United States Patent [191
`Sedlmeyer et al.
`
`[54] PROCESS FOR FINDING THE OVERALL
`MONITORING THRESHOLD DURING A BIT(cid:173)
`RATE-REDUCING SOURCE CODING
`
`[75]
`
`Inventors: Robert Sedhneyer. Ismaning; Andreas
`Brefort, Krefeld; Jens Groh, Miinchen;
`Wo1fgang Krafft. Miinchen; Klaus
`Rosinski. Miinchen; Detlef Wiese.
`Neufahrn; Gerhard Stoll. Zolling;
`Martin Link. Mi.inchen. all of Germany
`
`[73] Assignee: lnstitut Fuer Rundfunktechnik
`GmbH, Munich. Germany
`
`[21] Appl. No.:
`
`[22] PCT Filed:
`
`520,765
`Jul. 21, 1992
`
`[86] PCT No.:
`
`PCT/EP92/01658
`
`§ 371 Date:
`
`Sep. 17, 1993
`
`§ 102( e) Date: Sep. 17, 1993
`
`[87] PCT Pub. No.: W093/02508
`
`PCT Pub. Date: Feb. 4, 1993
`
`Related U.S. Application Data
`
`[63] Continuation of Ser. No.119,109, Sep.17, 1993, abandoned.
`Foreign Application Priority Data
`[30]
`
`[DE] Germany ............................. 4124493.l
`
`Jul 24, 1991
`Int CL 6
`................................. G IOL 7 /02; H04B 1/66
`[51]
`[52] U.S. Cl. ........................ 395/2.36; 395/2.14; 395/2.92
`[58] Field of Search ...................... 39512. 2.1. 2.35-2.39.
`395/2.92. 2.14. 2.15; 381/36-40
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`I llll llllllll Ill llll lllll lllll IDll lllll 1111111111111111111111111111111
`US0057403 l 7 A
`[111 Patent Number:
`[ 451 Date of Patent:
`
`5,740,317
`Apr. 14, 1998
`
`llfl 990 Theile et al. ........................... 395/2.36
`4,972,484
`5,353,375 10/1994 Gotu et al .............................. 395/2.39
`
`FOREIGN PATENT DOCUMENTS
`
`0176243
`0420745A2
`0424162A2
`88/04117
`88/10035
`
`4/1986
`4/1991
`4/1991
`611988
`12/1988
`
`European Pat. Off ..
`European Pat. Off. .
`European Pat. Off ..
`WIPO.
`WIPO.
`
`Primary Emminer-Allen R. MacDonald
`Assistant Examiner-Donald L. Storm
`Attorney, Agent, or Firm-Spencer & Frank
`
`[57]
`
`ABSTRACT
`
`Source coding digitized audio signals includes providing
`time or spectral domain sampling values of the digitized
`audio signal. requantizing the sampling values according
`their permissible quantizing noise as determined by a coding
`and requantizing control signal. and multiplexing the control
`signal and the requantized sampling values into a time
`multiplex frame depending on the bit rate reduction
`employed. The coding and requantizing control signal is
`determined from the sampling values by determining the
`global masking effect from all relevant maskers which are
`tonal maskers and noise maskers, and which result from the
`sampling values, and from a silence audio threshold. The
`global masking effect is determined by segmenting masking
`edges of possible maskers and approximating in individual
`segments with lower order polynomials. and determining
`coefficients of the lower order polynomials. converting the
`maskers into logarithmic levels and using intensities of the
`maskers to determine the coefficients of the lower order
`polynomials. and determining the global masking threshold.
`step-wise, masker by masker, at individual possible base
`points. from the polynomials describing masking edges of
`the i)ossible maskers.
`
`4,862,346
`
`8/1989 Wagner et al.
`
`395/400
`
`18 Claims, 6 Drawing Sheets
`
`70
`
`'.IULTI·
`P~[XER
`
`·B
`
`Flt TER BANK Of!
