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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a frequency subband encoding apparatus, and more
particularly to a frequency subband encoding apparatus wherein mask levels
for individual frequency subbands are calculated from a digital audio
signal based on a psychoacoustic model and signal levels of the individual
frequency subbands are detected from samples of the individual frequency
subbands, and then signal to mask level ratios which are ratios of the
signal levels and the mask levels are calculated for the individual
frequency subbands and bit allocation amounts for the individual frequency
subbands are determined from the signal to mask level ratios, whereafter
samples of the individual frequency subbands are quantized with
quantization step numbers determined from the bit allocation amounts.
2. Description of the Related Art
As an example of a conventional frequency subband encoding apparatus, a
frequency subband encoding apparatus is described in the ISO/IEC 11172-3
standards (hereinafter referred to as "MPEG/Audio system"), Layer 1.
FIG. 5 shows in block diagram the conventional frequency subband encoding
apparatus. Referring to FIG. 5, the frequency subband encoding apparatus
includes a subband filter 1, a scale factor determinator 2, a quantizer 3,
an SMR calculator 4 for outputting, for each of frequency subbands, a
signal to mask level ratio (hereinafter referred to as SMR) which is a
ratio between a signal level of the frequency subband and a mask level for
the frequency subband based a psychoacoustic model, a bit allocator 15 for
determining a bit allocation amount, and a multiplexer 6.
The subband filter 1 outputs values of a PCM (Pulse Code Modulation) signal
7 divided into 32 frequency subbands. The sampling frequency of the output
signal of the PCM signal 7 is equal to 1/32 the sampling frequency of the
PCM signal 7. The scale factor determinator 2 determines, from an output
value of each of the frequency subbands of the subband filter 1, and
outputs a scale factor for use to perform normalization for each 12
samples of the frequency subband. The SMR calculator 4 calculates a mask
level for each of the individual frequency subbands from the PCM signal 7
based on a psychoacoustic model, calculates a signal level from samples of
each of the frequency subbands and the corresponding scale factor, and
calculates and outputs, for each frequency subband, a ratio (SMR) between
the signal level and the mask level, which is a ratio between the two
levels. The bit allocator 15 determines a bit allocation amount for each
of the frequency subband from the SMR of the frequency subband. The
quantizer 3 quantizes, with the bit allocation amount and the scale factor
for each of the frequency subbands, samples of the frequency subband and
outputs the thus quantized samples. The multiplexer 6 multiplexes the bit
allocation information from the bit allocator 15, the scale factors from
the scale factor determinator 2 and the quantized values from the
quantizer 3 and outputs a bit stream 8.
The number of bits included in one frame of the "MPEG/Audio Layer 1" system
depends upon the values of the bit rate and the sampling frequency. As an
example, when the bit rate is 192 kbps and the sampling frequency is 48
kHz, the number of bits of one frame is 1,536 bits in accordance with the
standards mentioned above.
The structure of one frame of the "MPEG/Audio Layer 1" system includes, for
example, as shown in FIG. 6, factors of a header, a CRC (Cyclic Redundancy
Code), a bit allocation, a scale factor, a sample and an ancillary bit.
The header normally includes 32 bits, and the bit allocation requires 4
bits for each frequency subband and totally requires 128 (4.times.32)
bits/ch. The other factors vary depending upon conditions. The header
includes a bit representing whether or not a CRC is present. When a CRC is
present, 16 bits are required for the CRC. The scale factor requires, when
the bit allocation to each frequency subband is not equal to 0, 6 bits for
each frequency subband. The sample requires, when the bit allocation to
each frequency subband is not equal to 0, 12 bits for each one bit of the
bit allocation. The ancillary bit includes bits provided by a number equal
to the remaining number when a total number of bits mentioned above is
subtracted from a total bit number of one frame.
A maximum bit number allocated by bit allocation processing exhibits a
maximum value when the ancillary bit is not allocated intentionally
without a CRC. The total bit number of the scale factor and the sample
which is an allocatable bit number of one frame in the aforementioned case
wherein the bit rate is 192 kbps and the sampling frequency is 48 kHz is
1,376 (1,536-(32+128)) bits.
