Arithmetic encoding/decoding of a multi-channel information signal

A data compression apparatus is disclosed for data compressing a plurality of information signals. The data compression apparatus comprises input terminals (1,2) for receiving the information signals, prediction units (6,8) for carrying out a prediction step on the plurality of information signals so as to obtain a plurality of prediction signals, probability signal determining units (12,14) for generating in response to the plurality of information signals a corresponding plurality of probability signals, a first switching unit (4) for each time selecting a symbol in one a plurality of input signals applied to the first switching unit, a second switching unit (10) for each time selecting a probability signal corresponding to the symbol selected by the first switching unit, a control signal generator unit (24) for generating switching control signals for the first and second switching units (4,10), a lossless coding unit (18) and an output terminal (22) for supplying said data compressed output signal.

FIELD OF THE INVENTION
 The invention relates to the field of lossless data compression.
 BACKGROUND OF THE INVENTION
 The invention relates to a data compression apparatus for data compressing
 a plurality of at least two digital information signals. The invention
 also relates to a data compression method, to a data expansion apparatus,
 to a transmission apparatus provided with the data compression apparatus,
 to a record carrier obtained with the transmission apparatus and to a
 receiver apparatus provided with the data expansion apparatus.
 The data compression and expansion aimed at hereafter is data
 compression/expansion based on lossless coders, more specifically
 arithmetic coders/decoders.
 Lossless coding using arithmetic coders has been extensively described in
 document F. Bruekers et al, "Improved lossless coding of 1-bit audio
 signals", presented at the 103rd Convention of the AES, Sep. 26-29, 1997,
 preprint 4563 (I-6). Further, G. G. Langdon, "An introduction to
 arithmetic coding", IBM J. Res. Develop., Vol, 28, pp. 135-149, March 1984
 and P. G. Howard et al., "Arithmetic coding for data compression", in
 Proc. of the IEEE, 1994 give an extensive introduction into data
 compression and expansion using arithmetic coders/decoders.
 SUMMARY OF THE INVENTION
 The invention aims at providing data compression/expansion on a plurality
 of digital information signals. The data compression apparatus in
 accordance with the invention includes
 input apparatus for receiving the plurality of information signals,
 prediction apparatus for carrying out a prediction step on the plurality of
 information signals so as to obtain a plurality of prediction signals,
 probability signal determining apparatus for generating in response to the
 plurality of prediction signals a corresponding plurality of probability
 signals,
 first switching apparatus for each time selecting a symbol in one of a
 plurality of input signals applied to the first switching means,
 second switching apparatus for each time selecting a probability signal
 corresponding to the symbol selected by the first switching means,
 control signal generator apparatus for generating switching control signals
 for the first and second switching means,
 lossless coding apparatus having an input for receiving symbols selected by
 the first switching apparatus, for carrying out a lossless encoding step
 on symbols, so as to obtain a data compressed output signal at an output,
 the lossless encoding apparatus including an entropy encoder for carrying
 out the lossless encoding step on the input signal in response to the
 probability signals selected by the second switching apparatus,
 output apparatus for supplying the data compressed output signal.
 In this way, only one arithmetic coder is required for encoding a plurality
 of digital information signals. Further, smaller buffers are required, for
 the reason that each time one symbol of the plurality of digital
 information signals is multiplexed into a serial datastream that is
 supplied to the arithmetic coder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIG. 1 shows a first embodiment of a data compression apparatus for
 arithmetically encoding a multi-channel information signal. The apparatus
 of FIG. 1 is adapted to encode a 2-channel information signal, such as the
 left and right hand signal components of a stereo audio signal. The signal
 components can be in digital form, such as in the form of a bitstream
 signal or in the form of an n-level digital signal, where n is larger than
 2. The apparatus includes two inputs 1 and 2 for receiving the left and
 right hand signal components, respectively. The inputs 1 and 2 are coupled
 to an a-terminal and a b-terminal, respectively of a switch 4, as well as
 to an input of a prediction filter 6 and 8, respectively. An output of the
 prediction filter 6 is coupled to an a-terminal of a switch 10 via a
 probability determining unit 12. An output of the prediction filter 8 is
 coupled to the b-terminal of the switch 10 via a probability determining
 unit 14. A c-terminal of the switch 4 is coupled to an information signal
 input 16 of an arithmetic coder 18. The c-terninal of the switch 10 is
 coupled to a probability signal input 20 of the arithmetic coder 18. The
 output of the arithmetic coder 18 is coupled to an output terminal 22 of
 the apparatus. A control unit 24 is available for generating a switching
 control signal to the switches 4 and 10.
