US006151578A
`6,151,578
`(11) Patent Number:
`115
`United States Patent
`Bourcet et al.
`[45] Date of Patent:
`Nov. 21, 2000
`
`
`[54]
`
`[75]
`
`SYSTEM FOR BROADCASTOF DATA IN AN
`AUDIO SIGNAL BY SUBSTITUTION OF
`IMPERCEPTIBLE AUDIO BAND WITH DATA
`
`Inventors: Patrice Bourcet, Mey; Denis Masse,
`Rosselange; Bruno Jahan, Montigny
`les Metz, all of France
`
`[73] Assignee: Telediffusion de France, Paris, France
`
`[21] Appl. No.:
`[22]
`PCT Filed:
`[86]
`PCT No.:
`
`08/952,998
`Jun. 3, 1996
`PCT/FR96/00833
`
`[87]
`
`§ 371 Date:
`Nov. 21, 1997
`§ 102(e) Date: Nov. 21, 1997
`PCT Pub. No: WO96/38927
`PCTPub. Date: Dec. 5, 1996
`‘
`‘
`°
`[30]
`Foreign Application Priority Data
`Jun. 2,1995
`[FR]
`France wceeseeseeeeeeee 95 06727
`[SL]
`Tint, Ca? acccccccsecccssseesssseesssseeesssees H04H 9/00
`
`[52] U.S. Ch occ.
`704/500; 380/253
`[58] Field of Search oo... 704/501, 229,
`704/230, 500; 370/214; 380/253; 455/2
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`9/1995 Jensen et al. ices cece 380/253
`5,450,490
`eeeeeeeeee 455/2
`11/1996 Fardeau et ab.
`oe
`5,574,962
`
`
`.....ceeeeeeeeeeeee 455/2
`5,581,800 12/1996 Fardeau et al.
`
`. 380/253
`5,764,763
`6/1998 Jensen et al.
`...
`
`7/1998 Fardeau etal. s.sssseesseseseeseecees 455/2
`5,787,334
`accccccccssssssssssessseees 341/51
`5,945,932
`8/1999 Smith et al.
`
`FOREIGN PATENT DOCUMENTS
`
`.
`.
`
`0 245 037
`0 372 601
`38 06 411
`
`11/1987 European Pat. Off.
`6/1990
`EuropeanPat. Off.
`9/1989 Germany .
`.
`.
`.
`Primary Examiner—David R. Hudspeth
`Assistant Examiner—Harold Zintel
`Attorney, Agent, or Firm—Nilles & Nilles SC
`[57]
`ABSTRACT
`A system for broadcasting data (D) that can transmit infor-
`mation in the passband of a broadcast audio-frequency
`signal (S). The system can determineat least one frequency
`band (F',;, ... , F'>4) and the amplitude (A’,;, . .. , A’3,) of
`the audio-frequency signal (S). The system comparesthis
`amplitude with an auditory masking level (Nm(13), ...,
`Nm(24)) associated with this frequency band and eliminates
`the frequency componentsof the audio-frequency signal in
`the frequency band if the amplitude of the signal is lower
`than the auditory masking level of the band. The system can
`insert the data in this frequency band at a level lower than
`or equal to the auditory maskinglevelof the frequency band.
`
`Sony v. MZ Audio
`
`5,319,735
`
`6/1994 Pruss et al. oe eeeceeeeereeeee 704/205
`
`8 Claims, 2 Drawing Sheets
`
`
`
`
`
`Sony Exhibit 1022
`Sony Exhibit 1022
`Sony v. MZ Audio
`
`

`

`U.S. Patent
`
`Nov. 21, 2000
`
`Sheet 1 of 2
`
`6,151,578
`
`A
`
`—*
`
`FIG.1
`
`A ( fs, f9,Ao) M(fo, Ao)
`
`0 f
`
`m
`
`fam
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`fim
`
`f2m fusfom f
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`Sheet 2 of 2
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`6,151,578
`
`Nov.21, 2000
`
`U.S. Patent
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`

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`6,151,578
`
`1
`SYSTEM FOR BROADCASTOF DATA IN AN
`AUDIO SIGNAL BY SUBSTITUTION OF
`IMPERCEPTIBLE AUDIO BAND WITH DATA
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The invention concerns the signal broadcasting field for
`signals including an audio-frequency component. More
`especially, it concerns a data broadcasting system.