The relationship between the allocated bit number for each of the frequency
subbands and the signal to noise ratio (S/N ratio) which depends upon the
allocated bit number is illustrated in FIG. 7. The bit numbers allocated
to the frequency subband range from 0 to 15 bits except one bit (not shown
in FIG. 7).
FIG. 8 is a flow chart illustrating bit allocation processing of the
conventional frequency subband encoding apparatus shown in FIG. 5. While
the MPEG/Audio system allows encoding of up to 2 channels, description is
given here of bit allocation processing for one channel. At step P1, a
frequency subband which exhibits a maximum SMR among SMR values of the 32
frequency subbands is detected, and the allocated bit number of the
frequency subband is increased by one step (FIG. 7) and a new SMR is
detected in accordance with the thus increased bit number. When bits are
allocated to the frequency subband at first (when 2 bits are allocated to
the frequency subband to which 0 bit has been allocated in FIG. 5),
totaling 30 bits are allocated including 6 bits allocated to the scale
factor and 24 bits allocated to the sample (2.times.12=24 since the audio
signal is sectioned into 12 samples with respect to the time base in one
frame). As a result, if the thus allocated bit number becomes equal to 15
bits (maximum bits in FIG. 7) which are a limit value, then the frequency
subband is removed from the object of comparison processing of the SMR so
that no allocation processing may occur later with the frequency subband.
When bits are to be allocated to any frequency subband to which bits (two
or more bits) have been allocated already, only 12 bits (1.times.12) are
allocated to the sample. The thus allocated bit number is subtracted from
the allocatable bit number.
At step P2, it is discriminated whether or not the allocatable bit number
is equal to or greater than 30, and if the allocatable bit number is equal
to or greater than 30, then the control sequence returns to step P1. On
the contrary if the allocatable bit number is smaller than 30, the control
sequence advances to step P3. At step P3, it is discriminated whether or
not the allocatable bit number is smaller than 12. If the allocatable bit
number is smaller than 12, then the processing is ended. However, if the
allocatable bit number is equal to or greater than 12, then the control
sequence advances to step P4. At step P4, frequency subbands to which bits
have not been allocated are removed from the object of comparison
processing of the SMR so that allocation processing may not occur later
with the frequency subbands.
Then at step P5, a frequency subband which exhibits a maximum SMR among the
SMR values of the 32 frequency subbands is detected, and the allocated bit
number of the frequency subband is increased by one step. As a result, if
the allocated bit number becomes equal to 15 which is the limit value,
then the frequency subband is removed from the object of comparison
processing of the SMR so that allocation processing may not thereafter
occur with the frequency subband. Further, the allocated bit number is
subtracted from the allocatable bit number. Then at step P6, it is
discriminated whether or not the allocatable bit number is equal to or
greater than 12. If the allocatable bit number is equal to or greater than
12, then the control sequence returns to step P5. However, if the
allocatable bit number is smaller than 12, then the processing is ended.
Comparison processing for repeating a fixed number of processing operations
is included in each of the processing operations at steps P1, P4 and P5.
Such processing can be reduced by describing a same process in series by a
necessary number of times. Therefore, the numbers of times of comparison
processing operations at the individual processing operations except
repetitive processing operations of the fixed numbers are such as
illustrated in FIG. 9.
The condition wherein the bit allocation processing amount described above
exhibits a maximum value is the case wherein the number of repetitions
from step P1 to step P2 exhibits a maximum value. This because, since the
bit allocation is one-sided to a certain frequency subbands, the bit
number of the scale factor is reduced while the bit number of the sample
is increased and consequently the processing number of times from step P1
to step P2 exhibits a maximum value. In the case described above wherein
the bit rate is 192 kbps and the sampling frequency is 48 kHz, the
repetition number exhibits a maximum value when 14 bits are allocated to
the frequency subbands 0 to 5, 13 bits are allocated to the frequency
subbands 6 to 7 and 0 bits are allocated to the other frequency subbands.
