JPH0548463A - Noise shaping a/d converter - Google Patents

Abstract

PURPOSE:To provide the efficient noise shaping A/D converter with low power consumption while satisfying a transmission characteristic based on the PCM coding system of a voice frequency band signal stipulated by the CCITT recommendations G.711. CONSTITUTION:This A/D converter is provided with a D/A converter 6 whose output voltage relative value is five values of + or -4, + or -1 and 0, an addition integration device 10 accumulating a difference between an output signal of the D/A converter 6 and an analog input signal X and a comparator 8 comparing an output signal 10a of the adder integration device 10 with a preset 4-value reference voltage and outputting a 5-value digital output signal Y and inputting the digital output signal Y to the D/A converter 6. Thus, the efficient noise shaping A/D converter with low power consumption is realized.

Description

Detailed Description of the Invention

[0001]

This invention relates to CCITT Recommendation G.264.
The present invention relates to an AD converter to which the PCM coding method for a voice frequency band signal described in No. 711 is applied, and to a noise shaping AD converter of a delta-sigma modulation method.

[0002]

2. Description of the Related Art CCITT (International Telegraph and Telephone Advisory Committee)
Recommendation G. In the PCM encoding system for audio frequency band signals described in No. 711, a 4-wire interface PC
Regarding the definition of the transmission / reception separation characteristic of the M encoding method, the G. 714
And a two-wire interface PCM.
Regarding the specification of transmission / reception separation characteristics of the encoding system, see G. 715. In the PCM coding method of the audio frequency band signal, a compression method by 13-line broken line approximation called A-law and a compression method by 15-line broken line approximation called μ-law are specified, and both coding methods are 8 The bit PCM data requires an accuracy of about 12 bits on the small amplitude side and an accuracy of about 7 bits on the large amplitude side.

When attempting to realize an AD converter based on these encoding methods, the accuracy is 13 bits to 14 bits.
A bit precision A-D converter is required. Conventionally, many successive approximation type A / D converters have been developed as this type of A / D converter. The reason for this is that the audio frequency band is relatively narrow, so that a successive approximation type configuration can be taken, and that there is the best balance in terms of power consumption and circuit scale.

However, this successive approximation type A / D converter requires a steep prefilter to limit the input signal band, and an SCF (Switched Capacitor) is required to integrate this prefilter into an IC. It is currently realized by using technology. In recent years, with the miniaturization of semiconductor process technology, an oversampling A-type converter using a delta-sigma modulation method is used as a means for exchanging analog data and digital data instead of the successive approximation type A-D converter. A D converter (hereinafter referred to as "delta-sigma modulation type A-D converter") has been developed. This delta-sigma modulation type A / D converter is one in which the quantization noise is transferred to a high frequency band and the quantization noise in the signal band is reduced.

This delta-sigma modulation type A-D converter is a high-precision A-D converter without using a multi-bit, high-precision D-A converter section used in a successive approximation type A-D converter or the like. D
Since conversion is possible and the sampling frequency is higher than that in the signal band, there is an advantage that the specifications required for the prefilter can be loose. Delta like this
The sigma modulation type AD converter is the successive approximation type AD described above.
Compared with the converter, the sampling frequency is high and the circuit scale of the digital part is large, so that there is a drawback that the circuit scale and power consumption increase as a whole, but in recent years, with the miniaturization of semiconductor process technology, Since there is a tendency for downsizing and low power consumption, and because it has good affinity with other signal processing LSIs, it is expected that more and more attention will be given from now on.

On the other hand, CCITT Recommendation G. When an AD converter is to be realized based on the PCM encoding method for audio frequency band signals described in 711, the accuracy is 1
It is necessary to realize a 3-bit to 14-bit precision AD converter. Therefore, if this is to be realized by the delta-sigma modulation type A / D converter, it is necessary to set the oversampling ratio to 128 times and to configure the secondary delta-sigma type modulator.

FIG. 3 is a block diagram showing the configuration of a conventional second-order delta-sigma type modulation AD converter. In FIG. 3, 1 and 2 are adders, 3 and 4 and 5 are delay devices, 6 and 7 are DA converters having binary output voltage values, and 80 is a comparator. Further, 10 and 11 are summing integrators, which are usually composed of active elements such as operational amplifiers.

The digital output signal Y which is a binary signal is
It is input to the DA converters 6 and 7 via the delay device 5, and is fed back to the addition integrators 10 and 11 as a binary analog signal. The amplitude of the binary analog signal output from the DA converters 6 and 7 is set to the same amplitude as the maximum amplitude of the analog input signal X. In the conventional second-order delta-sigma modulation type AD converter configured in this way, the comparator 8
An analog input signal X and a quadratic difference output of the quantization noise appear as a digital output signal Y from 0.