`2. Description of the Related Art
`The broadcasting field (broadcasting of TV or radio
`programmes,radiotelephony,etc.) is well known.
`A current
`tendency is to transmit,
`in addition to the
`programmes(or soundinthe telephonyfield), data useful for
`the broadcasting companies, for control organisations,or for
`listeners or viewers. This data can concern for example:
`help in selecting a radio or TV programme (example:
`automatic tuning aids, search for a radio station by
`name, search by type of programme, search by menu,
`etc.),
`information on the programme being broadcast or
`replayed after recording (for example the nameof the
`company whichcreated a programme, thetitle of a film
`broadcast by a TV channel, the record reference of a
`song broadcast bya radio station,etc.),
`service data in the analogue radiotelephonefield.
`We also remark the development of so-called interactive
`broadcasting systems which allow the viewersorlisteners to
`dialogue in a more orless efficient manner with the pro-
`gramme source. These means are used either to act on the
`content of the broadcast programme, or to play, bet or
`communicate on the subject of this same programme. Thus,
`a form of interactivity, via small devices simulating
`pseudodialogue with a programme designed for
`this
`purpose, recently appeared. A remote-sized unit gives the
`illusion ofinteractivity as it allows, for instance, to reply to
`a televised question/reply game as and when the questions
`are asked. Or again, an electronic device dissimulated in a
`fluffy toy allows the toy to react to a broadcast programme
`or a programmeplayed back on a video cassette recorder. In
`fact, the interactivity is not real as the string of good replies
`or the reactions of the toy follows preestablished sequences,
`common to the memory of the interactive device and the
`broadcast or played back programme. As the audiovisual
`sequence was prerecorded in accordance with a selected
`code, its execution is predictable and therefore the only
`information to be transmitted to the interactive deviceis the
`
`start signal and the exact timing of the questions/replies or
`the various possible reactions in the case of a toy.
`There is also a demand for the automatic identification of
`
`2
`digital systems. However, existing systems and equipment
`populations do notin general easily lend themselves well to
`this development and experience proves that, from a sales
`engineering viewpoint, the compatibility and the relative
`cost of the processes and devices to be implemented are
`critical factors when introducing a newservice.
`For the transmission of data concerning a broadcast
`programme, two techniques are currently used.
`The first technique consists in transmitting these data
`outside of the passband occupied by the signal of the
`transmitted programme(sound and possibly image). A solu-
`tion exists, for instance, in sound broadcasting by multiplex
`frequency modulation,
`in using the upper part of the
`multiplex, between 54 and 76 kilohertz. Another example
`consists in using the lines available during frameretrace for
`TV broadcasting. These techniques have drawbacks. The
`saturation of the frequency resources available for broad-
`casting limits the numberof users of these resources. Also,
`receivers adapted to the passbands used to transmit
`the
`emitted information are required.
`Another technique consists in transmitting the data in the
`passband ofthe signal of the transmitted programme; this
`technique does not require the use of dedicated frequency
`bands.It is therefore not necessary to use transmitters and
`receivers with a frequency adapted to transmit these dedi-
`cated frequency bands. Typically,
`the original signal
`(corresponding to the programme to be transmitted) is
`filtered at origin to eliminate the frequency components in a
`given frequency band andthe data is inserted in this band.
`The original signal is therefore deformed which may be
`unpleasant for a viewer or a listener not interested in the
`data. Therefore, the time dedicated to transmitting the infor-
`mation is limited by the broadcasters to the strict minimum
`which reduces the data flow rate accordingly. Thus, for
`interactive devices in the television field, the data is loaded
`globally, in one go, at the start of a given application.It is
`then impossible to adapt the data subsequent to a modifica-
`tion in the programme which must be run according to
`scheduled timing and without unexpected interruptions.