`TPANSFORMATION STAGE.
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`L
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`TRA'1SFORMAHGN STAG[
`
`VOLUME
`
`40 dB f----+--+-
`
`CALClJU\TION or
`AEQUANTIZATION
`~-----' ·.5 co~~TR01 INf-OAMATIQN
`
`IPR2016-01710
`UNIFIED EX1022
`
`

`
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`CONTROL INFORMATION
`
`REQUANTIZATION
`CALCULATION OF
`160
`
`CALCULATION OF
`r-50
`
`MASKING THRESHOLD ~
`
`PLEXER
`MULTI-
`
`7~
`
`MAXIMA OF SAMPLED VALUES
`
`DETERMINATION OF
`
`4·~
`
`70
`
`REQUANTIZATION OF ll!
`
`SAMPLED VALUES
`
`20
`
`130
`
`1
`
`2~
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`r2
`
`FIG. 1
`
`\5
`
`'
`
`-TRANSFORMATION STAGE
`
`140
`
`2a~
`
`I
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`"10 dB Lr-..---~---'----L-...lL_..o=-------'-------lL-.......::_____..i=--L-L.-----==-----'
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`
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`
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`
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`
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`
`40dB1------+-~--+~----+-+++-~-+-~-H-1----<-~~-H+--1--~__...~+--l
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`
`l 50dB~-t--t-~-t-~---lr-t-~+-~-l---+-------1--1-~_J_-_.__J
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`
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`
`70dBr--~~r-~-r----,-~~-i-~-.~~..--~~.-~--r-~-.
`
`FIG.2
`
`

`
`U.S. Patent
`
`Apr. 14, 1998
`
`Sheet 3 of 6
`
`5,740,317
`
`FIG.3
`
`TIME SAMPLING VALUES
`l
`
`'
`l PROVIDE SAMPLING VALUES OF I
`DIGITIZED AUDIO SAMPLES (BLOCK 10)
`I
`~
`I '
`DETERMINE THE MAXIMUM
`SIGNAL LEVELS FROM THE
`SAMPLING VALUES (BLOCK 30)
`
`I
`I
`
`TRANSFORMATION OF DIGITIZED AUDIO
`SIGNAL TO FREQUENCY DOMAIN, WHICH
`PRODUCES DISCRETE SAMPLING VALUES
`(BLOCK 40)
`
`I DETERMINE ALL RELEVANT MASKERS
`+
`' CONVERT LEVELS OF ALL RELEVANT
`MASKERS INTO LOGARITHMIC LEVELS
`
`I DETERMINE COEFFICIENTS OF
`
`LOWER ORDER POLYNOMIALS
`
`SEGMENT MASKING EDGES
`OF ALL RELEVANT MASKERS
`
`I
`I CALCULATE SEGMENTED MASKING
`
`EDGES OF ALL RELEVANT MASKERS
`
`DETERMINE GLOBAL MASKING THRESHOLC
`MASKER-BY-MASKER CONSIDERING THE
`RESTING THRESHOLD USING DIFFERENT
`SPECTRAL SPACING IN LOWER, MIDDLE
`AND UPPER FREQUENCY RANGE
`(BLOCK 50)
`
`DERIVE THE CODING AND REOUANTIZING
`CONTROL SIGNAL FOR THE SAMPLING VALUES
`UNDER CONSIDERATION OF THE DETERMINED
`GLOBAL MASKING THRESHOLD (BLOCK 60)
`
`QUANTIZE SAMPLING VALUES ACCORDING
`TO PERMISSIBLE QUANTIZING NOISE AND
`
`A CODING AND REOUANTIZING CONTROL -
`
`SIGNAL (BLOCK 20)
`
`MULTIPLEX CODING AND REOUANTIZING CONTROL SIGNAL AND SAMPLING VALUES
`MAXIMUMS AND QUANTIZED SAMPLING VALUES INTO A TIME MULTIPLEXED FRAME
`ACCORDING TO BIT RATEREDUCTION EMPLOYED (BLOCK 70)
`
`

`
`U.S. Patent
`
`Apr. 14, 1998
`
`Sheet 4 of 6
`
`5,740,317
`
`FIG. 4
`
`1270
`
`1030
`DETERMINE ALL MASKERS WHICH MAXIMUM MASKING
`EDGE LEVEL IS LOWER THAN THE LEVEL OF THE RESTING __)
`THRESHOLD. THESE MASKERS WILL BE EXCLUDED FROM
`THE MASKING THRESHOLD COMPUTATION.