[Equation 1] shows the system function of the conventional second-order delta-sigma modulation type AD converter.

[0010]

[Equation 1]

In [Equation 1], X (z) indicates an analog input signal X, Y (z) indicates a digital output signal Y, and Q (z) indicates quantization noise. As shown in [Equation 1], the quantization noise Q is subjected to (1-z ^-1 ) ² modulation and second-order noise shaping is performed. Therefore, if the sampling frequency is sufficiently higher than the pass band of the analog input signal X, the quantization noise Q is generated from the digital output signal Y by passing the digital output signal Y of the comparator 80 through a digital filter (not shown). Can be separated. As a result, a highly accurate digital value of the analog input signal X can be obtained.

[0012]

As described above, the CCI
TT Recommendation G. No. 711, an AD converter satisfying the transmission characteristics based on the PCM coding method of a voice frequency band signal is used as an oversampling delta-sigma modulation type A converter.
In order to realize it by the -D converter, it is necessary to set the oversampling ratio to 128 times and to configure the second-order delta-sigma type modulator.

Generally, the following three methods are considered as methods for reducing the quantization noise in the signal band. (a) Use a high-order delta-sigma modulator. (b) Increase the sampling rate. (c) Make the quantization level finer.

When a high-order delta-sigma modulator is used as in (a), the quantum noise in the signal band can be sent to a higher frequency band, and the efficiency of reducing the quantization noise in the signal band can be reduced. Is a good way. If the sampling rate is increased as in (b), theoretically it is possible to reduce in-band quantization noise, but this is also limited by the speed and power consumption of analog and digital elements. ..

As shown in (c), the quantization noise itself can be reduced by making the quantization level multivalued instead of 1 bit. According to the CCITT Recommendation G.264, the method shown in (a) to (b) is adopted.
It is possible to realize an A / D converter that satisfies the transmission characteristics based on the PCM encoding method of the voice frequency band signal described in 711.

However, the quantization noise reducing methods shown in (a) to (c) are not necessarily efficient methods. This is because CCITT Recommendation G. The transmission characteristics based on the PCM coding method of the voice frequency band signal described in No. 711 require accuracy of about 7 bits when the amplitude of the analog input signal is large, and about 12 bits when the amplitude of the analog input signal is small. This is because accuracy is required. That is, although high accuracy is required on the small amplitude side of the analog input signal, the conventional second-order delta-sigma modulation type AD
The converter, regardless of the amplitude of the analog input signal,
This is because the quantization noise is being reduced.

In view of the above problems, the object of the present invention is to provide C
CITT Recommendation G. A noise shaping AD converter that satisfies the transmission characteristics based on the PCM coding method of a voice frequency band signal described in No. 711 and has low power consumption and is efficient.

[0018]

The noise shaping AD converter of the present invention is a DA having an output voltage value of 5 values.
The converter, the signal accumulating means for accumulating the difference between the output signal of the DA converter and the analog input signal, and the output signal of the signal accumulating means are compared with a preset four-valued reference voltage. A digital output signal of a value and a comparator for inputting the digital output signal to the D-A converter are provided, and the relative ratio of the output voltage values of the D-A converter is ± 4, ± 1 and 0. It is a value.

[0019]

According to the structure of the present invention, the signal accumulating means allows the difference between the output signal of the DA converter and the analog input signal whose relative ratios of the output voltage values are five values of ± 4, ± 1 and 0. The output signal of the signal accumulating means by the comparator,
A four-valued reference voltage set in advance is compared to output a five-valued digital output signal, and the digital output signal is input to the DA converter. As a result, it is possible to reduce the absolute amount of the quantization noise generated when the analog input signal has a small amplitude, although the primary noise shaping is performed by the primary delta-sigma modulation.

Therefore, as compared with the conventional second-order delta-sigma modulation type AD converter, noise shaping can be performed with high efficiency by a simple circuit configuration.

[0021]

1 is a block diagram showing the configuration of a noise shaping AD converter according to an embodiment of the present invention. Figure 1
In the figure, 1 is an adder, 3 and 5 are delay devices, 6 is a D-A converter, and 8 is a comparator. Reference numeral 10 is an addition integrator composed of an adder 1 and a delay device 2 which are signal accumulating means, and is constituted by an active element such as an operational amplifier.

