JPH06291659A - A/d converter - Google Patents

Abstract

PURPOSE:To perform A/D conversion with substantially high precision by using an A/D converter which has low precision. CONSTITUTION:A de-emphasis circuit 1 de-emphasizes the high frequency characteristics of an analog signal and a delay circuit 1 delays the signal. Then a difference circuit 3 calculates the difference between the output of the de- emphasis circuit 1 and the output of a delay circuit 2 and the A/D converter 4 performs A/D conversion. Then the A/D-converted signal is integrated by a digital integration circuit 5 and a digital emphasis circuit 6 which has characteristics reverse to those of the de-emphasis circuit 1 emphasizes its high frequency characteristics.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an A / D converter suitable for use in converting an analog signal such as a video signal or an audio signal into a digital signal.

[0002]

2. Description of the Related Art Conventionally, a so-called flash type is generally known (used) as an A / D converter for converting a video signal as an analog signal into a digital signal. For example, an 8-bit precision flash A / D converter has 256 (= 2 ⁸ ) comparators, samples and holds an input analog signal, and compares it with 256 different thresholds S _{0 to} S _255. To do. The sampled and held level, the threshold value S _i to _{S i + 1 (i = 0,1} , ···, 255, however, the threshold S ₂₅₆ is, for example, infinity) when a level in the range of , And is converted into an 8-bit digital signal of level i, for example, and output.

[0003]

However, in this flash type A / D converter, when the bit precision is increased, the number of required comparators is increased and the circuit scale is increased. Therefore, considering the practicality, the bit precision of the flash A / D converter is limited to at most 8 bit precision.

Therefore, as a method of realizing a more accurate A / D converter, there is a method of repeating the A / D conversion processing a plurality of times, but this method has a problem that the circuit configuration becomes complicated.

Further, as a method for realizing a highly accurate A / D converter, for example, as described in Japanese Patent Laid-Open No. 3-114378, the difference between pixel signals as input analog signals is A / D converted. There is a way.

That is, by taking advantage of the fact that the level change of adjacent pixel signals is usually gentle, the difference between adjacent pixel signals is calculated to reduce the dynamic range of the signal to be A / D converted. There is a method of A / D converting the difference to substantially improve the bit precision.

However, in this method, when the levels of adjacent pixel signals change drastically, that is, when the image contains many high frequency components, even if the difference is taken, the dynamic range is hardly reduced (or remains unchanged), There was a problem that A / D conversion could not be performed accurately.

The present invention has been made in view of the above circumstances, and is intended to enable highly accurate and accurate A / D conversion without making the circuit complicated and large. is there.

[0009]

A / D according to claim 1
The converter is an A / D converter that converts an input analog signal into a digital signal, de-emphasizes the high frequency characteristics of the analog signal, and calculates the difference between those that are temporally close to each other. It is characterized in that it is A / D-converted to be converted into a digital signal, the digital signal is integrated, and the frequency characteristic in the high frequency band is emphasized.

An A / D converter according to a second aspect of the present invention provides a de-emphasis circuit 1 as a de-emphasis means for de-emphasising the high frequency characteristics of an input analog signal, and an output of the de-emphasis circuit 1. A delay circuit 2 as a delay means for delaying by a clock, a difference circuit 3 as a difference calculation means for calculating a difference between the output of the de-emphasis circuit 1 and the output of the delay circuit 2.
And an A / D converter 4 as A / D conversion means for A / D converting the output of the difference circuit 3 and the output of the A / D converter 4 delayed by one clock, and the delayed output and A / D
A digital integrator circuit 5 as an integrator that integrates the output of the A / D converter 4 by adding the output of the D converter 4, and a de-emphasis circuit that emphasizes the high frequency characteristics of the output of the digital integrator circuit 5. The digital emphasis circuit 6 as an emphasis means having a characteristic opposite to the de-emphasis characteristic of No. 1 is provided.

