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Claims  |
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We claim:
1. A coder for transforming an input signal into a low bit rate digital
output signal comprising:
a plurality of coding means, including respective output quantizers, for
independently receiving and transforming the input signal into respective,
compressed digital output signals having respective different numbers of
output bits as defined by the respective output quantizers thereof;
a plurality of candidate output signal determining means having respective,
different and unique quality evaluating standards, each said candidate
output signal determining means receiving the respective digital output
signals of said plurality of coding means and determining, in accordance
with the respective and unique quality evaluating standard thereof, which
of the plurality of received digital output signals satisfy the respective
and unique quality evaluating standard thereof and then, for the digital
output signals which satisfy the evaluating standard, determining which
thereof has the smallest number of output bits, and producing a candidate
output signal control output identifying the respective coding means for
the thus determined digital output signal and thereby designating the
digital output signal thereof as a candidate output signal of the coder
system, said plurality of candidate output signal determining means thus
providing a corresponding plurality of candidate output signal control
outputs each designating a candidate output signal and the respective
coding means;
power detecting means for detecting, and thereby obtaining a value
representing, the power level of said input signal to the coder and for
producing a corresponding power level control output;
control determining means for receiving the plurality of candidate output
signal control outputs of said respective plurality of candidate output
signal determining means and responsive to the power level control output
of said power detecting means for selecting one of the plurality of
candidate output signal control outputs and producing a corresponding
coding means selection control output which designates the thus selected
coding mean corresponding to the selected candidate output signal; and
output selecting means responsive to the coding means selection control
output for selecting the thus selected candidate output signal of the
thereby designated and respectively associated coding means as the actual,
low bit rate digital output signal of the coder system.
2. A coder for transforming an input signal into a low bit rate digital
signal, comprising:
a plurality of coding means, each comprising an output quantizer and said
plurality thereof having respective, different numbers of output bits, for
independently transforming said input signal into respective, compressed
digital output signals;
a plurality of decoding means, corresponding to and respectively associated
with said plurality of coding means, for receiving the respective
compressed digital output signals thereof and producing corresponding
decoded output signals;
a plurality of error obtaining means corresponding to and respectively
associated with said plurality of decoding means, each thereof for
obtaining an error, generated through the corresponding coding means and
the corresponding decoding means, by comparing said input signal and the
respective decoded output signals of the corresponding decoding means and
producing corresponding error output signals;
a plurality of S/N obtaining means, corresponding to and respectively
associated with said plurality of decoding means, each thereof for
obtaining a signal-to-noise ratio by comparing the power level of said
input signal and the corresponding power levels of the respective outputs
of the plurality of corresponding error obtaining means and producing
corresponding S/N ratio output signals;
S/N control candidate output signal determining means for determining the
output of a specific said coding means, among said plurality of coding
means, as a candidate output signal of said coder, in accordance with the
respective S/N ratio output signals of said plurality of S/N obtaining
means and producing a corresponding S/N ratio selector control output
signal;
error level control candidate output signal determining means for
determining the output of a specific said coding means, among the outputs
of said plurality of coding means, as a candidate output signal of said
coder, in accordance with the respective error output signals of said
plurality of error obtaining means and producing a corresponding
error-based selector control output signal;
control determining means for selecting one of said S/N control candidate
output signal determining means and said error level control candidate
output signal determining means as the means to determine the actual
output of the coder, in accordance with the power level of said input
signal; and
output selecting means for selecting the output of a specific one of said
plurality of coding means as the actual output of the coder in response to
the respective selector control output signal of said selected one of said
S/N control candidate output signal determining means and said error level
control candidate output signal determining means.
3. A coder according to claim 2, wherein said control determining means
selects said S/N control candidate output signal determining means as the
means to determine said actual output of the coder when said power level
of the input signal is greater than a predetermined value, and said
control determining means selects said error level control candidate
output signal determining means as the means to determine said actual
output of the coder when the power of the input signal is not greater than
said predetermined value.
