Noise shaper capable of generating a predetermined output pattern in no-signal condition

In a noise shaper comprising first and second cascaded integrators, a quantizer receiving an output signal of the second integrator, and a feedback path for feeding back an output of the quantizer to each of the first and second integrators, the first integrator includes an adder having a first input receiving art input signal applied to the first integrator, a delay circuit receiving an output of the adder to output a one-sample-delayed signal, and a multiplying circuit having an input connected to an output of the delay circuit for multiplying the one-sample-delayed signal outputted from the delay circuit, by a positive coefficient "0.999". An output of the multiplying circuit is connected to a second input of the adder. An output of the delay circuit constitutes the output of the first integrator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to noise shaper, and more particularly to 
noise shaper capable of stably performing its operation, when it is 
powered on, or when an input signal is brought into a no-signal condition, 
ceaselessly with a constant noise characteristics including an S/N ratio 
(signal-to-noise) and a distortion characteristics, without depending upon 
an initial value of an integrator included in the noise shaper. 
2. Description of Related Art 
Referring to FIG. 1, there is shown a block diagram of one example of a 
conventional noise shaper. 
The shown noise shaper is generally designated by the Reference Numeral 
100, and is constructed to receive a one-bit input signal V.sub.IN and a 
one-bit output signal V.sub.OUT. The noise shaper 100 comprises a first 
subtracter 11, a first integrator 102, a second subtracter 13, a second 
integrator 14, a quantizer 15, a delay 16 and a multiplier 17, connected 
as shown in FIG. 1. 
The first subtracter 11 has a minuend input connected to receive the input 
signal V.sub.IN and a subtrahend input connected to an output of the delay 
16, so as to output a difference between the input signal V.sub.IN and an 
output signal of the delay 16. The first integrator 102 has an input 
connected to an output of the first subtracter 11 so as to integrate an 
output signal of the first subtracter 11. The second subtracter 13 has a 
minuend input connected to an output of the first integrator 102 and a 
subtrahend input connected to an output of the multiplier 17, so as to 
output a difference between an output signal of the first integrator 102 
and an output signal of the multiplier 17. The second integrator 14 is 
connected to an output of the second subtracter 13 so as to integrate an 
output signal of the second subtracter 13. 
The quantizer 15 has an input connected to an output of the second 
integrator 14, and outputs "+1" when the output of the second integrator 
14 is larger than "0", and also outputs "-1" if it is not larger than "0", 
for the purpose of quantize the output of the second integrator 14. An 
output of the quantizer 15 constitutes the output signal V.sub.OUT of the 
noise shaper 100. The delay 16 has an input connected to the output of the 
quantizer 15, and delays the output signal V.sub.OUT of the noise shaper 
100, by one sample, so that a one-sample-delayed signal is fed back to an 
input of the multiplier 17 and the subtrahend input of the first 
subtracter 11. The multiplier 17 doubles the output of the delay 16. Thus, 
the one-sample-delayed signal is fed back to the first and second 
integrators 102 and 14. 
Referring to FIG. 2A, the first integrator 102 includes a first adder 111 
having a first input receiving an input signal applied to the first 
integrator 102, and a first delay circuit 112 receiving an output of the 
first adder 111 to feed back a one-sample-delayed signal to a second input 
of the first adder 111. An output of the first delay circuit 112 
constitutes the output of the first integrator 102. Accordingly, the first 
integrator 102 functions to integrate the output signal of the first 
subtracter 11 by obtaining a sum of a current data of the output signal of 
the first subtracter 11 shown in FIG. 1 and a data-before-one-sample (data 
stored before one sample) of the output signal of the first subtracter 11. 