`DETERMINE ALL MASKERS WHICH SIGNAL LEVEL IS LOWER 1020
`THAN THE MASKING EDGE LEVEL OF ANOTHER MASKER AT _.)
`THIS FREQUENCY POINT. THESE MASKERS WILL BE EX-
`CLUDED FROM THE THRESHOLD COMPUTATION.
`
`i
`
`DETERMINE SPECTRAL SPACING USING A HIGHER
`SPECTRAL RESOLUTION FOR LOWER FREQUENCIES
`AND A LOWER RESOLUTION FOR HIGHER FREQUENCIES .
`
`1290
`L.J
`
`1010
`__)
`
`(J) a:
`w
`~
`(J)
`<(
`:!!
`
`1-z :g;
`w
`_J w a:
`
`_J
`_J
`<(
`a:
`0
`u..
`I(cid:173)
`<(
`LU
`0...
`LU a:
`
`•
`BEGIN THE GLOBAL MASKING THRESHOLD DETERMINATION 1000
`v
`WITH THE HIGHEST FREQUENCY MASKER
`i
`
`t
`BEGIN THE DETERMINATION OF THE GLOBAL MASKING
`THRESHOLD FOR THE RESPECTIVE MASKER TOWARD
`UPPER FREQUENCIES FIRST AND THEN TOWARD
`LOWER FREQUENCIES
`
`•
`
`UPDATE THE LEVELS OF EACH 1300
`[__/
`SPECTRAL POINT OF THE
`MASKING THRESHOLD
`t
`1040
`ABORT THE GLOBAL THRESHOLD DETERMINATION IF THE V
`MASKING EDGE OF THE CURRENTLY PROCESSED MASKER
`FALLS BELOW A CERTAIN LEVEL
`
`1J
`
`ABORT THE GLOBAL THRESHOLD DETERMINATION IF THE
`MASKING EDGE OF THE CURRENTLY PROCESSED MASKER
`IS RUNNING DOWN TO THRESHOLD RELEVANCY
`
`ABORT THE GLOBAL THRESHOLD DETERMINATION IF THE
`MASKING EDGE OF THE CURAENTL Y PROCESSED MASKER
`FALLS BELOW THE LEVEL SPECIFIED BY THE
`RESTING THRESHOLD
`
`1050
`__)
`
`1060
`__)
`
`

`
`U.S. Patent
`
`Apr. 14, 1998
`
`Sheet 5 of 6
`
`5,740,317
`
`FIG.5
`
`TO CALCULATE THE GLOBAL MASKING THRESHOLD ONLY
`DISCRETE SPECTRAL LOCATJONS ARE USED
`
`1290
`
`
`11 20
`LJ
`
`TO CALCULATE THE GLOBAL MASKING THRESHOLD A
`30
`11
`SPECTRAL SPACING OF BASE POINTS IS DONE SO THAT LJ
`THE SPACING IS SMALL IN A LOWER FREQUENCY RANGE
`AND BECOMES GREATER IN A MIDDLE FREQUENCY
`RANGE AND GREATER IN AN UPPER FREQUENCY RANGE
`11
`40
`TO CALCULATE THE GLOBAL MASKING THRESHOLD
`PLACE THEIR SPECTRAL POINTS TO EXISTING SPECTRAL :_)
`POINTS OF THE FREQUENCY DOMAIN TRANSLATION
`
`FIG. 7
`
`CONVERT LEVELS OF ALL RELEVANT MASKERS INTO
`LOGARITHMIC LEVELS WHEREIN THE LOGARITHMIC
`LEVELS ARE QUANTIZED IN LEVEL STAGES
`
`CONVERT LEVELS OF ALL RELEVANT MASKERS INTO
`LOGARITHMIC LEVELS USING A TABLE TO CONVERT
`INTENSITIES TO LOGARITHMIC LEVELS
`
`CONVERT LEVELS OF ALL RELEVANT MASKERS INTO
`LOGARITHMIC LEVELS BY DIVIDING THE INTENSITY
`VALUES INTO MANTISSA AND EXPONENT. THE INTENSITY
`MANTISSA IS TRANSLATED VIA A TABLE AND THE
`RESULTING LOGARITHMIC LEVEL IS DETERMINED BY
`COMBINING THE TRANSLATED MANTISSA
`AND THE EXPONENT.