4. A coder according to claim 3, wherein said S/N control candidate output
signal determining means first determines the outputs of the coding means
for which the corresponding outputs of the S/N obtaining means satisfy a
predetermined S/N condition, and then determines, from among said coding
means satisfying said predetermined S/N condition, the output of the
coding means having the smallest number of output bits as said candidate
output signal of the coder.
5. A coder according to claim 4, wherein said predetermined S/N condition
is that the signal-to-noise ratio of the coding means output is greater
than a predetermined threshold value.
6. A coder according to claim 3, wherein said error level control candidate
output signal determining means first determines the outputs of the coding
means for which the corresponding outputs of the error obtaining means
satisfy a predetermined error condition when the coder means outputs are
each decoded through the corresponding decoding means and the
corresponding errors are each obtained in said error obtaining means, and
then determines, from among the outputs of said coding means satisfying
the predetermined error condition, the output of the coding means having
the smallest number of output bits, as said candidate output signal of the
coder.
7. A coder according to claim 6, wherein said predetermined error condition
for an error is that the error is not to be greater than a predetermined
maximum.
8. A coder for transforming an input signal into a low bit rate digital
signal, comprising:
a plurality of coding means, each comprising an output quantizer having a
different number of output bits, for independently transforming said input
signal into respective, compressed digital signals;
a plurality of decoding means corresponding to and respectively associated
with said plurality of coding means, for receiving the respective
compressed digital output signals thereof and producing corresponding
decoded output signals;
a plurality of error obtaining means corresponding to and respectively
associated with said plurality of decoding means, each thereof for
obtaining an error, generated through the corresponding coding means and
the corresponding decoding means, by comparing said input signal and the
respective, decoded output signals of the corresponding decoding means and
producing corresponding error output signals;
a plurality of S/N obtaining means, corresponding to and respectively
associated with said plurality of decoding means, each thereof for
obtaining a signal-to-noise ratio by comparing the power level of said
input signal and the power levels of the respective outputs of the
plurality of corresponding error obtaining means and producing
corresponding signal-to-noise ratio output signals;
S/N control candidate output signal determining means for determining the
output of a specific said coding means, among said plurality of coding
means, as a candidate output signal of said coder, in accordance with the
respective signal-to-noise ratio output signals of said plurality of S/N
obtaining means and producing a corresponding signal-to-noise ratio
selector control output signal;
error level control output candidate determining means for determining the
output of a specific said coding means, among the outputs of said
plurality of coding means, as a candidate output signal of said coder, in
accordance with the respective error signal outputs of said plurality of
error obtaining means and producing a corresponding error-based selector
control output signal;
means for defining a first power level threshold value and a second power
level threshold value which is less than the first threshold value;
control determining means for determining, as the actual output of the
coder, the output of the coding means which is determined by said S/N
control candidate output signal determining means when said power level of
the input signal is greater than the first power level threshold value,
for determining, as the actual output of the coder, the output of the
coding means which is determined by said error level control output
determining means when the power level of the input signal is not greater
than the second power level threshold value, and for determining, as the
actual output of the coder, the candidate output signal of the coding
means having a smaller number of output bits, as between said candidate
output signal determined by said S/N control candidate output signal
determining means and said candidate output signal determined by said
error level control candidate output signal determining means, when said
power level of the input signal is between said first and second power
level threshold values; and
an output selecting means for determining, as the actual output of the
coder, the output of the coding means determined by said control
determining means.
9. A coder for transforming an input signal into a low bit rate digital
signal, comprising:
a plurality of coding means in each of which a number of output bits can be
variably set, and each of which can carry out a predictive coding
independently;
a plurality of prediction gain obtaining means each for obtaining a
prediction gain in each of said plurality of coding means;
an bit rate calculating means for determining a plurality of successive
numbers as said numbers of output bits of said plurality of coding means,
based on a required value of the signal-to-noise ratio of the coder and
said prediction gains in said plurality of coding means;
a bit rate setting means for setting said plurality of successive numbers
in said plurality of coding means as said number of output bits,
respectively; and
an output selecting means for selecting an output of one of said plurality
of coding means, as the output of said coder, based on a predetermined
standard.