`Filtering means can of course be used at the receivers so as
`not
`to systematically pass on the sound or visual data
`received, this data then being transparentto the listener or
`the viewer. Nevertheless, we cannot ensure that the signal
`seen or heard by the viewer or listener will be the same as
`the original signal that he would have perceived before the
`insertion of the data.
`
`OBJECTS AND SUMMARYOF THE
`INVENTION
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`In view of the above, the purpose of the invention is to
`propose a system to allow transmission of data in the
`passband of a signal
`including an audio-frequency
`component, without modifying, in relation to the original
`audio-frequency signal, the signal perceived by the listener.
`a sound sequence, accompanied by an imageornot. For the
`The invention proposesto insert these data in the so-called
`broadcasters, this is used to check that a given programme
`masked frequency bands of the original audio-frequency
`is correctly broadcast on the frequency allocated to it; this
`signal, if these bands exist, that is at a level lower than the
`can become fairly complex when a national programmeis
`instantaneous auditory threshold due to the auditory mask-
`affected by regionalor local disconnectings. This also allows
`ing phenomena induced by the original audio-frequency
`the controlling bodies to count the broadcasting of works
`signal itself. The data transmitted are then inaudible, do not
`protected by copyrights or to check the conformity of the
`alter the original audio-frequency signal from a subjective
`broadcasting of commercials. Finally, for sample survey or
`viewpoint and do not require the use of frequency compo-
`audience evaluation organisations,
`it
`is used to rapidly
`nents located outside of the spectral band occupied by the
`identify that whichis actually listened to or seen byalistener
`original signal. The invention therefore proposes data trans-
`or a viewer. Today,
`to assess a radio audience, the only
`mission adapted to the use of existing receivers and trans-
`solution available is to conduct a sample survey by inter-
`mitters and subjectively not disturbing for the listener.
`viewing the consumers.
`All
`these applications are easy to incorporate when
`The invention thus concerns a data broadcasting system,
`designing new radio or TV broadcasting systems, especially
`this information being transmitted in the passband of a
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`6,151,578
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`3
`it
`broadcast audio-frequency signal characterised in that
`includes means for determining in at least one frequency
`band the amplitude of the audio-frequency signal and for
`comparing this amplitude with an auditory masking level
`associated with this frequency band, meansfor eliminating
`the frequency components from the audio-frequency signal
`in the said frequency band if the amplitude of the signal is
`lowerthan the auditory masking level of the said band, and
`meansfor inserting the said information into this frequency
`band at a level lower than or equal to the auditory masking
`level of the said frequency band.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Other features and advantages will appear in reading the
`description which follows, and which is to be read in
`conjunction with the appended drawings on which:
`FIGS. 1 and 2 represent diagrams showing the auditory
`masking phenomenon,
`FIG. 3 represents a data extraction device,
`FIG. 4 represents a data insertion device,
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`FIGS. 1 and 2 are amplitude versus frequency diagrams
`illustrating the auditory masking phenomenon which is a
`phenomenonofa physiologicalorigin.
`If we consider the hearing by a human being of an
`audio-frequency signal with a given frequency and
`amplitude, the auditory masking phenomenonisreflected by
`the non-perception, by this same human being, of audio-
`frequency signals transmitted simultaneously and with
`amplitudes lower than the given threshold levels.
`Thus,
`in reference to FIG. 1, if we consider a single-
`frequency signal at frequency fp,
`located in the audio-
`frequency spectrum (typically between 20 and 15,500 Hertz)
`and with an amplitude Aj, we can define an amplitude and
`frequency range M(f,, Aj) such that any single-frequency
`signal emitted simultaneously of frequency fs in a limited
`frequency range [f0m, f0M] where f,,,<f, and fo,,>f) and
`amplitude A<A (f£,, fp, Ap)<Ag is inaudible.
`The valuesfo,,,. foay are variable for a given frequency fp.
`In practice, the higher the amplitude A, the wider the range
`[fom> fos].
`It can also be noted that
`the range is not
`symmetrically in relation to f, and extends more widely in
`the range of frequencies greater than f,.