`
`1240
`
`11
`50
`_)
`
`11
`60
`_)
`
`11
`70
`:_)
`
`

`
`U.S. Patent
`
`Apr. 14, 1998
`
`Sheet 6 of 6
`
`5,740,317
`
`RG.6
`
`UPDATE THE LEVELS OF EACH SPECTRAL POINT OF THE
`GLOBAL MASKING THRESHOLD BY DOING AN
`INTENSITY ADDITION
`
`•
`UPDATE THE LEVELS OF EACH SPECTRAL POINT OF THE
`GLOBAL MASKING THRESHOLD BY DOING AN INTENSITY
`ADDITION USING A NOMOGRAM
`
`USE THE ABSOLUTE LEVEL DIFFERENCE BETWEEN THE
`A PREVIOUSLY DETERMINED GLOBAL MASKING
`THRESHOLD AND A MASKING EDGE OF THE CURRENTLY
`PROCESSED MASKER AS AN INPUT FOR THE NOMOGRAM
`
`1300
`
`70
`10
`lJ
`
`80
`10
`:_)
`
`90
`10
`LJ
`
`ADD THE NOMOGRAM FORMED LEVEL TO THE MAXIMUM
`LEVEL OF THE PREVIOUSLY DETERMINED GLOBAL
`MASKING THRESHOLD AND THE MASKING EDGE OF THE
`CURRENTLY PROCESSED MASKER.
`10
`LIMIT THE POSSIBLE NUMBER OF LEVEL ADDITION VALUES 11
`OF A NOMOGRAM TO A PRECALCULA TED NUMBER THAT _,)
`CORRESPONDS TO THE DESIRED ACCURACY
`
`00
`11
`LJ
`
`

`
`5,740,317
`
`10
`
`2
`coding. produces discrete spectral sampling values 5 (step
`1280). In the case of transformation coding, the spectral
`sampling values determined in the time/frequency transfor(cid:173)
`mation stage can be employed as sampling values 5 (path 2a
`5 shown in dashed lines). According to a procedure (step
`1220) specific to the invention to be described in greater
`detail below, a stage 50 calculates the global masking
`threshold 6 from sampling values 5 and possibly the maxi(cid:173)
`mum signal levels 4.
`For sub-band coding. a stage 30 additionally determines
`the maximum signal levels 4 in the individual sub-bands
`from the sampling values 2.
`In a stage 60, the above-mentioned coding and requan(cid:173)
`tizing control signal 7 is produced from the global masking
`15 threshold 6. Stage 60 is described in FIG. 3. information
`blocks 5.5 and 5.3, of the above-mentioned WO 88/04,117
`which is expressly referred to. In the mentioned information
`block 5.5. the relationship between maximum occurring
`(masking) sub-band level and minimum global masking
`20 threshold is determined (according to permissible quantizing
`noise), from which, in the subsequent information block 5.3
`, the sub-band association of the quantization (=resolution)
`is calculated.