10. A coder according to claim 9, further comprising:
a plurality of decoding means being provided corresponding to said
plurality of coding means;
a plurality of error obtaining means being provided corresponding to said
plurality of decoding means, and each obtaining an error which is
generated through a corresponding coding means and a corresponding
decoding means, based on said input signal and an output of a
corresponding decoding means; and
a plurality of S/N obtaining means being provided corresponding to said
plurality of decoding means, and each obtaining a signal-to-noise ratio,
based on the power of said input signal and the power of an output of a
corresponding error obtaining means;
wherein said predetermined standard comprises a first requirement that a
signal-to-noise ratio in the construction through a coding means and a
corresponding decoding means is not to be smaller than a predetermined
threshold, and a second requirement that an output having a smallest
number of output bits is to be selected among the outputs of the coding
means satisfying said first requirement.
11. A coder for transforming an analog input signal into a low bit rate
digital signal, comprising:
an analog to digital converter provided in an input stage, and converting
said analog input signal to a digital signal;
a plurality of coding means in each of which a number of output bits can be
variably set, and each of which can carry out a predictive coding
independently;
a plurality of prediction gain obtaining means each for obtaining a
prediction gain in each of said plurality of coding means;
a bit rate calculating means for determining a plurality of successive
numbers as said numbers of output bits of said plurality of coding means,
based on a required value of the signal-to-noise ratio of the coder, an
ADC signal-to-noise ratio determined by an input amplitude range and
resolution in said analog to digital converter, and said prediction gains
in the plurality of coding means;
a bit rate setting means for setting said plurality of successive numbers
in said plurality of coding means as the number of output bits,
respectively; and
an output selecting means for selecting an output of one of said plurality
of coding means as the output of said coder, based on a predetermined
standard.
12. A coder according to claim 11, wherein said plurality of successive
numbers are determined as outputs of 2k+1 coding means having number of
output bits i-k to i+k with a center value of the number of output bits i,
where the number i is determined by the equations, i=(S/N-PG+10.8-20
log.sub.10 AR)/6 when S/N.ltoreq.SNI, and i=(SNI-PG+10.8-20 log.sub.10
AR)/6 when S/N.gtoreq.SNI, S/N is said required value of the
signal-to-noise ratio of the coder, SNI is said ADC signal-to-noise ratio
in said analog to digital converter, PG is said prediction gain in each
coding means, and AR is said input amplitude range expressed by a root
mean square of the input signal level.
13. A coder according to claim 12, further comprising:
a plurality of decoding means being provided corresponding to said
plurality of coding means;
a plurality of error obtaining means being provided corresponding to said
plurality of decoding means, and each obtaining an error generated through
a corresponding coding means and a corresponding decoding means based on
said input signal and an output of a corresponding decoding means; and
a plurality of S/N obtaining means being provided corresponding to said
plurality of decoding means, and each obtaining a signal-to-noise ratio
based on the power of said input signal and the power of an output of a
corresponding error obtaining means;
wherein said predetermined standard includes a first requirement that a
signal-to-noise ratio in the construction through a coding means and a
corresponding decoding means is not to be smaller than a predetermined
threshold, and a second requirement that an output of a coding means
having a smallest number of output bits is to be selected as the output of
the coder among the outputs of the coding means satisfying said first
requirement when the power of the input signal is greater than a
predetermined value; and said predetermined standard further includes of a
third requirement that an error which is generated through a coding means
and a corresponding decoding means is to be smaller than a predetermined
maximum, and a fourth requirement that an output of a coding means having
a smallest number of output bits is to be selected as the output of the
coder among the outputs of the coding means satisfying said third
requirement when said power of the input signal is not greater than said
predetermined value.