 The prediction filter 6 realizes a prediction filtering on the input signal
 supplied to the input 1 so as to obtain a predicted version of the input
 signal, which is in the form of a multi value output signal. The
 probability determining unit 12 generates a probability signal in response
 to this multi value output signal. In an identical way, the prediction
 filter 8 realizes a prediction filtering on the input signal supplied to
 the input 2 so as to obtain a predicted version of the input signal, which
 is in the form of a multi value output signal. The probability determining
 unit 14 generates a probability signal in response to this multi value
 output signal. More specifically, the probability determining units 12 and
 14 each supply probability values indicating the probability of occurrence
 of each of the symbol values of the symbols occurring in the serial
 datastream of the input signals applied to the inputs 1 and 2,
 respectively.
 The control unit 24 controls the switches in such a way that they both are
 either in the position a-c or in the position b-c. Further, via the
 switches 4 and 10, alternately, symbols of the one or the other input
 signal, with their corresponding probability signals are supplied to the
 arithmetic coder 18. In this way, the two signal components are
 interleaved and subsequently encoded by the arithmetic coder 18.
 Various embodiments of the prediction filters 6,8 and the probability
 determining units 12,14 are possible. In one embodiment, the prediction
 filters and the probability determining units are static (fixed) unit, in
 the sense that their behaviour does not change in time. In a second
 embodiment, the filters 6,8 are adaptive, so that they can adapt
 themselves to changing input signal characteristics. In another
 embodiment, the probability determining units 12,14 can (also) be
 adaptive. When adaptive, the serial datastream of an input signal is
 subdivided into frames of information and for each frame, the prediction
 filter and the probability determining unit are adapted to the content of
 the frame: that is: the coefficients of the prediction filter are derived
 so as to obtain the best prediction of the signal content in the frame and
 the probabilities are derived by comparing the original digital
 information signal in the frame with the corresponding multi-value output
 signal of the prediction filter.
 For each of a plurality of subintervals in the value range of the multi
 value output signal, the probability of each of the n levels of the
 information signal is obtained by counting the number of times each of the
 n levels occurs, during a specific time interval (e.g. one frame), when
 the multi value output signal falls in the range of the subinterval. For
 example, if n=3 and the levels in the information signal are -1, 0, and
 +1, the multi value output signal of the prediction filter can lie within
 a range of e.g. -3 and +3. One subinterval of this range could e.g. be the
 range of 1 to 1.5. The counts obtained for each of the levels, when the
 multi value output signal falls in the range of 1 to 1.5 could e.g. be
 1000, 3000, and 6000 for the levels -1, 0, and +1, respectively. The
 probabilities would then be 0.1, 0.3, and 0.6, respectively, for those
 levels, when the multi value output signal falls in the subinterval with
 the range of 1 to 1.5.
 FIG. 2 shows a second embodiment of the data compression apparatus for
 arithmetically encoding a multi-channel information signal. The apparatus
 of FIG. 2 differs from the apparatus of FIG. 1, in that the apparatus of
 FIG. 2 includes quantizer units 26 and 28, having an input coupled to the
 outputs of the prediction filter units 6 and 8, respectively. Further,
 signal combination units 30 and 32, having first inputs coupled to the
 input terminals 1 and 2, respectively, second inputs coupled to the
 outputs of the quantizer units 26 and 28, respectively, and outputs
 coupled to the a-terminals of the switches 4 and 10, respectively.
 The functioning of the prediction filter unit 6, the probability
 determining unit 12', the signal combination unit 30 and the quantizer
 unit 26 is as follows. The quantizer unit 26 is adapted to quantize the
 multivalue signal of the prediction filter unit 6 into an n-level output
 signal, which is the predicted version of the n-level information signal,
 applied to the input terminal 1. For the case of a 1-bit bitstream signal,
 n=2, the predicted version of the information signal is also a signal in
 the form of a serial datastream of `zeroes` and `ones`. The signal
 combination unit 30 combines the information signal and the predicted
 version of the information signal in a subtractive way so as to obtain a
 residual signal, which is supplied to its output. The advantage of
 generating a residual information signal is that, by combining the n-level
 information signal and the predicted version thereof, the probability of
 symbols with zero level in the residual signal is significantly increased.
 This can simplify the subsequent data compression in the arithmetic coder
 18.