`The amplitude value A (f,, fp, Ag) varies with fs, f,, and
`Ao. In practice, the nearer f, is to fp, the higher the inaudi-
`bility threshold A (f,, fo, Ao).
`The auditory masking phenomenon has been knownfor
`several years. For further details, refer to the work entitled
`“Psychoacoustique” by E. Zwicker and R. Feldtkeller, Ed.
`Masson, 1981. The experimental results described in this
`work gave rise to a standardization (standard ISO/IEC
`11172-3).
`A masking level curve M(S) can be defined (shownby the
`dotted line on FIG. 2) for any signal S covering the audio-
`frequency spectrum [f,,,, f,,], where f,,=20 Hertz and f,,=15,
`500 Hertz. In the example shown on FIG. 2, two ranges[f,,,,,
`f,,7] and [f,,,,. f2,,] can be seen where the masking level
`curve M(S) has an amplitude higherthan thatof signal S. In
`concrete terms, this means that the spectral components in
`these ranges are inaudible for
`the human being.
`Consequently, the subjective auditory rendition of a signal S'
`identical to signal S outside of these ranges, and without
`frequency components in these ranges, will be identical to
`the rendition of signal S shown on FIG. 2.
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`4
`The modelling of the auditory masking phenomena has
`given rise to the dividing of the audio-frequency spectrum
`into twenty four separate ranges, called critical bands, such
`that the combination of the twenty four critical bands covers
`the frequency range between 20 Hertz and 15,500 kiloHertz.
`Eachcritical band B; (i integer index from 1 to 24) is defined
`by its central frequency fe and its width.
`The table below gives, for each critical band, the value of
`the central frequency and its width.
`
`Critical band
`Bl
`B2
`B3
`B4
`BS
`Bo
`B7
`B8
`B9
`B10
`Bll
`B12
`B13
`B14
`B15
`B16
`B17
`B18
`B19
`B20
`B21
`B22
`B23
`B24
`
`Central frequency
`fe (Hz)
`60
`150
`250
`350
`455
`570
`700
`845
`1000
`1175
`1375
`1600
`1860
`2160
`2510
`2925
`3425
`4050
`4850
`5850
`7050
`8600
`10750
`13750
`
`Bandwidth (Hz)
`80
`100
`100
`100
`110
`120
`140
`150
`160
`190
`210
`240
`280
`320
`380
`450
`550
`700
`900
`1100
`1300
`1800
`2500
`3500
`
`We can see that the critical bands have variable widths,
`the narrowest being the first critical band B,, which covers
`the lowest frequencies, and the widest being the twenty
`fourth critical band B,, which covers the highest frequen-
`cies.
`
`Foreachcritical band, standard ISOMEC 11172-3 defines
`a critical band masking level Nm(i) . This is an approxima-
`tion of the level of the curve of the masking level over the
`complete critical band (the real level of the masking level
`curve for a given signal can vary in a givencritical band) .
`The masking level Nm(i) is defined according to the mask-
`ing levels of the eight lower critical bands (Nm(i-8) to
`Nm(i-1)) if they exist, and the three upper bands (Nm(i+1)
`to Nm(i+3)), if they exist.
`We have Nm(i)=x Nm(j), where
`j positive integer index such that j e [i-8,..., i-1, i+1,
`22.5 143],
`Nim(j)=104%"O-4°O-VFNV20,
`Xnm(j)=20 log,.(Av(j))+5.69 dB (sound pressure),
`Av(j)=6.025+0.275*z(j) for the tonal lines,
`lines, where
`Av(j)=2.025+0.175*z(j) for the non-tonal
`Ay(j) is the masking index of j” critical band and j and
`2(j) the ratio of the j” critical band,
`Vi(j)=G-j-1)*(17-0.15* Xnm(j))+17, of j
`i-1, and
`Vi(it+1)=0.4*Xnm(i+18)+6,
`Vi(i+2)=17*Xnm(i+2)+6,
`Vi(it3)=34* Xnm(i+3)+6.