14. A coder according to claim 12, further comprising:
a plurality of decoding means being provided corresponding to said
plurality of coding means;
a plurality of error obtaining means being provided corresponding to said
plurality of decoding means, and each obtaining an error which is
generated through a corresponding coding means and a corresponding
decoding means, based on said input signal and an output of a
corresponding decoding means; and
a plurality of S/N obtaining means being provided corresponding to said
plurality of decoding means, and each obtaining a signal-to-noise ratio
based on the power of said input signal and the power of an output of a
corresponding error obtaining means;
wherein said predetermined standard includes a first requirement that a
signal-to-noise ratio in the construction through a coding means and a
corresponding decoding means is not to be smaller than a predetermined
threshold, and a second requirement that an output of a coding means
having a smallest number of output bits is to be selected as the output of
the coder, among the outputs of the coding means satisfying said first
requirement when the power of the input signal is greater than a first
predetermined value; said predetermined standard includes a third
requirement that an error which is generated through a coding means and a
corresponding decoding means is to be smaller than a predetermined
maximum, and a fourth requirement that an output of a coding means having
a smallest number of output bits is to be selected as the output of the
coder, among the outputs of the coding means satisfying said third
requirement when said power of the input signal is not greater than a
second predetermined value; and said predetermined standard includes a
fifth requirement that said signal-to-noise ratio satisfies said first
requirement and said error satisfies said second requirement, and a sixth
requirement that an output of a coding means having a smallest number of
output bits is to be selected as the output of the coder, among the
outputs of the coding means satisfying said fifth requirement when said
power of the input signal is between said first and second predetermined
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a variable rate coder for transforming an
input signal into a low and adaptively variable bit rate digital signal,
and, more particularly, to such a coder employing a plurality of coding
circuits each comprising an output quantizer having a different output bit
rate, and independently transforming the input signal into a compressed
digital signal.
Generally, when a voice signal or a picture signal is transmitted, the
original input signal is coded at the sender (i.e., transmitting) side by
a coding method to reduce the bit rate on the transmission line, and the
coded signal is received and decoded to regenerate the original signal in
the receiver side.
In the above transmission using coding, in addition to the requirement of a
low bit rate, a sufficient quality of the regenerated signal is required
according to the purpose of usage.
In particular, in communication systems which allow variations of bit rate,
such as a packet communication system, it is desirable to code an input
signal to a least bit rate signal which maintains a sufficient quality of
the regenerated signal from the view point of efficiency and quality.
(2) Description of the Related Art
One attempt to realize the above-mentioned coding wherein an input signal
is coded to a least bit rate signal which maintains a sufficient quality
of the regenerated signal, called a variable rate coder, is described by
T. Taniguchi, et al. in "ADPCM with a Multiquantizer for Speech Coding",
IEEE Journal on Selected Areas in Communications, Vol. 6, Feb. 1988, pp.
410-424.
The coder described in the above paper comprises a plurality of ADPCM coder
units, and the number of bits of the quantizer output in each coder unit
is different.
Input signals are coded in parallel in the plurality of coder units, the
quality of the coding in each coder unit is evaluated for each frame of
data, and the coder unit giving a lowest bit rate among the coder units
satisfying a predetermined quality requirement is selected to be used for
transmission.
To carry out the above evaluation (determination whether or not the output
of each coder unit satisfies the above predetermined requirement of
quality), two methods are described in the above paper.
In the one method, the signal-to-noise ratio (SNR) value is obtained for
each coder unit and each frame of data, and the obtained SNR value is
compared with a threshold of the SNR corresponding to the above
predetermined quality requirement.
In a coder which performs prediction coding, the difference between an
input signal and a prediction value of the input are quantized by the
quantizer, and the output of the quantizer is used as the output of the
coder. The SNR in the coder is expressed by a multiple (summation when the
SNR is expressed as a logarithm) of a prediction gain and a quantization
gain when the input signal is large enough to ignore the quantization step
size in the analog to digital converter which is provided in the input
stage of the coder. Therefore, when the prediction gain is large (the
above difference is small), only a small number of the quantization steps
in the quantizer (a small number of output bits of the quantizer) is
necessary to obtain the required SNR, and when the prediction gain is
small (the above difference is large), a large number of the quantization
steps in the quantizer (a large number of output bits of the quantizer) is
necessary to obtain the required SNR. Thus, the above-mentioned evaluation
of the SNR enables control of the output bit rate of the coder.
