JPH0779255B2 - Quantizer - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quantizer for converting a digital signal having a long word length into a digital signal having a short word length sampled at high speed.

2. Description of the Related Art In recent years, with the improvement of digital signal processing technology, signals that have been conventionally analog processed have been digitally processed.
Along with this, higher performance and lower cost of digital-analog converters have become more important. Noise-shaping quantizers are often used for these purposes. As a quantizer using noise shaping, for example, Japanese Patent Laid-Open No. 63-209334 discloses a quantizer of a multi-stage noise shaping type. By using this quantizer, stable high-order noise shaping can be performed without causing oscillation or the like. However, on the other hand, there is also a problem that the gradation of the quantizer output increases. Therefore, a method has been proposed in which this quantizer is improved so as to suppress an increase in gradation of the quantizer output. The block diagram is shown in FIG. 5 and is explained (for example, IEEE Journal of solid state circuit, Au).
g.1989, Vol.24, No.4).

The delay circuit 127 and the adder 126 form an integrator 102. Local quantizer 103, adder 128, delay circuit 4, subtractor 101,
Main loop 10 which is a single integral type noise shaping quantizer having a first-order shaping order by integrator 102
0 is configured. Further, the adder 120 and the delay circuit 121 configure the integrator 108, and the adder 122 and the delay circuit 123 configure the integrator 1.
10 are configured. The subtractors 107 and 109, the local quantizer 6, the integrators 108 and 110, and the delay device 112 constitute a sub-loop 106 which is a double integral type noise shaping quantizer having a quadratic shaping order. Subloop 106
Is given to the input / output difference of the local quantizer 103 by the subtractor 2. The output of the integrator 108 is the multiplier 130.
After being multiplied by the coefficient a, it is added to the input of the local quantizer 103 via the adder 128. Here, the input X is a 16-bit digital signal, and the local quantizer
103 and 6 perform the quantization as shown in Tables 1 and 2. The output is standardized at 16384.

Here, the quantization error generated by the local quantizer 103 is Vq1, and the quantization error generated by the local quantizer 6 is Vq1.
If q2, the relationship between the input X and the output W of the main loop 100 is expressed by the equation (1).

W = X + (1−z ⁻¹ ) · Vq1 (1) On the other hand, the equation (2) is established from the relationship between the input X ′ and the output W ′ of the subloop 106.

W ′ = X ′ + (1−z ⁻¹ ) ² · Vq2 (2) Since the output of the adder 2 is the input / output difference of the local quantizer 103, X ′ = − Vq1 (3) Therefore, after the output W'of the sub-loop 106 is differentiated by the differentiator 10 composed of the subtracter 13 and the delayer 14, and then added by the output W of the main loop 100 by the adder 12, Vq1 shown in the equation (1) is obtained. The term of is canceled out, and the relation between input and output X, Y as a whole is as shown in equation (4).

Y = X + (1−z ⁻¹ ) ³ · Vq2 (4) Here, the reason why this circuit operates stably despite the gradation of the sub-loop is ± 05 is as follows. That is, the value of the integrator 108 is fed back by the multiplier 130 to the local quantizer 103 via the adder 128. Therefore, when the value of the integrator 108 is large, the local quantizer 103
Since the input of is also large, the value of the subtractor 2 becomes a large negative value. This value is given to the integrator 108 via the subtractor 107, but since the other input of the subtractor 107 is 0.5 at the most, a large negative value is input to the integrator 108 and gradually increases. The output of the integrator 108 becomes small.

In this way, the value of the integrator 110 can be suppressed to a small value by feeding back to the main loop 100 in the direction in which the value of the integrator 108 is decreased, and the output gray level of the local quantizer 6 is lowered. Is what you can do.