`The calculation of global masking threshold 6 (step 1220)
`25 will now be described in greater detail with reference to FIG.
`2.
`
`1
`PROCESS FOR FINDING THE OVERALL
`MONITORING THRESHOLD DURING A BIT(cid:173)
`RATE-REDUCING SOURCE CODING
`
`This application is a continuation of application Ser. No.
`08/119,109, filed Sep. 17. 1993 (now abandoned).
`
`BACKGROUND OF THE INVENfION
`
`1. Field of the Invention
`The invention relates to a method of determining the
`global masking threshold in a bit rate reducing source
`coding process.
`2. Background Information
`To code digital audio signals by means of bit rate reducing
`coding methods, WO 88/04,117 discloses the calculation of
`the spectral masking threshold in order to obtain a requan(cid:173)
`tization rule.
`Since the signals to be transmitted are not composed of
`only a single tone but of a plurality of harmonics. the
`masking thresholds created by such signals differ consider(cid:173)
`ably. Their calculation requires a consideration of all rel(cid:173)
`evant tonal maskers and of all relevant noise maskers, each
`having frequency and level specific masking edges. Such an
`extensive consideration requires a correspondingly high
`calculating effort in the source coder which is justified only
`for a computer simulation but not for a real time realization.
`
`SUMMARY OF THE INVENfION
`
`In contrast thereto. it is the object of the invention to
`reduce the calculating effort for a bit rate reducing source
`coding process particularly for real time applications.
`Advantageous features and modifications of the method
`according to the invention are defined in the following
`description.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention will be described in greater detail with
`reference to the drawings. in which:
`FIG. 1 depicts a block circuit diagram of a source coder
`for implementing the method according to the invention;
`FIG. 2 depicts a frequency diagram including three
`maskers and the resting threshold whose joint masking effect
`results in the global masking threshold determined accord(cid:173)
`ing to the invention.
`FIGS. 3-7 are fiow charts of the inventive method.
`
`DEfAil...ED DESCRIPTION OF THE
`PREFERRED EMBODIMENf(S)
`In the block circuit diagram of FIG. 1. the digitized audio
`signal 1 at the input is fed, in the case of sub-band coding.
`to a polyphase filter bank 10 which produces sub-band
`sampling values 2 (step 1180). In the case of transformation
`coding. filter bank 10 is replaced by a time/frequency 55
`transformation stage which produces discrete. spectral sam(cid:173)
`pling values, for example. corresponding to a cosine or a fast
`Fourier transformation. Sampling values 2 are requantized
`in a quantizing stage 20 according to their permissible
`quantizing noise as determined by a coding and requantizing
`control signal 7 (step 1190). In order to form an output signal
`8, control signal 7 is fed, together with the requantized
`sampling values 3. to a multiplexer 70 which inserts signals
`3 and 7 into a time multiplex frame depending on the bit rate
`reduction method employed (step 1200).
`The digitized audio signal 1 at the input is also fed to a
`transformation stage 40 which, in the case of sub-band
`
`60
`
`65
`
`In the frequency diagram of FIG. 2. three maskers 100,
`200, 300 (step 1230) are plotted at 250 Hz, 1 KHz and 4
`KHz. showing their upper masking edges 101. 201 and 301,
`30 respectively. and their lower masking edges 102. 202 and
`302, respectively. FIG. 2 also shows the resting threshold
`400. Employing the procedure specific to the invention as
`described below, it is possible to advantageously determine
`the global masking threshold 6 from the interaction of the
`35 upper and lower masking edges 101. 201, 301. 102.202. 302
`and the resting threshold 400.
`To do this. in a preferred embodiment for the reduction of
`the calculating effort for the calculation of the global mask(cid:173)
`ing threshold. the following criteria are considered:
`40 (a) Each masker 100, 200, 300. as shown in FIG. 2, has an
`upper and a lower masking edge 101 and 102, 201 and
`202. 301 and 302. respectively. These masking edges are
`described by higher order polynomials. Since polynomial
`calculations are very complicated, these masking edges
`are segmented {step 1260) and these [segments] are
`approximated with lower order polynomials, for example.