In the other method, a noise (error) level is used for the evaluation
instead of the SNR.
As described in the above paper, there is a problem that a high bit rate
output is selected for a low level input signal when evaluation based on
the SNR is used, and for a high level input signal when evaluation based
on the noise level is used.
Generally, an analog to digital converter is provided in the sampling stage
of a coder for converting an analog input signal to a digital signal,
where a fixed amplitude range for input signals (dynamic range) is
assigned at the input of the analog to digital converter, and the step
size (resolution) in the analog to digital converter is constant. The
constant step size in the analog to digital converter limits the SNR in
the analog to digital converter, and thus the SNR in the total coder
output. In particular, when the input level becomes lower, it becomes
difficult to obtain the required SNR value.
As mentioned, the above control of the output bit rate of the coder using
the SNR evaluation is effective when the input signal is large enough to
ignore the quantization step size in the analog to digital converter.
However, when an SNR value in the analog to digital converter which is
determined by the factor: (input signal level)/(quantization step size in
the analog to digital converter), is comparable with the SNR value
required for the total coder (i. e., required for a signal regenerated in
the receiver side through the coder and a corresponding decoder), the
above control of the output bit rate of the coder using the evaluation of
SNR is not effective because an increase in the quantization gain in the
quantizer can recover only a decrease in the prediction gain, but cannot
recover the SNR degraded in the analog to digital converter. Nevertheless,
in the conventional variable rate coder, the above control of the output
bit rate of the coder using the evaluation of SNR, i. e., an increase in
the output bit rate of the coder when the SNR of the coder becomes lower,
is carried out even when the SNR of the coder is degraded by the SNR in
the analog to digital converter due to a low level input.
In the coder wherein the above control of the output bit rate is carried
out based on the noise level instead of the SNR, the aforementioned
problem of a high bit rate for a high level input is caused by a constant
threshold level for noises which requires a high SNR for a high level
input.
Therefore, in the conventional variable rate coders, even if the output bit
rate of the coder is controlled based on the SNR or the noise level, there
is a problem that a high bit rate, which does not contribute to the
quality of the transmitted signal, is output according to the variation of
the input level.
Further, in the conventional variable rate coder as described in the
aforementioned paper by Taniguchi et al., a plurality of coder units are
provided, wherein the number of the coder units is determined by the
extent of the variation of the necessary bit rates to obtain a required
quality, and an input signal is coded in parallel in all the coding units.
Therefore, there is a problem that the scale of the hardware is very
large.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a variable rate coder for
achieving optimal coding according to a variation of an input level from
the viewpoint of quality and bit rate.
Another object of the present invention is to provide a variable rate coder
for achieving optimal coding according to a variation of an input level
from the viewpoint of quality and bit rate, with a reduced hardware size.
According to the first embodiment of the present invention, there is
provided a coder for transforming an input signal into a low bit rate
digital signal, and wherein the coder comprises a plurality of coding
means, a plurality of candidate output signal determining means, a control
determining means, and an output selecting means. The plurality of coding
means each comprises an output quantizer having a different number of
output bits, and independently transforms the above input signal into a
respective, compressed digital signal. The plurality of candidate output
signal determining means each determines an output of one of the above
plurality of coding means as a candidate output of the (variable rate)
coder, (according to the present invention) by an individual, respective
determining method, in which method, the quality of a signal which is
regenerated from an output of each coding unit is evaluated by an
individual and respective predetermined quality evaluating standard, and
then the output of the coding means having the smallest number of output
bits, among the candidate outputs of the coding means satisfying the
respective above standards, is determined as the actual output of the
coder. The control determining means selects the aforesaid one of the
above output candidate output signal determining means as the means to
determine the actual output of the coder, according to the power of the
input signal. The output selecting means selects the output of the thus
determined one of the plurality of coding means as the actual output of
the coder, according to the output of the candidate output signal
determining means which is selected by the control determining means.