Referring to FIG. 2B, the second integrator 14 includes a second adder 121 
having a first input receiving an input signal applied to the second 
integrator 14, and a second delay circuit 122 receiving an output of the 
second adder 121 to feed back a one-sample-delayed signal to a second 
input of the second adder 121. An output of the second adder 121 
constitutes the output of the second integrator 14. Accordingly, the 
second integrator 14 functions to integrate the output signal of the 
second subtracter 13 by obtaining a sum of a current data of the output 
signal of the second subtracter 13 shown in FIG. 1 and a 
data-before-one-sample (data stored before one sample) of the output 
signal of the second subtracter 13. 
Now, operation of the noise shaper 100 shown in FIG. 1 will be described. 
The first integrator 102 integrates a difference (which is the output 
signal of the first subtracter 11) between the current data of the input 
signal V.sub.IN and a data-before-one-sample of the output signal 
V.sub.OUT (which is the output signal of the delay 16). The second 
integrator 14 integrates a difference (which is the output signal of the 
second subtracter 13) between the output of the first integrator 102 and 
the data (which is the output of the multiplier 17) obtained by doubling 
the data-before-one-sample of the output signal V.sub.OUT. As mentioned 
above, the quantizer 15 outputs "+1" when the output of the second 
integrator 14 is larger than "0", and also outputs "-1" if it is not 
larger than "0". 
Accordingly, assuming that a quantization noise generated in the quantizer 
15 is "Q", a relation between an input signal V.sub.IN and an output 
signal V.sub.OUT of the noise shaper 100 can be expressed by the following 
formula: 
EQU V.sub.OUT (z)=V.sub.IN (z)+(1-z.sup.-).sup.2 .multidot.Q(z)(1) 
Consequently, an output spectrum of the noise shaper 100 becomes a spectrum 
formed by superposing a signal obtained by the second-order 
differentiation of the quantization noise, on the input signal of the 
noise shaper 100. Namely, the quantization noise is shaped and superposed 
in a high frequency region, so that the sum of the noise in a signal band 
is remarkably reduced. 
However, the above mentioned conventional noise shaper 100 have the 
following disadvantage when an input signal is put in a no-signal 
condition. 
Now, assuming that both the output of the first delay circuit 112 included 
in the first integrator 102 and the output of the second delay circuit 122 
included in the second integrator 14 are "0" (zero) in an initial 
condition, and also assuming that the output signal of the delay 16 in the 
feedback loop is "1", the output signal V.sub.OUT of the noise shaper 100 
changes in an output signal pattern "-1", "-1", "1", "-1", "1", "1", "-1", 
"-1", . . . . On the other hand, assuming that the output of the first 
delay circuit 112 included in the first integrator 102 is "1", the output 
signal V.sub.OUT of the noise shaper 100 changes in a different output 
signal pattern "-1", "1", "-1", "1", "1", "-1", "-1", "1", . . . . 
Accordingly, the output pattern of the noise shaper 100 varies dependently 
upon the initial values of the first integrator 102 and the second 
integrator 14. 
Here, the term "initial value" is used to mean an output value of an 
integrator such as the first integrator 102 and the second integrator 14, 
at the moment the input signal V.sub.IN becomes a no-signal condition. 
Ordinarily, when the voltage supply is powered on, it is possible to fix or 
reset to "0" (zero) the initial value of the output signal of each of the 
first integrator 102 and the second integrator 14 included in the noise 
shaper 100. In operation, however, if the input signal V.sub.IN applied to 
the noise shaper 100 suddenly becomes a no-signal condition, the initial 
values of the output signals of the first integrator 102 and the second 
integrator 14 included in the noise shaper 100 have various values. 
On the other hand, the output signal V.sub.OUT of the noise shaper 100 is 
applied to a succeeding D/A (digital-w-analog) converter (not shown) where 
it is converted into an analog signal. In general, the D/A converter has 
characteristics different from ideal characteristics, because of variation 
in analog elements constituting the D/A converter. Ordinarily, the 
characteristics of the D/A converter is dependent upon the digital signal 
applied to the D/A converter. 