As an example which includes such bit allocation as described above, in
such SMR values as illustrated in FIG. 10, the processing operations from
step P1 to step P2 are repetitively executed by 101 times, and the
processing operations from step P5 to step P6 are executed once. As a
result, comparison processing occurs by a total number of 3,501 times. The
bit allocator 15 which performs such bit allocation processing as
described above in the conventional frequency subband encoding apparatus
of FIG. 5 is realized, for example, using a microprocessor.
In the conventional frequency subband encoding apparatus described above,
the processing of the bit allocator includes a large number of comparison
processing operations. Therefore, the conventional frequency subband
encoding apparatus is disadvantageous in that, in order to encode an audio
signal on the predetermined real time basis, a microprocessor and other
hardware elements having a high processing capacity are required and a
very high cost is required for the apparatus.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a a frequency subband
encoding apparatus wherein the amount of comparison operation processing
of a bit allocator is minimized.
In order to attain the object described above, according to the present
invention, there is provided a frequency subband encoding apparatus,
comprising a subband filter for dividing a digital audio signal into a
plurality of frequency subbands, a signal to mask level ratio calculator
for calculating mask levels for the individual frequency subbands from the
digital audio signal based on a psychoacoustic model, detecting signal
levels of the individual frequency subbands from samples of the frequency
subbands and outputting signal to mask level ratios which are ratios
between the signal levels and the mask levels for the individual frequency
subbands, a bit allocator for determining a reference noise to mask level
ratio (NMR) which is a value obtained by subtracting, from a signal to
mask level ratio of the entire frequency band, a signal to noise ratio
calculated from a reference bit number to be allocated to one of the
frequency subbands which exhibits a maximum signal to mask level ratio and
adjusting the reference NMR to perform bit allocation, and a quantizer for
quantizing samples of the individual frequency subbands with quantization
step numbers calculated from bit allocation amounts to the individual
frequency subbands. The bit allocator temporarily allocates, to those of
the frequency subbands which have higher signal to mask level ratios than
the reference NMR, bit numbers corresponding to differences between the
signal to mask level ratios and the reference NMR and then adjusts the
reference NMR to perform bit allocation repetitively until a remaining bit
number after the temporary bit allocation exhibits a minimum value.
In the frequency subband encoding apparatus, a reference NMR is calculated
from a maximum SMR and a reference bit number, and temporary bit
allocation is performed based on the reference NMR. Then, by adjusting the
reference NMR, temporary bit allocation is performed until an allowable
bit number is reduced to a minimum value. Thereafter, allocation
processing of a remaining bit number or releasing processing of an
insufficient bit number is performed. By this process, the number of
comparison processing operations in bit allocation processing can be
reduced comparing with that required with the conventional frequency
subband encoding apparatus. According to a preferred form, the number of
comparison processing operations can be reduced from 3,501 with the
conventional frequency subband encoding apparatus to 272 with the
frequency subband encoding apparatus of the present invention.