Here, the possible values of W, that is, the gradation is −2, −1, ...,
There are 5 ways of +2 (5 values), and the possible value of W'is -0.
Since it is a binary value of 5, +0.5, the possible values of Y are -3, -2,
…, 7 values of +3. That is, the input signal is 7-valued (3
It shows that it is compressed to a little bit). Also,
Equation (4) shows that the quantization error in the low frequency band is driven to the high frequency band. Therefore, by configuring as shown in FIG. 5, the output is made without damaging the dynamic range of the input digital signal. The bit number of the digital signal can be compressed, and a dynamic range of about 118 dB can be obtained by operating this circuit with 64 times oversampling.

However, in the above configuration, after the value of the integrator 108 in the subloop 106 is determined, the output of the integrator 108 → the multiplier
130 → adder 128 → local quantizer 103 → subtractor 2 → subtractor 1
The calculation must be performed again through the path of 07 → integrator 108 → subtractor 109 → integrator 110 → local quantizer 6, which requires a very long calculation time. Moreover, since the feedback is applied from the output of the integrator 108 in the first stage, and the integrator 110 after that has no feedback, it is almost impossible to prevent the integrator 110 from oscillating or overflowing. Therefore,
For example, there is a problem that it is difficult to use a sub-loop having a shaping order of 3 or higher.

In view of the above problems, the present invention requires less calculation time,
It is an object to provide a quantizer that can obtain the same effect.

Means for Solving the Problems To achieve the above object, a quantizer according to the present invention comprises a first local quantizer for quantizing an input signal, and a first local quantizer for noise shaping of a given input. Noise shaping type quantizer, a second local quantizer for quantizing an input signal, a detecting means for detecting a quantization error generated by the second local quantizer, and a detecting means for detecting the quantizing error. A feedback circuit for multiplying the output by a predetermined transfer function and feeding it back,
The quantization error generated by the first noise shaping quantizer and the output of the feedback circuit are added and input to the second local quantizer, and the polarity of the output of the feedback circuit and the first A control means for adding / subtracting the output value of the first local quantizer and a predetermined value based on the polarity of the quantization error generated by the local quantizer,
The output of the second local quantizer is differentiated according to the order of the first noise shaping quantizer, the differential output is added to the output of the control means, and the addition result is taken out as an output. It was done.

Function As described above, the output of the local quantizer in the main loop is incremented by +1 or -1 according to the output of the feedback circuit. Therefore, the quantization error generated by the main loop must be the same as the output of the feedback circuit. Become polar. Therefore, the absolute value of the input level of the local quantizer in the sub-loop to which the addition result of this value and the output of the feedback circuit is input is always smaller than that of the output of the feedback circuit, and the gradation of the local quantizer in the sub-loop is reduced. Can be reduced. Further, since the output of the feedback circuit is fixed in advance, the time required for the calculation can be shortened.

Examples Hereinafter, the present invention will be described with reference to the drawings.

FIG. 1 shows an embodiment of a quantizer according to the present invention. Referring to this figure, 1 is a local quantizer, which quantizes an input signal. The relationship between input and output is as shown in Table 3.

The output is standardized by 11264. Reference numeral 5 denotes a control circuit, which adds +1 or -1 to the output of the local quantizer 1 according to the output of the code detector 11 which will be described later.
The configuration is as shown in the figure. That is, the adder
20 the output of the code detector 11 and the output of the local quantizer 1
Add Q1 and the limiter 21 gives a result of -3 to +3
I try not to exceed. 6 is a local quantizer,
The relationship between the input and output is shown in Table 4.

Reference numeral 9 is a feedback circuit, and its transfer function H (z) is as shown in the expression (5), and specifically has a configuration as shown in FIG.