As shown in FIG. 1, noise shaping A
The -D converter includes a DA converter 6 having a relative ratio of output voltage values of 5 values of ± 4, ± 1 and 0, and this DA converter 6
5 and a digital output signal of 5 values by comparing the output signal 10a of the addition and integrator 10 with a preset 4-value reference voltage. And a comparator 8 which outputs Y and inputs the digital output signal Y to the DA converter 6.

Noise shaping A constructed in this way
The operation of the -D converter will be described. With the addition integrator 10,
The difference between the analog input signal X and the output signal of the D / A converter 6 is input to the adder 1 via the delay device 3 to be accumulated. The analog output accumulated by the adding integrator 10, that is, the output signal 10 a thereof is input to the comparator 8. The comparator 8 compares the voltage value of the output signal 10a of the addition integrator 10 with a four-valued reference voltage, and outputs a five-valued digital output signal Y.

The comparator 8 has a reference voltage in advance as
4 of ± 1/2 and ± 1/16 with respect to the maximum amplitude of the analog input signal X (hereinafter referred to as “input maximum amplitude”)
The value is set. When the output signal 10a of the adding integrator 10 has a value of ½ or more of the maximum input amplitude, “1” is output, and when it has a value of 1/16 to ½ of the maximum input amplitude. "1/4" is output to, and when the value is from -1/16 to 1/16 of the maximum input amplitude, "
0 "is output, and the input maximum amplitude is -1/2 to -1/16.
"-1/4" is output when the value is "1", and "-1" is output when the value is less than -1/2 of the maximum input amplitude. Further, the five-valued digital output signal Y which is the output signal of the comparator 8 is given as an input signal to the DA converter 6 via the delay device 5. The DA converter 6 converts the digital output signal Y of the comparator 8 input via the delay device 5 into an analog signal. The relative ratios of the output voltage values of the D-A converter 6 are five values of ± 4, ± 1 and 0. This allows
First-order noise shaping is performed on the quantization noise generated by the quantization of the analog input signal X.

Noise shaping A constructed in this way
The system functions of the -D converter are shown in [Equation 2], [Equation 3] and [Equation 4]. The analog input signal X is X (z)
, The digital output signal Y is Y (z), and the quantization noise is Q (z). When the maximum input amplitude is 1/4 or more,

[0026]

[Equation 2]

It becomes When the maximum input amplitude is 1/4 or less,

[0028]

[Equation 3]

It becomes Furthermore, when the maximum input amplitude is very small,

[0030]

[Equation 4]

It becomes 2 shown in [Formula 1] described in the conventional example
Compared to the following noise shaping, when the amplitude of the analog input signal X is large as shown in [Equation 2], the quantization noise in the input signal band is large, but when the amplitude of the analog input signal X is small. As shown in [Equation 3] and [Equation 4], the absolute amount of the quantization noise itself decreases, and the quantization noise in the input signal band decreases in proportion thereto.

Further, when the amplitude of the analog input signal X is large as shown in [Equation 2], the quantization in the input signal band is greater than the quadratic noise shaping shown in [Equation 1] described in the conventional example. Since the noise becomes large, the S / N (signal-to-total noise ratio) is worse than that of the conventional A / D converter, but CCIT is applied to the PCM coding method of the voice frequency band signal.
Recommendation G. As described in No. 711, when the amplitude of the analog input signal X is large, there is no problem because the accuracy of about 7 bits is sufficient.

Next, FIG. 2 shows the analog input signal X in the noise shaping AD converter of one embodiment of the present invention and the conventional second-order delta-sigma type modulation AD converter.
It is a figure which shows the relationship between the amplitude of S, and S / N (signal-to-total noise ratio). The oversampling ratio was 128 times. Also, the frequency of the analog input signal X is 1125.
A sine wave of [Hz] was input, and the input amplitude was changed from 0 [dB] to -60 [dB]. However, the characteristics shown in FIG. 2 are for a signal in which high-frequency noise is removed by passing the digital signals output by the comparators 8 and 80 shown in FIGS. 1 and 3 through a fifth-order digital LPF.

In FIG. 2, the outside of the shaded area is CCIT.
Recommendation G. It is a standard according to 714, and it can be seen that both the conventional AD converter and the noise shaping AD converter of the embodiment satisfy this characteristic. Also,
What is indicated by a dotted line is the characteristic of an AD converter using a normal first-order delta-sigma modulator, and in this case, CC
ITT Recommendation G. The standard according to 714 cannot be satisfied.