`z(j) is a constant defined for each critical band and
`2(1)=0.62 dB, z(2)=1.8 dB, .,3)=2.4 dB, 2(4)=3.6 dB, 2(5)=
`
`from i-8 to
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`

`

`6,151,578
`
`6
`remote for interactive television programmes. The data
`audio-frequency signal could be collected, at the reception
`device, either acoustically by a simple microphone (placed
`beside the loudspeakerof the radio receiver), or electrically
`using an appropriate connector (such as an audio recording
`output).
`In reference to FIG. 4, we will describe, as an example, a
`data insertion device 1, the information being in this case
`binary data.
`To transmit the data in the audio-frequency signal of a
`radio or TV programme, we replace, in certain frequency
`bandsofthis signal, the signal by a digital modulation. This
`transmission is preferably made at a level lower than the
`masking levels of these frequency bands in order to ensure
`the inaudible character of the transmitted information. Also,
`this transmission is preferably made when these masking
`levels are sufficiently high to ensure a satisfactory signal-
`to-noise ratio in relation to the broadcasting channel.
`In an example,
`the data to be transmitted could be
`organised into frames consisting of a start word and a
`defined numberof data words. Also, a frame could be chosen
`includinga start word, a variable numberof data words, and
`an end word.
`The data insertion device 1 shown on FIG. 4 includes an
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`4.7 dB, 2(6)=5.8 dB, z(7)=6.7 dB, z(8)=7.7 dB, z(9)=8.9 dB,
`z(10)=10.0 dB, z(11)=10.9 dB, z(12)=12.0 dB, z(13)=13.1
`dB, z(14)=14.0 dB, z(15)=14.9 dB, z(16)=15.8 dB, z(17)=
`16.7 dB, z(18)=17.7 dB, z(19)=18.8 dB, z(20)=19.8 dB,
`2(21)=20.9 dB, 2(22)=22.2 dB, z(23)=23.2 dB and 2(24)=
`23.9 dB.
`
`In general, the most masked critical bands are the high
`frequency bandsof the audio-frequency spectrum whichare
`masked by the low frequency bands, statistically more
`powerful.
`After this brief look at the auditory masking phenomenon
`and its modelling, an example of the implementation of the
`invention will now be described consisting of transmitting
`the data in the passband of a broadcast audio-frequency
`signal.
`The data can be either analog (musical patterns for
`example) or digital (that is binary data). The data may
`concern broadcast audio-frequency signals (for example the
`nameof a radio station or the references of musical works
`transmitted by this station) andits purpose is to be perceived
`by the auditor, for example via a liquid crystal display. This
`data could also be service data for the signal broadcaster or
`the controlling authorities and be imperceptible to the lis-
`tener.
`
`input 2 to receive the original audio-frequencysignal S to be
`In the remainder of the description given as an example,
`transmitted (song, voiceof a host, etc.), an input 3 to receive
`it will be assumed that the data are binary data. These data
`the data D to be transmitted, and an output 4 to deliver an
`will be relevant, for example, to the programmesbroadcast
`audio-frequency output signal S' produced from the original
`by a radio station.
`audio-frequency signal S and the data D.
`in the direction of
`A radio station generally transmits,
`The audio-frequency signal S is filtered by a bench of
`these listeners, audio-frequency signals modulated by con-
`twelve bandpass filters FPB' 13 to FPB' 24, preferably
`ventional amplitude or frequency modulation techniques.
`complex, receiving at input the audio-frequency signal S.
`These audio-frequency signals could be a song, a signature
`tune, the voice of a host, etc.
