place of DL3 in the second embodiment.

(Effects of the Invention) As described above, by utilizing the present invention, a low-cost, low-power consumption, and substantially high-precision A-D conversion device can be realized, and its effects are significant.

[Brief explanation of drawings]

FIGS. 1 and 2 show the first embodiment of the present invention. A block diagram showing the second embodiment, FIG. 3 is a diagram explaining the configuration of a general A-D converter, FIG. 4 is a block diagram showing a conventional example, and FIG. 5 is an analog signal that does not have high frequency components. 6th diagram illustrating how bit precision improves in a signal obtained by A-D converting a signal.
The figure is a block diagram showing an example of a digital low-pass filter. 1: A-D converter, 2.7.8: Digital low pass filter (LPF). 3: Delay circuit (DL). 4.9: Comparison circuit. 5.10: Judgment circuit. 6.1 1: Electronic switch. 1st 4th figure 5 times (A) A7c) 7 "Enp - / Register b■

Claims (1)

[Claims] 1. A digital low-pass filter with a predetermined cutoff frequency that limits and smoothes the frequency band of the A-D converted digital signal output; A means for detecting a level difference between the level difference between the level difference and the output passed through the digital low pass filter, and when this level difference is less than a predetermined value, the output passed through the digital low pass filter is selected; When the value is greater than the above value, the above digital low-pass filter is not passed.
1. An A/D converter comprising means for selecting and outputting a D-converted digital signal output. 2. A plurality of digital low-pass filters provided in parallel, each having a different cutoff frequency, to limit and smooth the frequency band of the A-D converted digital signal output, and the above-mentioned A-D converted digital signal output and the above-mentioned plurality of A means for detecting a level difference between the outputs of the respective outputs passed through the digital low-pass filters, and a means for detecting the level differences between the outputs of the A-D converted digital signals that are not passed through the digital low-pass filters or the outputs of the plurality of above-mentioned signals according to these level differences. Any one of the respective outputs passed through a digital low-pass filter
An A/D converter comprising means for selecting and outputting two outputs.