H (z) = − 2z ⁻¹ + z ⁻² (5) That is, the input is given to the delay circuit 41, the difference between the output doubled by the multiplier 44 and the output of the delay circuit 42 is subtracted.
43 required. 11 is a code detector, which is a subtractor 2
Of the main loop, that is, -Vq1 which is the output of the main loop and the output β of the feedback circuit 9, are input, and a 2-bit signal C (C1, C0) is output according to the signs of these inputs. Actually, the most significant bit (hereinafter referred to as MSB) of the output of the subtractor 2 and the output MSB of the feedback circuit 9 are given to the code detector 11, and the configuration is as shown in FIG. ing. here,
30 is an exclusive OR gate. Reference numeral 31 is an AND gate, and the signal β is input to the AND gate 31 with its polarity inverted. With this configuration, if −Vq1 ≧ 0 and β <0, then C = 0 −Vq1 <0 and β ≧ 0, then C = 0 −Vq1 ≧ 0 and β If ≧ 0, C = 1−Vq1 <0, and if β <0, C = −1 is output.

Next, the operation of the circuit shown in FIG. 1 will be described. The adder 3, the local quantizer 1, the control circuit 5, the subtractor 2, and the delay unit 4 constitute a main loop of single integration type noise shaping. If the quantization error generated by both the local quantizer 1 and the control circuit 5 is Vq1, the output of the subtractor 2 is -V.
It becomes q1. Since this value is fed back to the input via the delay unit 4, the output W of the control circuit 5 is as shown in the equation (6).

W = X + (1−z ⁻¹ ) · Vq1 (6) On the other hand, the local quantizer 6, the adder 7, the subtractor 8, and the feedback circuit 9 constitute a double integral type noise shaping subloop. The local quantizer 6 used in this embodiment outputs ± 0.5 as described above. Further, the feedback amount β by the feedback circuit 9 is up to three times the quantization error by the local quantizer 6, as is clear from the transfer function. Since the quantization error generated by the local quantizer 6 is 0.5 or less during normal operation, the maximum value of the feedback amount β by the feedback circuit 9 is 1.5.

Here, considering the input to the local quantizer 6, this value is obtained by subtracting the quantization error Vq1 generated by the main loop from the feedback amount β by the feedback circuit 9.

When the feedback amount β and the sign of the input (-Vq1) of the sub-loop are matched by the No. d detector 11, if the sign is positive, 1 is added to the local quantizer 1 and the output of the subtracter 2 (- The value of Vq1) becomes negative.

If the sign is negative, 1 is subtracted from the local quantizer 1, and the value of the output (-Vq1) of the subtractor 2 becomes positive.

Thus, the input (-Vq1) of the adder 7 and the sign of β are always different. The absolute value of the value of Vq1 is 0.5 on average.
Therefore, the input of the local quantizer 6 is within ± 1.0, and the occurrence of distortion can be suppressed. Therefore, as in the prior also in this circuit example, the quantization error local quantizer 6 is generated as Vq2, W '= - Vq1 + (1-z -1) 2 · Vq2 ... (7) , and the sum the output Y of the vessel 12 will be Y = W + (1-z -1) · W 1 = X + (1-z -1) 3 · Vq2 ... (8) , and the third-order noise shaping obtained. In this case, the output of the local quantizer 1 is 7 from -3 to +3.
Since this is a value, and the output of the local quantizer 6 is a binary value of ± 0.5, the final output Y is a nine value from -4 to +4, and the same shaping effect as in the conventional example can be obtained.

Here, considering the signal transmission path, as can be seen from FIG. 3, the output of the feedback circuit 9 is fixed regardless of the output of the code detector 11, and only the sign of the output of the subtractor 2 matters. Become. Here, even if the output of the feedback circuit 9 and the output of the subtractor 2 have different signs, the signal transmission path is as follows: the sign detector 11 → the control circuit 5 → the subtractor 2 → the adder 7 → the subtraction It is significantly shorter than that of the device 8 or the local quantizer 6. As for the feedback to the main loop, the feedback is applied so that the value to be input to the local quantizer 6 becomes small, so that the feedback is applied to the entire sub loop, and the degree of freedom in designing the feedback circuit 9 is increased. Therefore, even if the characteristic of the feedback circuit 9 is a third order or higher, the circuit can operate stably. That is, for example, the local quantizer 6 shown in Table 4 may be used, and the transfer function of the feedback circuit 9 may be of a higher order as shown in the equation (9).