`The analytical processing of the signal S facilitates the
`calculation of the amplitudes. Each complexfilter produces
`The invention proposes to calculate, from the audio-
`at output the real part (R',, to R',,) and the imaginary part
`frequency signal to be transmitted, for one or morecritical
`(I';; to I',,) of the audio-frequency signal S in the frequency
`bandsBi of the audio-frequency spectrum, the maskinglevel
`or levels of this or these critical bands. If, for a critical band,
`band(called F',, to F',,) thatit lets through. As will be seen,
`the bank of complex bandpass filters FPB',,
`to FPB',,
`the masking level is higher than the level of the audio-
`enables the components of the audio-frequency signal S in
`frequency signal,
`the corresponding part of the audio-
`frequency signal can be eliminated without a difference
`the frequency bands F',, to F',,, to be eliminatedto insert the
`data. These frequency bands (F',; to F',,) are bands included
`perceptible to the listener. The invention proposes to insert
`data (we shall speak of data audio-frequency signals) in a
`in the critical bands B,, to B,,. An amplitude calculation
`wayinaudible to the listener into this critical band or a part
`element OAC1calculates the amplitudes A’, (j integer index
`from 13 to 24) from signals R', andI’; delivered bythefilters
`of this critical band to replace the original audio-frequency
`FPB',, to FPB',,.
`signal (provided that, of course,
`the level of the audio-
`
`frequency signal of the data is lower than the critical band The audio-frequency signalSis also filtered by a bank of
`masking level). For the reception of the transmitted signal,
`twenty bandpassfilters FPB, to FPB,,, preferably complex,
`it is sufficientto filter the signal received as a function of the
`receiving at input the audio-frequency signal S. Each com-
`plexfilter produces at outputthe real part (R; to R,,,) and the
`critical bands to separate the data audio-frequency signal
`imaginarypart(I,to I,,) of the audio-frequency signal S in
`and process the transmitted data.
`Wecan see that the flow rate of the transmitted informa-
`the frequency band thatit lets through. The bank of complex
`tion cannot in practice be fixed, the original signal (and
`bandpassfilters FPB; to FPB,, enables the masking levels
`therefore the corresponding critical band masking levels
`of the critical bands B,; to B,, to be calculated. This
`Nm(i)) being a priori variable over time both in frequency
`calculation is done from an amplitude calculation element
`OAC2calculating the amplitudes A, (i integer index from 5
`and in amplitude.
`A data transmission system according to the invention
`to 24) from signals R, and I, delivered byfilters FPB, to
`will mainly include a data insertion device (an example of
`FPB,,. These amplitudes are delivered to a calculating
`which is shown on FIG. 4) and the data reception device (an
`processor ON calculating the masking levels Nm(13) to
`Nm(24).
`example of which is shown on FIG. 3). Typically, the data
`insertion device could be used either at the sound or visual
`to A',, and the masking levels
`The amplitudes A',,
`Nm(13) to Nm(24) are delivered to a control element OC
`which will compare them two at a time to determine if two
`amplitudes A',, and A',, exist lower than the corresponding
`masking levels Nm(j1) and Nm(j2) (j1 and j2 being two
`different integer indexes between 13 and 24). If this is the
`case,there is at least two frequency bands F’,, and F’,, in the
`audio-frequency spectrum for which signal S is inaudible.
`
`the audio-
`broadcasting final control room stage or at
`frequency signal production stage. The data reception device
`will include for example a received data display device(if
`the data are intendedfor the listener) and/or a storage device
`(if the data are dedicated for example to a deferred audi-
`ometry control). The reception device could also include a
`device for retransmitting information, for example to a game
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`6,151,578
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`7
`Signal S can then be filtered to eliminate these spectral
`components in these two frequency bands F’,, and F',,.
`To do this, the real components, called R', and R',, are
`subtracted from the signal S in these two frequency bands
`F';, and F';. of the original signal S. These two components
`R', and R', are delivered via a multiplexing device MUXP
`receiving the components R',; to R',,4, each of these com-
`ponents being weighted so that all but two of them (R';, and
`R',.) are cancelled. This MUXP device is controlled by the
`control element OC. These components (for example we
`have R',=R',, and R',=R',,) are then subtracted from the
`signal S (this signal having been delayed to take into account
`the time required to pass through the filters and the multi-
`plexing device) in two adders SM1 and SM2sothat an
`audio-frequency signal S'M=S-R',-R', is produced. This
`audio-frequency signal S'M is subjectively identical, for the
`listener perceivingit, to signal S.