According to the second embodiment of the present invention, there is
provided a coder for transforming an input signal into a low bit rate
digital signal, wherein the coder comprises a plurality of coding means, a
plurality of decoding means, a plurality of error obtaining means a
plurality of S/N obtaining means, an S/N control candidate output signal
determining means, an error level control candidate output signal
determining means, a control determining means, and an output selecting
means. The plurality of coding means each comprises an output quantizer
having a different number of output bits, and independently transforms the
above input signal into a respective, compressed digital signal. The
plurality of decoding means are provided corresponding to the above
plurality of coding means. The plurality of error obtaining means are
provided, corresponding to the above plurality of decoding means, and each
obtaining an error which is generated through a corresponding coding means
and a corresponding decoding means, based on the above input signal and an
output of the corresponding decoding means. The plurality of S/N obtaining
means are provided corresponding to the above plurality of decoding means,
and each obtains a signal-to-noise ratio based on the power of the above
input signal and the power of an output of a corresponding error obtaining
means. The S/N control candidate output signal determining means
determines an output of one of the coding means, among the outputs of the
above plurality of coding means, as the candidate output signal of the
coder, according to the outputs of the above plurality of S/N obtaining
means. The error level control candidate output signal determining means
determines an output of one of the coding means, among the outputs of the
above plurality of coding means, as the candidate output signal of the
coder, based on the outputs of the above plurality of error obtaining
means. The control determining means-which determines, as the output of
the coder, an output of a coding means determined by the above S/N control
output determining means when the power of the input signal is greater
than a first predetermined value--determines, as the actual output of the
coder, an output of a coding means determined by the above error level
control candidate output signal determining means when the power of the
input signal is not greater than a second predetermined value, and
determines, as the output of the coder, an output of a coding means having
a smaller number of output bits between the above output determined by the
above S/N control candidate output signal determining means and the above
output determined by the above error level control candidate output signal
determining means, when the above power of the input signal is between the
above first and second predetermined values. The output selecting means
determines, as the actual output of the coder, the output of the coding
means which is determined by the above control determining means.
According to the third embodiment of the present invention, there is
provided a coder for transforming an input signal into a low bit rate
digital signal, and wherein the coder comprises a plurality of coding
means, a plurality of prediction gain obtaining means, a bit rate
calculating means, a bit rate setting means, and an output selecting
means. In each of the plurality of coding means a number thereof output
bits can be variably set, and each of can carry out predictive coding
independently. Each of the plurality of prediction gain obtaining means
obtains a prediction gain, in each of the above plurality of coding means.
The bit rate calculating means determines a plurality of successive
numbers as the above numbers of output bits of the above plurality of
coding means, based on a required value of the signal-to-noise ratio of
the coder and the above prediction gains in the above plurality of coding
means. The bit rate setting means set the above plurality of successive
numbers in the above plurality of coding means as the above number of
output bits, respectively. The output selecting means select an output of
one of the above plurality of coding means, as the output of the above
coder, based on a predetermined standard.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 shows a first embodiment of the present invention;
FIG. 2 shows a second embodiment of the present invention;
FIG. 3 shows a third embodiment of the present invention;
FIG. 4 shows the construction of the bit rate presetting circuit; and
FIG. 5 shows the construction of the coder.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before describing the preferred embodiment of the present invention, the
basic principle of the first aspect of the present invention is explained
below.
As described in the summary of the invention, according to the first
embodiment of the present invention, an input signal is coded in parallel
in the plurality of coding means, and the output of one of the plurality
of coding means, which is selected by one of the plurality of output
determining means, is used as the output of the (variable rate) coder. The
above one of the output determining means is further selected by the
control determining means according to the power of the input signal.