As mentioned above, since the output signal V.sub.OUT of the noise shaper 
100 in the no-signal condition takes various different patterns, the 
analog signal obtained from the D/A converter similarly assumes different 
characteristics. Because of this, the conventional noise shaper 100 has 
adopted an approach of detecting whether or not the input signal V.sub.IN 
is "0" (zero), for forcibly bringing the output signal pattern to for 
example "1", "-1", "1", "-1", . . . , when the input signal V.sub.IN is 
"0" (zero). For realizing this approach, it is necessary to add an extra 
circuit for detecting whether or not the input signal V.sub.IN is "0" 
(zero). In addition, a delay in time from the moment the input signal 
V.sub.IN becomes "0" (zero) until the moment it is actually detected that 
the input signal V.sub.IN is "0" (zero), is a problem such a system. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a noise 
shaper which has overcome the above mentioned defect of the conventional 
one. 
Another object of the present invention is to provide a noise shaper 
capable of simply outputting a single predetermined output signal pattern 
in the no-signal condition. 
The above and other objects of the present invention are achieved in 
accordance with the present invention by a noise shaper comprising at 
least two cascaded integration means, a quantizing means receiving an 
output signal of a final stage integration means of the at least two 
cascaded integration means, and a feedback means for feeding back an 
output of the quantizing means to each integration means of the at least 
two cascaded integration means, at least one integration means of the at 
least two cascaded integration means being configured to add a current 
data of an input signal applied to the at least one integration means, 
with data obtained by multiplying a data-before-one-sample of the input 
signal applied to the at least one integration means, by a positive 
coefficient smaller than "1", the other integration means of the at least 
two cascaded integration means being configured to add a current data of 
an input signal applied to the same integration means, with a 
data-before-one-sample of the input signal applied to the same integration 
means. 
As mentioned above, in the noise shaper in accordance with the present 
invention, the at least one integration means of the at least two cascaded 
integration means is configured to add a current dam of an input signal 
applied to the at least one integration means, with data obtained by 
multiplying a data-before-one-sample of the input signal applied to the at 
least one integration means, by a positive coefficient smaller than "1". 
Namely, the at least one integration means constitutes an incomplete 
integrator, which outputs an output signal which gradually becomes "0" 
(zero) with the lapse of time, when no signal is applied, namely, when the 
input signal is put in a no-signal condition. 
On the other hand, the other integration means is constructed in the form 
of a complete integrator. The initial value of the other integration means 
in the form of the complete integrator, influences the output signal of 
the quantizing means in a transient condition, but becomes a steady 
condition with the lapse of time. 
Accordingly, when the input signal becomes the no-signal condition, a 
single predetermined pattern is outputted from the noise shaper, 
regardless of the initial value of each of the integration means included 
in the noise shaper. 
In a preferred embodiment, the noise shaper in accordance with the present 
invention includes a first subtracting means having a first input 
receiving an input signal, a first integrating means receiving an output 
of the first subtracting means, a second subtracting means having a first 
input receiving an output of the first integrating means, a second 
integrating means receiving an output of the second subtracting means, a 
quantizing means receiving an output of the second integrating means, and 
for outputting an output signal, and a feedback means for feeding back the 
output of the quantizing means to a second input of each of the first and 
second subtracting means, the first integrating means being configured to 
add a current data of the output of the first subtracting means with data 
obtained by multiplying a data-before-one-sample of the output of the 
first subtracting means by a positive coefficient smaller than "1", the 
second integration means being configured to add a current data of the 
output of the second subtracting means with a data-before-one-sample of 
the output of the second subtracting means. 
In a specific embodiment, the feedback means includes a delay means for 
delaying the output of the quantizing means by one sample. In another 
specific embodiment, the feedback means includes a delay means for 
delaying the output of the quantizing means by one sample and for 
supplying an output of the delay means to the second input of the first 
subtracting means, and a multiplying means for multiplying the output of 
the delay means by a positive coefficient so as to apply the multiplied 
output to the second input of the second subtracting means. 