[0011]

In the A / D converter of the present invention, the high frequency characteristics of the analog signal are de-emphasized, and the difference between the temporally adjacent ones is A / D converted. Then, after integrating the A / D converted signal,
Emphasis the frequency characteristics of the high frequency range. Therefore, the dynamic range of the difference between analog signals that are temporally close to each other is reduced by the amount of de-emphasis, so that substantially accurate A / D conversion can be performed.

[0012]

1 is a block diagram showing the configuration of an embodiment of an A / D converter according to the present invention. De-emphasis circuit 1
For example, as shown in FIG. 2, one end of a resistor R ₁ is connected to one end of a series circuit in which a capacitor C and a resistor R ₂ are connected in series, and the other end of the series circuit, that is, a resistor R ₂ , The transfer function H1 (s) (s is Laplace) where one end not connected to the capacitor C is grounded, the other end of the resistor R ₁ is an input terminal, and the connection point between the resistor R ₁ and the capacitor C is an output terminal. Variable) is configured as a low-pass filter represented by the formula H1 (s) = (1 + Ts) / (1 + 8Ts) (1) where T = CR ₂ = C (R ₁ + R ₂ ) / 8, and is input. De-emphasis the high frequency characteristics (f characteristic) of the analog signal (remove the high frequency characteristic).

The delay circuit 2 is, for example, a so-called sample and hold circuit, which is equivalent to one sampling clock of the A / D converter 4 and has a high frequency f in the de-emphasis circuit 1.
Delays special dropped analog signals. The delay circuit 2 may be configured as a so-called LC circuit in which, for example, a plurality of coils are connected in series and a capacitor whose one end is grounded is connected to the connection point between the coils.

The difference circuit 3 calculates the difference between the analog signal delayed by one sampling clock in the delay circuit 2 and the output of the de-emphasis circuit 1. A / D
The converter 4 A / D converts the output of the difference circuit 3 into a digital signal having a predetermined number of bits N.

As shown in FIG. 3, for example, the digital integrator circuit 5 includes a calculator 11 for adding an input signal (digital signal) and an output of the amplifier 13, and a delay for delaying the output of the calculator 11 by one sample clock. And an amplifier 13 that amplifies the output of the delay unit 12 by K times, and integrates the input digital signal. The digital emphasis circuit 6 includes, for example, as shown in FIG. 4, an arithmetic unit 21 that adds an input signal (digital signal) and an output of the amplifier 25, an arithmetic unit 22 that calculates a difference between the input signal and the output of the amplifier 23, An amplifier 23 that amplifies the output of the arithmetic unit 21 by a predetermined number (7/8 times in FIG. 4), a delay unit 24 that delays the output of the arithmetic unit 21 by one sample clock, and an output of the delay unit 24 This digital filter is composed of an amplifier 25 that amplifies by a factor of C, has a characteristic opposite to the de-emphasis characteristic of the de-emphasis circuit 1, and emphasizes the high frequency characteristic of the input digital signal.

In the A / D converter configured as described above, when an analog signal is input to the de-emphasis circuit 1, the analog signal is input based on the transfer function H1 (s) shown in the equation (1). The high frequency characteristic of the signal is reduced.

That is, in this case, the dynamic range of the high frequency component (the signal component that changes drastically) of the analog signal is
It is multiplied by 1/8.

Here, in the de-emphasis circuit 1, as shown in FIG.
FIG. 5B shows an example of the step response when the step function shown in FIG.

The analog signal whose high frequency f characteristic is reduced by the de-emphasis circuit 1 is output to the delay circuit 2 and the difference circuit 3. In the delay circuit 2, the input analog signal is delayed by one sampling clock of the A / D converter 4 and output to the difference circuit 3. The difference circuit 3 takes the difference between the analog signal delayed by one sampling clock in the delay circuit 2 and the analog signal from the de-emphasis circuit 2.

Here, by taking the difference between the analog signal delayed by one sampling clock and the original analog signal, the part of the analog signal with a gradual temporal change, that is, the low-frequency component of the analog signal, , Close to 0, and therefore the dynamic range of the low frequency component of the analog signal is considerably reduced (at least smaller than that of the high frequency component described above).