Thus, when the characteristic (particularly the applicability or
suitability) of the quality control --using the individual predetermined
evaluation standard by each of the output determining means--depends on
the power of the input signal (or input signal level), the most suitable
or applicable one of the quality controls using the individual
predetermined evaluation standards by the output determining means is
selected by the control determining means according to the power of the
input signal (or input signal level).
FIG. 1 shows the first embodiment of the present invention.
In FIG. 1, reference numerals 111, 121, and 131 each denote a coding unit,
112, 122, and 132 each denote a decoding unit, 61, 62, and 63 each denote
an adder, 64, 65, 66, and 67 each denote a power calculator, 21, 22, and
23 each denote a divider, 4a denotes a control select circuit, 24 denotes
an S/N evaluating control circuit, 3a denotes a noise evaluating control
circuit, 51 denotes a selector, 52 denotes a selector and multiplexer, 7
denotes a transmission line, 8 denotes a demultiplexer, and 9 denotes a
decoding unit.
Among the above elements, the demultiplexer 8 and decoding unit 9 are
included in the receiver side, while all the other elements (except the
above elements in the receiver side) and the transmission line 7,
constitute a the sender side.
A input signal (denoted by SIN in FIG. 1) is input in parallel into the
coding units 111, 121, and 131.
The coding units 111, 121, and 131, which constitute the aforementioned
plurality of coding means, have different numbers of output bits (for
example, three, four, and five, respectively), and the coding units 111,
121, and 131 in FIG. 1 each carry out, for example, ADPCM coding. The
output (coded) signals (each denoted by SCOD1, SCOD2, and SCOD3) from the
coders 111, 121, and 131, are input into the selector and multiplexer 52.
the decoding units 112, 122, and 132 respectively decode the output signals
(each denoted by SCOD1, SCOD2, and SCOD3) from the corresponding coders,
and therefore, each of the decoding units 112, 122, and 132 regenerates
the input signal suffering a possible degradation by the coding and
decoding through the corresponding coding unit and decoding unit,
respectively.
The outputs of the decoding units 112, 122, and 132 (the above-mentioned
regenerated signals, respectively denoted by SDEC1, SDEC2, and SDEC3) are
subtracted from the real input signal in the adders 61, 62, and 63,
respectively, and thus, the error signals which are each generated through
the coding units 111, 121, and 131, and the decoding units 112, 122, and
132, respectively, are obtained.
The resultant error signals are input into the power calculators 65, 66,
and 67, respectively, and thus, the respective powers (denoted by PWR1,
PWR2, and PWR3) of the above error signals are obtained.
On the other hand, the power (denoted by PWR0) of the input signal is
calculated in the power calculator 64, and, signal-to-noise ratios
(denoted by SN1, SN2, and SN3) for the pairs of the coding units and the
corresponding decoding units 111 and 112, 121 and 122, and 131 and 132,
are respectively calculated in the dividers 21, 22, and 23.
The above signal-to-noise ratios SN1, SN2, and SN3 from the dividers 21,
22, and 23 are input into the S/N evaluating control circuit 24.
The S/N evaluating control circuit 24 corresponds to one of the
aforementioned condidate output signal determining means in the first
embodiment of the present invention, and determines the output of one of
the coding units 111, 121, and 131, as a condidate output signal of the
coder (i.e., a condidate output of the sender side structure as shown in
FIG. 1), according to the above signal-to-noise ratios SN1, SN2, and SN3.
In detail, the S/N evaluating control circuit 24 first determines those
outputs of the coding units corresponding to the outputs of the dividers
which satisfy a predetermined S/N condition, and then determines the
output of the coding unit having the smallest number of output bits, among
the coding units satisfying the predetermined S/N condition, as a
condidate output of the coder.
The above S/N condition is, for example, that the signal-to-noise ratio
must be greater than a predetermined threshold. The threshold value
(denoted by SNTH in FIG. 1) is predetermined to be, for example, 36 dB.