The above and other objects, features and advantages of the present 
invention will be apparent from the following description of preferred 
embodiments of the invention with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 3, there is shown a block diagram of a first embodiment 
of the noise shaper in accordance with the present invention. In FIG. 3, 
elements corresponding to those shown in FIG. 1 are given the same 
Reference Numerals, and explanation thereof will be omitted for 
simplification of the description. 
As seen from comparison between FIGS. 1 and 3, a noise shaper 10 shown in 
FIG. 3 is different from the noise shaper 100 shown in FIG. 1 only in that 
the first integrator 102 of the noise shaper 100 is replaced with a first 
integrator 12 which has a multiplying circuit in its feedback loop. 
As shown in FIG. 3, the first integrator 12 includes a first adder 21 
having a first input receiving an input signal applied to the first 
integrator 12 (namely, the output of the first subtracter 11), a first 
delay circuit 22 receiving an output of the first adder 21 to output a 
one-sample-delayed signal, and a multiplying circuit 23 having an input 
connected to an output of the first delay circuit for multiplying the 
one-sample-delayed signal outputted from the first delay circuit 22, by a 
positive coefficient "0.999", an output of the multiplying circuit 23 
being connected to a second input of the first adder 21. An output of the 
first delay circuit 22 constitutes the output of the first integrator 12, 
namely, is connected to the minuend input of the second subtracter 13. 
Accordingly, the first integrator 12 constitutes an incomplete integrator 
which functions to integrate the output signal of the first subtracter 11 
by obtaining a sum of a current data of the output signal of the first 
subtracter 11 and the 0.999-multiplied data-before-one-sample (data stored 
before one sample) of the output signal of the first subtracter 11. 
In the noise shaper 10 as mentioned above, accordingly, assuming that a 
quantization noise generated in the quantizer 15 is "Q", a relation 
between an input signal V.sub.IN and an output signal V.sub.OUT of the 
noise shaper can be expressed by the following formula: 
EQU V.sub.OUT (z)=(z.sup.-1 /P(z)).sqroot.V.sub.IN 
(z)+(1-z.sup.-1)(1-0.999.multidot.z.sup.-1).multidot.Q(z)/P(z)(2) 
where P(z)=0.001.multidot.z.sup.-2 +0.001.multidot.z.sup.-1 +1 
In the above mentioned formula (2), P(z) is substantially "1" independently 
of frequency. Consequently, an output spectrum of the noise shaper 10 
becomes a spectrum formed by superposing a signal obtained by the 
second-order differentiation of the quantization noise, on the input 
signal V.sub.IN of the noise shaper 10. Namely, the quantization noise is 
shaped and superposed in a high frequency region, so that the sum of the 
noise in a signal band is remarkably reduced, without substantially 
deteriorating the noise shaping characteristics of the conventional noise 
shaper, 
In the noise shaper 10 as shown in FIG. 3, when the input signal V.sub.IN 
becomes the no-signal condition, since the first integrator 12 has a leak 
integration construction so as to accumulatively integrate the 0.999 times 
the integration amount, the output (integration amount) of the first 
integrator 12 becomes "0" (zero) with the lapse of time. 
On the other hand, the initial value of the second integrator 14 in the 
form of a complete integrator influences the output signal of the 
quantizer 15 in a transient condition, but becomes a steady condition with 
the lapse of time. 
Accordingly, when the input signal V.sub.IN becomes the no-signal 
condition, a single predetermined pattern is outputted from the noise 
shaper 10, regardless of the initial value of each of the two integrators 
12 and 14 included in the noise shaper 10. 
Referring to FIG. 4, there is shown is a block diagram of a second 
embodiment of the noise shaper in accordance with the present invention. 
In FIG. 4, elements corresponding to those shown in FIG. 3 are given the 
same Reference Numerals, and explanation thereof will be omitted for 
simplification of the description. 