The above and other objects, features and advantages of the present
invention will become apparent from the following description and the
appended claims, taken in conjunction with the accompanying drawings in
which like parts or elements are denoted by like reference characters.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a frequency subband encoding apparatus showing
a preferred embodiment of the present invention;
FIGS. 2(a) and 2(b) are flow charts illustrating bit allocation processing
of the frequency subband encoding apparatus of FIG. 1;
FIG. 3 is a flow chart illustrating temporary bit allocation processing of
the frequency subband encoding apparatus of FIG. 1;
FIG. 4 is a view illustrating comparison operation amounts at individual
steps of operations in the bit allocation processing of the frequency
subband encoding apparatus of FIG. 1;
FIG. 5 is a block diagram showing a conventional frequency subband encoding
apparatus;
FIG. 6 is a diagrammatic view showing a structure of a frame of the
MPEG/Audio Layer 1 system;
FIG. 7 is a diagrammatic view illustrating the relationship between the
allocated bit number and the S/N ratio;
FIG. 8 is a flow chart illustrating conventional bit allocation processing;
FIG. 9 is a diagrammatic view illustrating comparison operation amounts at
individual steps of operations in the conventional bit allocation
processing illustrated in FIG. 8; and
FIG. 10 is a diagrammatic view illustrating an example of SMR values for
individual frequency subbands when the comparison operation processing
amount exhibits a maximum value in the conventional bit allocation
processing illustrated in FIG. 8.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1, there is shown a frequency subband encoding
apparatus to which the present invention is applied. The frequency subband
encoding apparatus includes, similarly to the conventional frequency
subband encoding apparatus described hereinabove with reference to FIG. 5,
a subband filter 1, a scale factor determinator 2, a quantizer 3, an SMR
calculator 4 for outputting, for each of frequency subbands, a signal to
mask level ratio (SMR) which is a ratio between a signal level of the
frequency subband and a mask level for the frequency subband based a
psychoacoustic model, a bit allocator 5 for determining a bit allocation
amount, and a multiplexer 6. The present frequency subband encoding
apparatus is different from the conventional frequency subband encoding
apparatus of FIG. 5 only in that the bit allocator 5 operates in a manner
different from that of the bit allocator 15. Therefore, only operation of
the bit allocator 5 will be described in detail below.
A procedure of bit allocation processing of the bit allocator 5 is
illustrated in FIGS. 2(a), 2(b) and 3. Referring first to FIG. 2(a), first
at step S1, a frequency subband which exhibits a maximum signal to mask
level ratio (SMR) among 32 frequency subbands is detected. A value
obtained by subtracting, from the maximum SMR, an S/N ratio which
corresponds to a number of bits (reference bit number) to be allocated to
the frequency subband which has the maximum SMR is determined as reference
NMR. At step S2, to those frequency subbands whose SMR is higher than the
reference NMR, bit numbers corresponding to the differences of the SMR
values from the reference NMR are temporarily allocated.
Details of the temporary allocation processing are illustrated in FIG. 3.
Referring to FIG. 3, first at step S201, "0" is substituted into a
variable SB, and then at step S202, a temporary allocation bit number is
determined from the difference of an SMR corresponding to the variable SB
from the reference NMR. Upon such determination of the allocation bit
number, one of 16 different bit allocation levels is selected based on the
relationship between the S/N ratio and the bit number illustrated in FIG.
7. Consequently, by employing the present searching technique, an
allocated bit number can be determined by five comparison processing
operations. At step S203, a used bit number determined from the thus
determined bit number is subtracted from an allocatable bit number, and an
SMR determined from the allocated bit number is subtracted from the SMR of
the frequency subband. Then at step S204, the variable SB is incremented
by one, and then at step S205, it is discriminated whether or not the
variable SB is equal to 32. If the variable SB is not equal to 32, then
the control sequence returns to step S202, but if the variable SB is equal
to 32, then the temporary allocation processing of FIG. 3 is ended and the
control sequence returns to the original routine.
Referring back to FIG. 2(a), a remaining bit number is substituted into a
variable NEW at step S3. Then at step S4, it is discriminated whether or
not the allocatable bit number after such bit allocation as described
above is smaller than 0. When the allocatable bit number is equal to or
greater than 0, the control sequence advances to step S5, but when the
allocatable bit number is smaller than 0, the control sequence advances to
step S9. At step S5, it is discriminated whether or not the reference bit
number is equal to 14 which is smaller by one than a maximum allocatable
bit number. When the reference bit number is 14, the temporary allocation
processing is ended and the control sequence advances to step S15
illustrated in FIG. 2(b). On the contrary when the reference bit number is
not equal to 14 at step S5, the control sequence advances to step S6, at
which the reference bit number is increased by one step and the reference
NMR is calculated again, and then temporary bit allocation is performed.
Then at step S7, the variable NEW is substituted into another variable OLD
and the allocatable bit number is substituted into the variable NEW. At
step S8, it is discriminated whether or not the allocatable bit number is
greater than 0. If the allocatable bit number is greater than 0, the
control sequence returns to step S5, but otherwise, the control sequence
advances to step S13.