The difference signal output from the difference circuit 2 (the difference between the analog signal delayed by one sampling clock in the delay circuit 2 and the analog signal from the de-emphasis circuit 2) is input to the A / D converter 4. Is
A / to a digital signal with a predetermined bit precision (number of bits)
D converted.

Here, in the differential signal input to the A / D converter 4, the dynamic range of the high frequency component (signal component that changes drastically) of the input analog signal is 1/8.
This is a signal that has been doubled and whose dynamic range of the low-frequency component (a signal component that changes gently) has been considerably reduced as described above.

Therefore, the dynamic range of the differential signal is 1/8 (= 2 ^-3 ) times the dynamic range of the input analog signal, and therefore, it is reduced by 3 bits. .

From the above, in this case, the A / D converter 4
In the A / D by the difference processing in the difference circuit 3.
Considering the 1-bit resolution deterioration of the converter 4, A
2 bits more than the bit precision N of the / D converter 4 (N
+2) A / D conversion processing with bit precision is substantially performed.

That is, the low bit precision A / D converter can perform the A / D conversion processing equivalent to that of the high bit precision A / D converter.

The digital signal from the A / D converter 4 is input to the digital integration circuit 5. In the digital integrator circuit 5, as shown in FIG. 3, the digital signal delayed by one sample clock by the delay unit 12 and multiplied by K (K = 1 in this embodiment) by the amplifier 13 and A
The addition value with the digital signal from the / D converter 4 is calculated by the calculator 11, that is, the digital signal from the A / D converter 4 is expressed by the formula H2 (z) = 1 / (1 + Kz ⁻¹ ) = 1 / (1 + z ⁻¹ ) ... (2) is integrated based on the transfer function H2 (z) and output.

Therefore, the digital integrating circuit 5 outputs an analog signal before the difference is taken by the difference circuit 2, that is, a digital signal corresponding to the analog signal output from the de-emphasis circuit 1.

The digital signal from the digital integration circuit 5 is input to the digital emphasis circuit 6. In the digital emphasis circuit 6, as shown in FIG.
Delayed by one sampling clock with the delay unit 24,
The added value of the digital signal multiplied by C in the amplifier 25 and the digital signal from the digital integration circuit 5 is calculated by the calculator 2
Calculated by 1. The output of the calculator 21 is
It is input to the amplifier 23 and the delay device 24, multiplied by 7/8 in the amplifier 23, and then subtracted from the digital signal from the digital integration circuit 5 by the arithmetic unit 22 and output.

That is, in the digital emphasis circuit 6, the equation H3 (z) = (1/8) (1 + 8Cz ^-1 ) / (1 + Cz ^-1 ) (3) where C = T (= CR ₂ = C Based on the transfer function H3 (z) represented by (R ₁ + R ₂ ) / 8), the high frequency characteristics of the digital signal from the digital integration circuit 5 are emphasized.

That is, in the digital emphasis circuit 6, the transfer function H1 of the de-emphasis circuit 1
Based on the transfer function H3 (z) as an inverse function of (s) (however, in the present embodiment, the gain is different), the high frequency characteristic of the digital signal from the digital integrator circuit 5 is emphasized, As a result, the high frequency f characteristics reduced by the de-emphasis circuit 1 are compensated and output.

Here, the digital emphasis circuit 6
6B shows an example of the step response when the step function shown in FIG. 6A is input.

As described above, since not only the low-range dynamic range of the input analog signal but also the high-range dynamic range is reduced, even if the A / D converter 4 has low accuracy. , High precision and accurate A
/ D conversion processing becomes possible.

However, due to the emphasis in the digital emphasis circuit 6, the high frequency resolution of the digital signal is somewhat lowered. However, when this A / D converter is applied to A / D conversion of, for example, a video signal or an audio signal, human vision or hearing has a high resolution for a high frequency component of the video signal or the audio signal, respectively. Since it is not high, the deterioration of the high frequency resolution of the signal due to emphasis is practically no problem. Therefore,
This A / D converter has excellent cost performance,
It can be said that it is a frequency adaptive decomposition type A / D converter.

Further, if the A / D converter 4 has a high precision, the A / D conversion process with higher precision can be performed.