A noise shaper 40 of the second embodiment is different from the first 
embodiment only in that the first integrator 12 of the first embodiment is 
replaced with a first integrator 42 which includes a multiplying circuit 
configured to multiply the received signal by a coefficient "0.99". 
More specifically, the first integrator 42 shown in FIG. 4 includes a first 
adder 51 having a first input receiving an input signal applied to the 
first integrator 42 (namely, the output of the first subtracter 11), a 
first delay circuit 52 receiving an output of the first adder 51 to output 
a one-sample-delayed signal, and a multiplying circuit 53 having an input 
connected to an output of the first delay circuit 52 for multiplying the 
one-sample-delayed signal outputted from the first delay circuit 52, by a 
positive coefficient "0.99", an output of the multiplying circuit 53 being 
connected to a second input of the first adder 51. An output of the first 
delay circuit 52 constitutes the output of the first integrator 42, 
namely, is connected to the minuend input of the second subtracter 13. 
Accordingly, the first integrator 42 as shown in FIG. 4 constitutes an 
incomplete integrator which functions to integrate the output signal of 
the first subtracter 11 by obtaining a sum of a current data of the output 
signal of the first subtracter 11 and the 0.99-multiplied 
data-before-one-sample (data stored before one sample) of the output 
signal of the first subtracter 11. 
In the above mentioned noise shaper shown in FIG. 4, accordingly, assuming 
that a quantization noise generated in the quantizer 15 is "Q", a relation 
between an input signal V.sub.IN and an output signal V.sub.OUT of the 
noise shaper 10 can be expressed by the following formula: 
EQU V.sub.OUT (z)=(z.sup.-1 /P(z)).multidot.V.sub.IN 
(z)+(1-z.sup.-1)(1-0.99.multidot.z.sup.-1).multidot.Q(z)/P(z)(3) 
where P(z)=0.01.multidot.z.sup.-2 +0.01.multidot.z.sup.-1 +1 
In the above mentioned formula (3), P(z) is substantially "1" independently 
of frequency. Consequently, an output spectrum of the noise shaper 40 
becomes a spectrum formed by superposing a signal obtained by the 
second-order differentiation of the quantization noise, on the input 
signal V.sub.IN of the noise shaper 40. Namely, the quantization noise is 
shaped and superposed in a high frequency region, so that the sum of the 
noise in a signal band is remarkably reduced, without substantially 
deteriorating the noise shaping characteristics of the conventional noise 
shaper. 
In the noise shaper 40 shown in FIG. 4, when the input signal V.sub.IN 
becomes the no-signal condition, since the first integrator 42 has a leak 
integration construction so as to accumulatively integrate the 0.99 times 
the integration amount, the output (integration amount) of the first 
integrator 42 becomes "0" (zero) with the lapse of time. 
On the other hand, the initial value of the second integrator 14 in the 
form of a complete integrator influences the output signal of the 
quantizer 15 in a transient condition, but becomes a steady condition with 
the lapse of time. 
Accordingly, when the input signal V.sub.IN becomes the no-signal 
condition, a single predetermined pattern is outputted from the noise 
shaper 10, regardless of the initial value of each of the two integrators 
42 and 14 included in the noise shaper. 
Here, it is to noted that, the larger the integration leak of the first 
integrator 42 is, namely, the smaller the multiplying coefficient in the 
first integrator 42, the time required until the output pattern becomes 
steady, becomes long, but the deterioration of the S/N ratio can be 
reduced. 
Referring to FIG. 5, there is shown a block diagram of a third embodiment 
of the noise shaper in accordance with the present invention. In FIG. 5, 
elements corresponding to those shown in FIG. 3 are given the same 
Reference Numerals, and explanation thereof will be omitted for 
simplification of the description. 
As will be seen from comparison between FIGS. 3 and 5, a noise shaper 70 of 
the third embodiment shown in FIG. 5 is different from the first 
embodiment shown in FIG. 3, in which the multiplier 17 shown in FIG. 3 is 
omitted, and the first integrator 12 shown in FIG. 3 is replaced with a 
first integrator 72 as shown in FIG. 5. 