`linear equations (step 1250).
`(b) Since, for a calculation of the global masking threshold
`6. the masking edges of the individual maskers may
`possibly contain level dependencies, the intensities cal(cid:173)
`culated from the transformation of the audio signals into
`the frequency domain must be recalculated into logarith(cid:173)
`mic levels (step 1240). The logarithm formation is nor-
`mally also calculated with a higher order polynomial and
`is thus too complicated for realization. Since it is
`sufficient, however, to calculate the logarithm with limited
`accuracy. the number of logarithmic level stages con(cid:173)
`tained in the table is reduced according to the invention to
`a small number. These logarithmic levels are stored in a
`table which is then employed instead of the polynomial
`calculation (step 1160). If the logarithm formation is
`realized with the aid of splitting the intensities into
`mantissa and exponent, the logarithmic levels of the
`mantissa are stored in a table which is then employed
`instead of the polynomial calculation (step 1170).
`(c) Not all maskers are relevant for the calculation of the
`global masking threshold since one masker may cover
`
`45
`
`50
`
`

`
`5,740,317
`
`20
`
`3
`another masker. The masking edge of such a covered
`masker lies far below the global masking threshold with
`respect to level or intensity and thus no longer has a
`noticeable effect on the global masking threshold. For that
`reason. these non-relevant maskers are sorted out in a 5
`stage 50 and are no longer utilized to calculate the global
`masking threshold 6 (step 1020).
`(d) All maskers whose masking edges. with respect to
`intensity or level. lie so far below the resting threshold
`400 of the human auditory system that the masking 10
`resulting from the masking curve of the masker and the
`resting threshold is not significantly greater than the
`resting threshold itself. are not relevant for the calculation
`of the global masking threshold since the masking edge of
`such a masker lies far below the global masking threshold 15
`6 with respect to intensity or level and thus no longer has
`a noticeable effect on the global masking threshold.
`Therefore, these non-relevant maskers are also sorted out
`in stage 50 and are no longer utilized for the calculation
`of the global masking threshold 6 (step 1030).
`( e) It is not possible in principle to calculate a 'lcontinuous
`curve in a digital system with numerical methods. The
`spectral base points for the calculation of the global
`masking threshold 6 are therefore fixed in such a way that
`they are calculated only at discrete spectral locations (step 25
`1120).
`(f) With the aid of psychoacoustics. the spectral resolution
`required for a calculation of the global masking threshold
`6 can be reduced with respect to the masking threshold to
`a limited number of base points. The spectral base points 30
`for the calculation of the global masking threshold 6 are
`therefore fixed in such a way that they have a closer
`spectral spacing in the lower frequency range than in the
`upper frequency range (step 1270 and step 1130).
`(g) For a calculation of the global masking threshold 6. the 35
`audio signal must be reproduced in the frequency domain
`with the aid of a transformation (stage 40. FIG. 1) in order
`to pennit a spectral analysis of the audio signal. The
`spectral base points for the calculation of global masking
`threshold 6 are thus fixed in such a manner that they come 40
`to lie on the base points of this transformation (step 1140).
`Due to the greater spectral distance between the base
`points for the Calculation of the masking threshold in the
`upper frequency range. only some of the base points of the
`transformation are employed there.
`(h) The global masking threshold 6 is calculated step by step.
`masker by masker. at its base points (step 1270). Since a
`masker generally masks to a greater degree toward higher
`frequencies than toward lower frequencies. the step-wise
`calculation of the global masking threshold 6 begins with so
`the highest frequency masker (step 1000) so that the
`interruption (=abortion) criterion described in the follow(cid:173)
`ing paragraph comes to bear as early as possible.