`The assembly formed of the bandpassfilters F'13 to F'24,
`the multiplexing device MUXP and the adders SM1 and
`SM2 acts as an adaptive band-stop filter vis-a-vis signalS.
`The frequency bands F',, and F';, being freed to allow
`insertion of data D, we will now look atthis insertion.
`Conventionally,
`the binary data D will first of all be
`conditioned. Note that this conditioning operation is in any
`event independentof the freeing of the frequency bandsF’,
`in the audio-frequency signal S. The data D to be transmitted
`are conditioned in a device MFB soasto be transmitted in
`required frame form (that is by inserting start and possibly
`end words, redundant codes, etc.). Then, two data audio-
`frequency signals S$, and S, will be produced by means of
`a modulator MOD. The digital modulation used will be for
`example a QPSK (Quadrature Phase Shift Keying)
`modulation,
`the conditioned data, NRZ (Non Return to
`Zero) coded, modulating the phase of two frequencycarriers
`included in bandsF';, and F',., preferably corresponding to
`the centre frequencies of the used bands F’,, and F',, (which
`allows the complete width of these bands to be used to
`transmit the data audio-frequency signals S, and S.). This
`modulation step requires of course knowledge, via the
`control element OC, of the frequency bands freed in the
`spectrum of signal S.
`In parallel with the freeing of the bands F’,, and F’,., the
`masking levels Nm(13) and Nm(24) are delivered by the
`element ON to a multiplexing device MUXN which will
`produce two levels N'm=Nm(j1) and N"m=Nm(j2)at output.
`In orderto take the modulation chosen to producesignals S,
`and S, into account, two coefficients N' and N" will be
`produced from coefficients N'm and N"m using an automatic
`gain control device CAG. By meansof two multiplexers M1
`and M2, two data audio-frequency signals S',=N'*S, and
`S',=N"*S, will then be produced. By summingsignals S',,
`S', and S',, in to adders SM3 and SM4,a signal S'=S-(R', +
`R',)+(S',+S8'5) is produced. The signal S' produced includes
`both the audible audio-frequency componentsof the original
`audio-frequency signal S and the data D (represented by S',
`and S',) which are inaudible.
`Oncesignal S' has been produced, it will conventionally
`be modulated according to known techniques before being
`transmitted to the listeners’ receivers.
`Note that as the gain applied to signals S, and S, is only
`proportional to the masking levels of the j,_,, and j>_,, bands
`F',, and F',,, the amplitude level of signals S', and S', could
`be greater than the amplitude levels of the components of
`signal S which were removed.
`Preferably, bands F',, to F',, have same width to ensure
`a fixed transmitted data flow rate irrespective of the bands
`F',, to F',, used to transmit them. Asame type of modulation
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`can thus be used irrespective of the bands freed in signal S.
`In the example shown, the possibility of transmitting the
`data in the last twelve critical bands, from critical band B,,
`(fc=1860 Hz)to critical band B,, (fc=13750 Hz) is provided
`for. As has been seen, this information is transmitted in two
`bands each located in one of the twelve critical bands. Of
`course,
`the higher
`the number of bands F',; used
`simultaneously, the higher the transmitted data flow rate. A
`data insertion device usingall freeable bands F', can there-
`fore be made. Nevertheless, it can be seen that simultaneous
`use of a reduced numberof bands F’; enables the distortion
`probability of the original audio-frequency signal
`to be
`reduced if this signal varies to a high extent from one
`moment
`to another (although this probability is low on
`account of the temporal masking of the humanear).
`Whateverthe critical band or bands in whichthe data are
`inserted, it is easy to understand that the band or bands F,
`used within these critical bands have a width lower than or
`equal to the width of the correspondingcritical bands.
`In the example shown,the first bench of bandpassfilters
`consists preferably of bandpass filters F',,
`to F',, with
`bandwidths equal to 280 Hz at -3 decibels. This width
`correspondsto the width of the critical band usable to insert
`the data which has the lowest width, that is the width of the
`thirteenth critical band (of course, it is supposedhere that the
`carrier frequencies used to produce the data audio-frequency
`signals are equal to the central frequencies of the critical
`bands). There is therefore little reason to use data transmis-
`sion in the lowercritical bandsas their widths are lower and
`
`this would limit the maximum permissible flow rate.