As shown in FIG. 5, the first integrator 72 includes a first adder 81 
having a first input receiving an input signal applied to the first 
integrator 72 (namely, the output of the first subtracter 11), a first 
delay circuit 82 receiving an output of the first adder 81 to output a 
one-sample-delayed signal, and a multiplying circuit 83 having an input 
connected to an output of the first delay circuit 82 for multiplying the 
one-sample-delayed signal outputted from the first delay circuit 82, by a 
positive coefficient "0.999", an output of the multiplying circuit 83 
being connected to a second input of the first adder 81. An output of the 
first adder 81 constitutes the output of the first integrator 72, namely, 
is connected to the minuend input of the second subtracter 13. The first 
delay circuit 82 and the multiplying circuit 83 are located only in a 
feedback path to the first adder 81. 
Accordingly, the first integrator 72 constitutes an incomplete integrator 
which functions to integrate the output signal of the first subtracter 11 
by obtaining a sum of a current data of the output signal of the first 
subtracter 11 and the 0.999-multiplied data-before-one-sample (dam stored 
before one sample) of the output signal of the first subtracter 11. 
In the noise shaper 70 as mentioned above, accordingly, assuming that a 
quantization noise generated in the quantizer 15 is "Q", a relation 
between an input signal V.sub.IN and an output signal V.sub.OUT of the 
noise shaper 70 can be expressed by the following formula: 
EQU Vhd OUT(z)=(z.sup.-1 /P(z)).multidot.V.sub.IN 
(z)+(1-z.sup.-1)(1-0.999.multidot.z.sup.-1).multidot.Q(z)P(z)(4) 
where P(z)=0.001.multidot.z.sup.-1 +1 
In the above mentioned formula (4), P(z) is substantially "1" independently 
of frequency. Consequently, an output spectrum of the noise shaper 70 
becomes a spectrum formed by superposing a signal obtained by the 
second-order differentiation of the quantization noise, on the input 
signal V.sub.IN of the noise shaper 70. Namely, the quantization noise is 
shaped and superposed in a high frequency region, so that the sum of the 
noise in a signal band is remarkably reduced, without substantially 
deteriorating the noise shaping characteristics of the conventional noise 
shaper. 
In the noise shaper 70 as shown in FIG. 5, when the input signal V.sub.IN 
becomes the no-signal condition, since the first integrator 72 has a leak 
integration construction so as to accumulatively integrate the 0.999 times 
the integration amount, the output (integration amount) of the first 
integrator 72 becomes "0" (zero) with the lapse of time. 
On the other hand, the initial value of the second integrator 14 in the 
form of a complete integrator, influences the output signal of the 
quantizer 15 in a transient condition, but becomes a steady condition with 
the lapse of time. 
Accordingly, when the input signal V.sub.IN becomes the no-signal 
condition, a single predetermined pattern is outputted from the noise 
shaper 70, regardless of the initial value of each of the two integrators 
72 and 14 included in the noise shaper 70. 
As seen from the above, the noise shaper in accordance with the present 
invention is such that, when the input signal is brought into the 
no-signal condition, the output signal pattern is put in a predetermined 
single pattern without deteriorating the S/N ratio. Accordingly, when the 
output of the noise shaper in accordance with the present invention is 
connected to a D/A converter, the D/A converter can operate stably, even 
if the characteristics of the D/A converter is different from an ideal 
characteristics because of variations in analog elements included in the 
D/A converter. 
Furthermore, the noise shaper in accordance with the present invention 
makes unnecessary an extra circuit for detecting whether or not the input 
signal is "0" (zero). 
The invention has thus been shown and described with reference to the 
specific embodiments. However, it should be noted that the present 
invention is in no way limited to the details of the illustrated 
structures but changes and modifications may be made within the scope of 
the appended claims.