`(i) In the step-wise calculation of the global masking thresh(cid:173)
`old 6. the calculation always starts With a calculation. for 55
`the respective masker. of its spectral masking edge toward
`upper frequencies and then toward lower frequencies
`(step 1010). This pennits an early interruption of the
`calculation of the masking percentage which. by way of
`the masking edge of the respective masker. contributes to 60
`global masking threshold 6. This interruption takes place
`as soon as the effect of the masking edge of the respective
`masker on the previously calculated global masking
`threshold 6 falls below a certain measure (=level) (step
`1040).
`(j) the calculation of the effect of the masking edge of a
`masker and the global masking threshold 6 is interrupted
`
`45
`
`65
`
`4
`as soon as the intensity or the level of the masking edge
`of the masker at the momentarily calculated base point of
`the global masking threshold 6 falls below a certain
`measure so that it no longer has a noticeable effect on the
`global masking threshold 6 (step 1050).
`(k) The calculation of the effect of the masking edge of a
`masker on the global masking threshold 6 is interrupted as
`soon as the intensity or the level of the masking edge of
`the masker at the momentarily calculated base point of the
`global masking threshold 6 drops a certain degree below
`the intensity or the level of the resting threshold 400 and
`thus no longer has a noticeable effect on the global
`masking threshold 6 (step 1060).
`(1) The global masking threshold 6 is composed. as
`described above. of the masking effect of different indi(cid:173)
`vidual maskers 100. 200. 300 and is formed by adding the
`intensities of the masking edges 101. 102. 201. 202. 301.
`302 of these individual maskers (step 1070). This inten(cid:173)
`sity addition normally requires a considerable amount of
`calculations since. based on logarithmic levels. an addi(cid:173)
`tion of intensities requires repeated exponentiation and
`logarithm formations. The addition of the intensities is
`thus effected with the aid of a nomogram (step 1080). The
`input value for the nomogram is the absolute value of the
`level difference between the previously calculated global
`masking threshold 6 and the masking edge of the momen(cid:173)
`tarily considered masker (step 1090). The resulting output
`value of the nomogram is a logarithmic level which is
`added to the maximum level formed from the previously
`calculated global masking threshold 6 and the masking
`edge of the masker presently under consideration (step
`1100). Since the accuracy required for the intensity addi(cid:173)
`tion is limited. the number of possible level addition
`values is reduced to a low number (step 1110). These
`values can be calculated in advance for the nomogram and
`can be employed for the truly occurring absolute level
`differences.
`Of the above-mentioned sections (a) to (1) only some of
`the sections may be employed. if required. as defined in the
`dependent claims.
`We claim:
`1. A method of determining a global masking threshold
`used for source coding digitized audio signals having sam(cid:173)
`pling values, comprising:
`providing the sampling values of the digitized audio
`signals to a quantizer. the sampling values being one of
`time or spectral domain sampling values. the sampling
`values having permissible quantizing noise;
`requantizing the sampling values with the quantizer
`according to the permissible quantizing noise thereof in
`response to a coding and requantizing control signal;
`multiplexing the coding and requantizing control signal
`and the sampling values requantized in said requantiz(cid:173)
`ing step. into a time multiplexed frame in accordance
`with a bit rate reduction employed;
`wherein the coding and requantizing control signal is
`derived from the sampling values by detennining a
`global masking threshold using all relevant maskers
`which are tonal maskers and noise maskers. and which
`result from the sampling values. and using a resting
`threshold, the global masking threshold being deter(cid:173)
`mined by the following steps:
`(a) converting levels of all relevant maskers into loga(cid:173)
`rithmic levels and using intensities of the maskers to
`determine the coefficients of lower order polynomi(cid:173)
`als;
`(b) segmenting masking edges of all relevant maskers
`in individual segments with the lower order polyno(cid:173)
`mials; and
`
`

`
`5,740,317
`
`5
`(c) determining the global masking threshold, step(cid:173)
`wise, masker by masker, beginning with a highest
`frequency masker, at individual possible base points,
`from the lower order polynomials describing mask(cid:173)
`ing edges of the possible maskers, taking into con-
`sideration the resting threshold, using a different
`spectral spacing in lower. middle and upper fre(cid:173)
`quency ranges.