The noise shaping characteristic of the second-order delta-sigma modulation type AD converter described in the conventional example has the amplitude of the analog input signal X of -30 [dB] as compared with the embodiment.
As described above, the S / N ratio is good. However, G.G. If data compression is performed by the PMC encoding method described in 711, logically -30
For input amplitudes above [dB], the S / N is 44 [d
B] or more, so that in practice, -30 [d
It is meaningless to secure an S / N of 44 [dB] or more with an amplitude of B] or more. Further, it is understood that for the input amplitude of -30 [dB] or less, the embodiment has substantially the same characteristics as the conventional example.

Next, the noise shaping A of this embodiment is performed.
-D converter and conventional second-order delta-sigma type A shown in FIG.
Compare with the -D converter in terms of power consumption.
Generally, when an adding integrator is configured by active elements such as operational amplifiers, a relatively large amount of power is required due to requirements for slew rate, settling time, voltage gain, etc., but as is clear from FIGS. 1 and 3, The conventional A-D converter requires two addition integrators, but only one A-D converter according to the embodiment is required, and power consumption can be reduced.

Further, in the configuration of the conventional AD converter shown in FIG. 3, the amplitude of the input signal a to the second-stage addition integrator 11 is about twice the amplitude of the input signal X, It is necessary for the addition integrator 11 to have a large dynamic range. On the other hand, the AD converter of the embodiment shown in FIG. 1 has comparatively loose specifications required for the addition integrator 10, and can be configured by one addition integrator 10, but instead has four comparators. You need one. However, since the comparator 8 is used as an open loop, a compensating capacitor for oscillation prevention is unnecessary, and low power consumption can be achieved.

As explained above, according to the noise shaping AD converter of the embodiment, the relative ratio of the output voltage values is ± 4, ± 1 by the addition integrator 10 serving as the signal accumulating means.
The difference between the output signal of the DA converter 6 and the analog input signal X, which are five values of 0 and 0, is accumulated, and the comparator 8 outputs the output signal 10a of the addition integrator 10 and a preset four-value reference. The voltage is compared to output a five-valued digital output signal Y, and the digital output signal Y is output to the DA converter 6
To enter. As a result, it is possible to reduce the absolute amount of the quantization noise generated when the analog input signal X has a small amplitude, although it is the primary noise shaping by the primary delta-sigma modulation. Therefore, the conventional 2
Compared with the delta-sigma modulation type AD converter, the circuit configuration is simple and the power consumption can be reduced.

[0039]

According to the noise shaping A / D converter of the present invention, the output of the D / A converter in which the relative ratio of the output voltage values is five values of ± 4, ± 1 and 0 by the signal accumulating means. The difference between the signal and the analog input signal is accumulated, and a comparator compares the output signal of the signal accumulating means with a preset 4-value reference voltage to output a 5-value digital output signal and the digital output. The signal is input to the DA converter. By this, 1 by the first-order delta-sigma modulation
Although it is the next noise shaping, the absolute amount of the quantization noise generated when the analog input signal has a small amplitude can be reduced.

Therefore, as compared with the conventional second-order delta-sigma modulation type AD converter, noise shaping can be performed with high efficiency by a simple circuit configuration. as a result,
CCITT Recommendation G. It is possible to obtain an efficient noise shaping A / D converter that satisfies the transmission characteristics of the PCM coding method for the audio frequency band signal described in No. 711 and has low power consumption.

[Brief description of drawings]

FIG. 1 is a noise shaping A- according to an embodiment of the present invention.
It is a block diagram which shows the structure of a D converter.

FIG. 2 is a noise shaping A- according to an embodiment of the present invention.
D converter and conventional second-order delta-sigma type modulation type A-
Amplitude and S / N of analog input signal X in D converter
It is a figure which shows the relationship with (signal-to-total noise ratio).

FIG. 3 is a block diagram showing a configuration of a conventional second-order delta-sigma type modulation AD converter.

[Explanation of symbols]

 6 DA converter 8 Comparator 10 Addition integrator (Signal accumulating means) X Analog input signal Y Digital output signal

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroyuki Nakahira 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Toshiyuki Shono, 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (1)

[Claims] 1. A DA converter having an output voltage value of 5 values, a signal accumulating means for accumulating a difference between an output signal of the DA converter and an analog input signal, and a signal accumulating means of the signal accumulating means. A comparator for outputting a five-value digital output signal by comparing the output signal with a preset four-value reference voltage and inputting the digital output signal to the DA converter; A noise shaping A / D converter in which the relative ratio of the output voltage values of the converter is five values of ± 4, ± 1 and 0.