On the other hand, at step S9, it is discriminated whether or not the
reference bit number is equal to 2 prior by one step to the non-allocated
condition. If the reference bit number is equal to 2, then the temporary
allocation processing is ended and the control sequence advances to step
S15 of FIG. 2(b). If the reference bit number is not equal to 2 at step
S9, then the control sequence advances to step S10, at which the reference
bit number is decreased by one step and the reference NMR is calculated
again, and then bit releasing is performed. Then at step S11, the released
bit number is added to the allocatable bit number, and at step S12, it is
discriminated whether or not the allocatable bit number is equal to or
greater than 0. If the allocatable bit number is smaller than 0, the
control sequence returns to step S9, but on the contrary if the
allocatable bit number is equal to or greater than 0, the control sequence
advances to step S13. At step S13, the variable OLD and an absolute value
of the variable NEW are compared with each other, and if the absolute
value of the variable NEW is greater, then the variable OLD is substituted
into the remaining bit number and also the bit allocation information is
returned to the value of the variable OLD. Thereafter, the control
sequence advances to step S15 of FIG. 2(b).
Referring now to FIG. 2(b), at step S15, the remaining bit number is
checked. If the remaining bit number is smaller than 0, then the control
sequence advances to step S16, but if the remaining bit number is equal to
or greater than 0, the control sequence advances to step S19. At step S16,
those frequency subbands to which bits have not been allocated yet are
removed from the object of comparison processing of the SMR. Then at step
S17, a frequency subband which exhibits a minimum SMR is detected, and the
allocated bit number of the frequency subband is decreased by one step.
Subsequently at step S18, it is discriminated whether or not the allocable
bit number is smaller than 0. If the allocatable bit number is smaller
than 0, then the control sequence returns to step S17, but if the
allocatable bit number is equal to or greater than 0, then the control
sequence advances to step S21.
On the other hand, at step 19, it is discriminated whether or not the
allocable bit number is smaller than 30. When the allocatable bit number
is smaller than 30, the control sequence advances to step S21, but when
the allocatable bit number is equal to or greater than 30, the control
sequence advances to step S20. At step S20, a frequency subband which
exhibits a maximum SMR is detected, and the allocated bit number of the
frequency subband is increased by one step, whereafter the control
sequence returns to step S19.
At step S21, it is discriminated whether or not the allocatable bit number
is smaller than 12. If the allocatable bit number is smaller than 12, then
the processing is ended. On the contrary if the allocatable bit number is
equal to or greater than 12, then the control sequence advances to step
S22, at which those frequency subbands to which bits have not been
allocated yet are removed from the object of the comparison processing of
the SMR. Then at step S23, a frequency subband which exhibits a maximum
SMR is detected, and the allocated bit number of the frequency subband is
increased by one step. Subsequently at step S24, it is discriminated
whether or not the remaining bit number is smaller than 12. If the
remaining bit number is smaller than 12, then the processing is ended, but
on the contrary if the remaining bit number is equal to or greater than
12, then the control sequence returns to step S23.
FIG. 4 illustrates comparison operation numbers which are comparison
processing amounts of the various processing operations in the present
embodiment. The comparison operation numbers in the present embodiment are
calculated in the condition in which the comparison calculation amount by
the conventional frequency subband encoding apparatus exhibits a maximum
value. Here, the reference bit number is selected to be 9 bits. From step
S4, the control sequence advances to step S5, and the processing
operations at steps S5 to S8 are repeated by five times. From step S15,
the control sequence advances to step S19, and from step S19, the control
sequence advances to step S20. Then, when the control sequence advances to
step S19 for the third time, the control sequence advances from step S19
to step S21, from which the control sequence comes to an end.
Consequently, the total number of comparison operations is 272. Meanwhile,
the result of the bit allocation processing is the same as that obtained
by the conventional frequency subband encoding apparatus.
Having now fully described the invention, it will be apparent to one of
ordinary skill in the art that many changes and modifications can be made
thereto without departing from the spirit and scope of the invention as
set forth herein.
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