`2. A method according to claim 1. wherein the step-wise
`determination of the global masking threshold includes
`always determining first, for a respective masker, a spectral
`masking edge towards upper frequencies and then determin(cid:173)
`ing an edge towards lower frequencies.
`3. A method according to claim 1. further comprising
`detecting maskers whose masking edges have essentially no 15
`effect on a determination of the global masking threshold
`because of masking edges of adjacent maskers, wherein the
`detected maskers are not considered in determining the
`global masking threshold.
`4. A method according to claim 1. further comprising
`detecting maskers whose masking edges lie far below the
`global masking threshold with respect to level or intensity,
`wherein the detected maskers are not considered in deter(cid:173)
`mining the global masking threshold.
`5. A method according to claim 1. further comprising
`detecting the effect on the determination of the global
`masking threshold of a masker and interrupting the deter(cid:173)
`mination as soon as the effect of the masking edge of the
`masker on the determination of the global masking threshold
`falls below a certain level.
`6. A method according to claim 1. further comprising
`detecting the level or the intensity of the masking edge of a
`masker at a momentarily determined base point of the global
`masking threshold, and interrupting the determination of the
`global masking threshold when the detected level or inten- 35
`sity falls below a certain level, wherein a level or an
`intensity, respectively, of the masking edge has essentially
`no effect on the determination of the global masking thresh(cid:173)
`old.
`7. A method according to claim 1, further comprising 40
`detecting the level or the intensity, respectively, of the
`masking edge of a masker at a momentarily determined base
`point of the global masking threshold, and interrupting the
`determination of the global masking threshold when the
`detected level or intensity drops a certain degree below an 45
`intensity or level, respectively. of the resting threshold.
`8. A method according to claim 1, further comprising
`adding intensities of the masking edges of the individual
`
`30
`
`5
`
`6
`maskers during the step-wise determination of the global
`masking threshold.
`9. A method according to claim 8, wherein the step of
`adding intensities is effected with a nomogram.
`10. A method according to claim 9. further comprising
`determining an absolute value of a level difference between
`a previously determined global masking threshold and a
`masking edge of a momentarily considered masker, and
`using the determined value as an input value for the nomo-
`10 gram.
`11. A method according to claim 9, further comprising
`forming a maximum level or intensity value. respectively,
`from a previously determined global masking threshold and
`a masking edge of a momentarily considered masker. and
`adding the formed level or intensity value to an output
`value of the nomogram.
`12. A method according to claim 8. further comprising
`limiting possible intensities or level addition values to a
`precalcutated number that corresponds to a desired accuracy.
`13. A method according to claim 1. further comprising
`20 determining spectral base points for calculation of the global
`masking threshold so that the spectral base points for
`calculation of the global masking threshold lie only at
`discrete spectral locations.
`14. A method according to claim L further comprising
`25 selecting a spectral spacing of base points for a determina(cid:173)
`tion of the global masking threshold so that the spacing is
`smaller in a lower frequency range than in a middle fre(cid:173)
`quency range and is greater in an upper frequency range than
`in the middle frequency range.
`15. A method according to claim 1. further comprising
`reproducing a digitized audio signal in a frequency domain,
`and
`fixing spectral base points for determination of the global
`masking threshold so that the spectral base points for
`determination of the global masking threshold lie on
`base points of a reproduction.
`16. A method according to claim l, further comprising
`quantizing logarithmic levels in level stages.
`17. A method according to claim 16, further comprising
`converting with a table intensities to logarithmic levels.
`18. A method according to claim 17, wherein the table
`contains a number of associations between intensity values
`and logarithmic level stages and the method further com-
`prises reducing the number of associations by dividing
`intensity values into mantissas and exponents, and by stor(cid:173)
`ing only the mantissas.
`
`* * * * *