If this transfer function is used, a fourth-order noise shaping effect can be obtained in the low frequency range, and a dynamic range of about 118 dB can be obtained by operating this circuit with 32 times oversampling.

In the above embodiments, the local quantizer 1 which outputs 5 values from -3 to +3 is used, but of course, the present invention is not limited to this, and 6 values or more or 5 values or less are used. It may be one. Further, the case where the transfer function of the feedback circuit 9 shown in the equation (5) or (9) is used and the local quantizer 6 having a binary or ternary output is shown. Of course, it is not limited to this. Furthermore, the main loop does not have to be a single-integration type noise shaping circuit, in short, the quantization error generated in this loop is input to the sub-loop, and the feedback amount of the quantization error in the sub-loop and the sub-loop are input. It is good if the values from the main loop have opposite polarities.

As described above, the present invention has the first local quantizer for quantizing the input signal, and the first noise shaping quantizer for performing noise shaping of the given input, A second local quantizer for quantizing the input signal, a detection means for detecting a quantization error generated by the second local quantizer, and an output of the detection means multiplied by a predetermined transfer function. A feedback circuit for feeding back, and adding the quantization error generated by the first noise shaping quantizer and the output of the feedback circuit and inputting to the second local quantizer, Control means for adding / subtracting the output value of the first local quantizer and a predetermined value based on the polarity of the output of the feedback circuit and the extreme positiveness of the quantization error generated by the first local quantizer. And providing the output of the second local quantizer to the first noise Of the local quantizer in the sub-loop by differentiating according to the shaping order of the S-shaped quantizer, adding the differentiated output and the output of the control means, and taking out the addition result as an output. Since the number of tones is small, the gradation of the quantizer as a whole can be reduced. Further, even if a sub-loop feedback circuit having a quadratic degree or more is used, the oscillation of the feedback circuit can be suppressed without increasing the gray scale, and the time required for the calculation can be shortened. Is to have.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of a quantizer according to the present invention, FIG. 2 is a block diagram showing a concrete embodiment of a control circuit 5, and FIG. 3 is the same embodiment. 4 is a block diagram showing the details of the feedback circuit in FIG. 4, FIG. 4 is a block diagram showing a concrete embodiment of the code detector 11, and FIG. 5 is a block diagram showing a conventional quantizer. 1,6 ... local quantizer, 4 ... delay circuit, 5 ... control circuit, 9 ... feedback circuit, 10 ... differentiator, 11 ... code detector.

Continuation of the front page (56) Reference JP 62-265810 (JP, A) JP 54-99545 (JP, A) JP 61-84914 (JP, A) JP 63-161713 (JP , A) JP-A-3-289709 (JP, A) JP-A-3-289810 (JP, A) JP-A-3-289808 (JP, A) JP-A-4-56407 (JP, A) JP-A 4-30618 (JP, A) JP-A-4-30619 (JP, A) JP-A-4-30620 (JP, A) JP-B 6-83150 (JP, B2)

Claims (2)

[Claims] 1. A first noise-shaping quantizer for noise-shaping a given input, and a first local quantizer for quantizing an input signal, and quantizing an input signal. A second local quantizer, a detecting means for detecting a quantization error generated by the second local quantizer, and a feedback circuit for feeding back an output of the detecting means by multiplying a predetermined transfer function by feedback. Then, the quantization error generated by the first noise shaping quantizer and the output of the feedback circuit are added and input to the second local quantizer, and the polarity of the output of the feedback circuit is added. And a control means for adding / subtracting an output value of the first local quantizer and a predetermined value based on the polarity of the quantization error generated by the first local quantizer, and the second local quantum. The output of the quantizer is the first noise shaping quantizer Differentiated depending on shaping order, adds the output of the control means and its differential output, the quantizer which is to extract the addition result as an output. 2. The quantizer according to claim 1, wherein the value after the output value of the first local quantizer is controlled by the control means is within a predetermined value.