Further, by using a low precision A / D converter 4, the cost of the device can be reduced. Further, since the circuit configuration is simple, the circuit can be downsized and the processing speed can be increased easily.

As shown in this embodiment, when the de-emphasis circuit 1 is composed of an analog (low pass) filter and the digital emphasis circuit 6 is composed of a digital filter, the de-emphasis circuit 1
It has to be possible to realize a digital filter having the inverse characteristic of the emphasis characteristic (in the present embodiment, the transfer function H1 (s) shown in the equation (1)).
That is, the transfer function of the de-emphasis circuit 1 is given by the equation (1)
However, it is not limited to what is shown by (4), but is restricted by the feasibility of the digital filter as the digital emphasis circuit 6.

Further, in the present embodiment, the amplification factor K in the amplifier 13 of the digital integrating circuit 5 shown in FIG.
= 1, but when the DC component of the signal is not required, for example, when A / D converting an audio signal, K is set to a value smaller than 1 and the analog system (de-emphasis circuit 1, delay circuit 2) of the circuit is set. , The accuracy of the difference circuit 3) can be relaxed.

Further, in the case of A / D converting the video signal, the digital integrator circuit 5 is reset in the H cycle (horizontal cycle), so that the circuit can be processed in the same manner as in the above audio signal. The analog precision of can be loosened.

When this A / D converter is used as a pair with the D / A converter in a closed system, an analog signal obtained by D / A conversion without the digital emphasis circuit 6 is provided. In addition, analog emphasis processing can be performed.

Further, in this embodiment, the difference circuit 3
The output of is input to the A / D converter 4 as it is.
ingfor minimum distortion ", J.Max, IRE Trans. Infor
Non-linear processing as described in mation Theory, Vol. IT-6, pp7-12, March 1960, etc., and then A / D
You may make it input into the converter 4.

[0041]

As described above, according to the A / D converter of the present invention, the high frequency characteristics of the analog signal are de-emphasized, and the difference between the temporally adjacent ones is A. / D conversion. Then, after the A / D converted signal is integrated, the frequency characteristic of the high frequency band is emphasized. Therefore, the dynamic range of the analog signal is
Since the size is small, it is possible to perform A / D conversion with substantially high accuracy.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of an A / D converter of the present invention.

FIG. 2 is a more detailed circuit diagram of the de-emphasis circuit 1 of the embodiment of FIG.

FIG. 3 is a more detailed circuit diagram of the digital integrator circuit 5 of the embodiment of FIG.

FIG. 4 is a digital emphasis circuit 6 of the embodiment of FIG.
3 is a more detailed circuit diagram of FIG.

5 is a waveform diagram showing a step response of the de-emphasis circuit 1 of the embodiment of FIG.

FIG. 6 is a digital emphasis circuit 6 of the embodiment of FIG.
5 is a waveform diagram showing the step response of FIG.

[Explanation of symbols]

 1 De-emphasis circuit 2 Delay circuit 3 Difference circuit 4 A / D converter 5 Digital integration circuit 6 Digital emphasis circuit

Claims (2)

[Claims] 1. An A / D converter for converting an input analog signal into a digital signal, wherein the high-frequency characteristics of the analog signal are de-emphasized, and among them, those which are temporally close to each other. An A / D converter characterized in that the difference is A / D-converted to be converted into a digital signal, the digital signal is integrated, and the high-frequency characteristics thereof are emphasized. 2. De-emphasis means for de-emphasizing the high frequency characteristics of the input analog signal, delay means for delaying the output of the de-emphasis means by one clock, and output of the de-emphasis means. Difference calculating means for calculating the difference from the output of the delay means, A / D converting means for A / D converting the output of the difference calculating means, and delaying the output of the A / D converting means by one clock. ,
An integrating means for integrating the output of the A / D converting means by adding the delayed output and the output of the A / D converting means, and enhancing the high frequency characteristic of the output of the integrating means. An A / D converter comprising: an emphasis means having a characteristic opposite to the de-emphasis characteristic of the de-emphasis means.