Thus, if, for example, the signal-to-noise ratios SN2 and SN3 from the
dividers 22 and 23 are above 36 dB, and the signal-to-noise ratio SN1 from
the divider 21 is below 36 dB, first, it is determined that the coding
units 121 and 131 satisfy the above S/N condition, and then the output of
the coding unit 121 is determined as a candidate output of the coder
because the number of output bits (four) of the coding unit 121 is smaller
than the number of output bits (five) of the coding unit 122, at the stage
of the S/N evaluating control circuit 24. The result of the determination
is output in the form of a selector control signal (denoted by ESN in FIG.
1), and is applied to an input terminal of selector 51.
On the other hand, the powers of the error signals PWR1, PWR2, and PWR3 are
input into the noise evaluating control circuit 3a.
The noise evaluating control circuit 3a corresponds to another of the
aforementioned candidate output signal determining means in the first
embodiment of the present invention, and determines the output of one of
the coding units 111, 121, and 131 as a candidate output of the coder
(i.e., a candidate for the output of the actual sender side structure as
shown in FIG. 1), according to the above powers of the error signals PWR1,
PWR2, and PWR3.
In detail, the noise evaluating control circuit 3a first determines the
outputs of the coding units corresponding to the outputs of the dividers
which satisfy a predetermined error condition, and then determines the
output of the coding unit having the smallest number of output bits, among
the coding units satisfying the predetermined error condition, as a
candidate output of the coder.
The above error condition is, for example, that the power of the error
signal must be smaller than a predetermined maximum value. The maximum
value (denoted by NTH in FIG. 1) is predetermined to be for example, -75
dBm0.
Thus, if, for example, the powers of the error PWR2 and PWR3 from the power
calculators 66 and 67 are below -75 dBm0, and the powers of the error
signal PWR1 from the power calculator 65 is over -75 dBm0, first, it is
determined that the coding units 121 and 131 satisfy the above error
condition, and then the output of the coding unit 121 is determined as a
candidate output of the coder because the number of output bits (four) of
the coding unit 121 is smaller than the number of output bits (five) of
the coding unit 122, at the stage of the noise evaluating control circuit
3a. The result of the determination is output in the form of a selector
control signal (denoted by EN in FIG. 1), and is then applied to a
corresponding input terminal of selector 51.
The control select circuit 4a corresponds to the aforementioned control
determining means in the first embodiment of the present invention, and is
implemented by a comparator.
The comparator 4a compares the above-mentioned power PWR0 of the input
signal SIN with a threshold level (denoted by SPTH in FIG. 1) of the power
of the input signal. The result of the comparison is output in the form of
a selector control signal (denoted by ED in FIG. 1), and is then applied
to a corresponding control input terminal of selector 51.
The above threshold level of the power of the input signal is set to, for
example, -50 dBm0.
Thus, when the power of the input signal is above the above threshold level
SPTH, the control output ESN from the S/N evaluating control circuit 24 is
selected in the selector 51. The output of the selector 51 (denoted by
SEL) is applied to the selector and multiplexer 52 as a control signal for
the selector portion of the selector and multiplexer 52. The selector and
multiplexer 52 selects one of the output signals from the respective
coding units 111, 121, and 131 according to the control signal supplied
from the selector 51, and multiplexes the selected output of a coding unit
and the above control signal for transmission through the transmission
line 7. Thus, the aforementioned control of the output bit rate of the
coder using the evaluation of SNR is carried out for selecting a number of
output bits in the coder (for selecting a coding unit).
When the power of the input signal is below the above threshold level SPTH,
the control output EN from the noise evaluating control circuit 3a is
selected in the selector 51. Similar to the above case, the output of the
selector 51 (denoted by SEL) is applied to the selector and multiplexer 52
as a control signal for the selector portion of the selector and
multiplexer 52. The selector and multiplexer 52 selects one of the output
signals from the coding units 111, 121, and 131, according to the control
signal supplied from the selector 51, and multiplexes the selected output
of a coding unit and the above control signal for transmission through the
transmission line 7. Thus, the aforementioned control of the output bit
rate of the coder, using the evaluation of noise levels, is carried out
for selecting the number of output bits in t | | |