`The bankoffilters F',, to F',, is preferably achieved by
`multi-rate filtering thus giving a constant propagation time
`and a limited number of operations.
`The second bankoffilters F; to F,, is preferably obtained
`from reconstructible bandpassfilters (that is filters such that
`the sum of the filtered output signals is the same as the input
`signal before filtering) the envelopes of which correspond to
`the critical bands.In other words,it is interesting to calculate
`as finely as possible the critical band maskinglevels to avoid
`producing data audio-frequency signals which could be
`audible.
`
`The binary information is for example grouped into words
`of thirty-two bits. A transmitted frame will
`include for
`example a start word, coded over thirty-two bits and a data
`word ofthirty-two bits. The start word consists for example
`of the first nine bits comprising a lock-on ramp used in the
`reception device,
`the next twenty-three bits forming the
`synchronisation word. The data word consists for example
`of three bytes representing the data and a last redundancy
`byte for an error correction code if such a code is used. This
`organisation of the information frames corresponds to a
`transmission of information on the time frames of the
`
`audio-frequency signal lasting 256 milliseconds, which cor-
`respondsto the time required to transmit the sixty-four bits,
`that is two data frames. This enables a maximumbinary flow
`rate of 500 bits per second to be attained.
`Preferably, the data frames are transmitted provided that
`the masking levels of the critical bands used to insert the
`data are greater than the minimum energy level providing
`resistance to the disturbances induced by the channel.
`Although not specified, it is of course preferable not to
`free frequency bandsin the original audio-frequency signal
`when there is no data to be transmitted. For this,
`it
`is
`sufficient to cancel the signals produced at the output of the
`multiplexing device MUXP. Thus, even if the masking level
`of the original signal varies rapidly and extensively, there is
`no risk of disturbing the original signal by suppressing
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`6,151,578
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`9
`audible frequencies. Once the data transmission has been
`made, progressive cancellation of the output signals of the
`multiplexing device MUXPwill preferably be carried out to
`reduce the probability to make the “filling in” audible.
`If the maskinglevel of the original audio-frequency signal
`drops and the start word has been transmitted, transmission
`will be preferably continued to facilitate data processing at
`the level of the reception device. If the data are coded over
`thirty-twobits, this is not very troublesome on accountof the
`temporal auditory masking.
`The data extraction device 5 shown on FIG. 3 includes an
`input 6 to receive the audio-frequency signal S'.
`The audio-frequency S' is filtered by a bank of twelve
`bandpassfilters FPB",, to FPB",, with envelopes identical
`to the twelve filters FPB",;,
`to FPB",,. Twelve audio-
`frequency signals S',, to S',, are thus produced correspond-
`ing to the spectral components of signal S in bands F’,, to
`F',, whereit is likely that we shall find the data inserted by
`a device similar to the one described as reference on FIG. 4.
`Device 5 includes a bank of twelve demodulators
`DEMOD,, to DEMOD,,, each demodulator being associ-
`ated with one of the bandpassfilters. Once the signals have
`been demodulated, they are sampled in samplers EC,, to
`EC,, associated with the clock recovery devices RC,, to
`RC,, to produce the binary data.
`Once the audio-frequency signals have been sampled, the
`binary data produced are processed in the recognition ele-
`ments RTB,,
`to RTB,,
`to determine if these data are
`representative of the transmitted data (in which case the
`synchronisation bits of the start word will be present) orif
`these data correspondto nothing (the probability being fairly
`low that bits corresponding to the synchronisation bits of a
`start word can be produced by sampling from any audio-
`frequency signal).
`Of course, if the data transmitted are not digital data but
`analog data, such as a musical pattern for example, the data
`insertion and extraction devices will be adapted to suit. In
`particular, it will not be necessary to use the modulation,
`demodulation and sampling devices. These will be replaced
`by means for converting the data to be inserted into fre-
`quencies to adapt the frequencies of the data to the frequen-
`cies freed in the insertion device.
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