 The probability determining unit 12' now generates one or more probability
 values (the number of which is dependent of n), indicating the probability
 that a symbol of the residual information signal supplied by the
 combination unit has a predetermined value. This (these) probability
 value(s) is (are) thus supplied to the arithmetic coder, together with the
 corresponding symbol in the residual signal so as to enable data
 compression in the arithmetic coder 18.
 It should be noted that the probability values for data compressing the
 residual signal are obtained from the prediction signal from the
 prediction filter 6 and not from the residual signal itself. This has the
 advantage that a higher data compression rate can be obtained with the
 arithmetic coder 18.
 As stated above, both the filter 6 and the unit 12' can be static, or they
 can be adaptive. The prediction filter 6 realizes a prediction filtering
 on the n-level information signal so as to obtain the multi value output
 signal. The multi value output signal lies within a range of eg. +3 and
 -3. The quantizer 26 receives the multi value output signal and generates
 a predicted version of the n-level information signal therefrom, eg. (if
 n=3 and the levels are -1, 0 and +1) by allocating a value of `+1` if the
 multi value output signal is eg. larger than +1, by allocating a value
 `-1` if the multi value prediction signal is smaller than -1 and by
 allocating a value `0`, if the multi value prediction signal lies between
 -1 and +1.
 The residual information signal can have values `-2`, `-1``0`, `+1` and
 `+2`. For each of a plurality of subintervals in the value range of the
 multi value output signal, it is determined what the probability is that
 the corresponding value of the residual signal is eg. `+1`. In the
 adaptive embodiment of the unit 12', this can be realized by counting the
 number of `+2`s, `+1`s, `0`s, `-1`s and `-2`s occurring in the residual
 information signal during a specific time interval, when the multi value
 output signal falls in one of such ranges. The probabilities thus obtained
 for the various values in the multi value output signal are subsequently
 supplied as the probability signal to the arithmetic coder 18.
 The functioning of the prediction filter unit 8, the probability
 determining unit 14=, the signal combination unit 32 and the quantizer
 unit 28 is identical to the functioning described above and therefore
 needs no further explanation.
 After multiplexing of both residual signals by the switch 4 into a
 composite residual signal, and the multiplexing of the probabilities by
 the switch 10, the arithmetic coder 18 carries out a data compression on
 the composite residual signal, resulting in a data compressed output
 signal which is supplied to the terminal 22, for transmission via a
 transmission medium or for recording.
 FIG. 3 shows an embodiment of a data expansion apparatus for decoding the
 data compressed signal supplied by the encoder of FIG. 1 to the decoder
 via the transmission medium or a record carrier, such as an optical record
 carrier. FIG. 3 shows an expansion apparatus having an input terminal 40
 coupled to an input of an arithmetic decoder 42 that carries out an
 arithmetic decoding step on the data compressed composite information
 signal, under the influence of a probability signal p, supplied to a
 control input 44 so as to generate a replica of the composite information
 signal which is supplied to an output 46. The output 46 of the decoder 42
 is coupled to a c-terminal of a switch 48, which has an a-terminal coupled
 to an output terminal 52 as well as to an input 52 of a prediction filter
 56. Further, a b-terminal of the switch 48 is coupled to an output
 terminal 54 as well as to an input of a prediction filter 58. The output
 of the prediction filter 56 is coupled to an input of a probability
 determining unit 60. The probability determining unit 60 has an output
 which is coupled to an a-terminal of a switch 50. The output of the
 prediction filter 58 is coupled to an input of a probability determining
 unit 62. The probability determining unit 62 has an output which is
 coupled to an b-terminal of the switch 50. The c-terminal of the switch 50
 is coupled to the control input 44 of the decoder 42.
 Further, a control unit 64 is available for supplying a switching control
 signal for the controllable switches 48 and 50.
 The data expansion apparatus of FIG. 3 is complementary to the data
 compression apparatus of FIG. 1, in the sense that, if the prediction
 filters and the probability determining units in the apparatus of FIG. 1
 are fixed, the prediction filters and the probability determining units in
 the apparatus of FIG. 3 are fixed, or that, if the prediction filters and
 the probability determining units in the apparatus of FIG. 1 are adaptive,
 that also the filters and the probability determining units in the
 apparatus of FIG. 3 are adaptive.
 When the filters are adapted, it is required to transmit side information
 from the compression apparatus to the expansion apparatus. Such side
 information can include the filter coefficients. When, the probability
 determining units are adaptive, it is also required to transmit side
 information to the expansion apparatus, such side information including
 information concerning the conversion of the multi-level information
 signal supplied by the filters into the probabilities generated in
 response thereto by the probability determining units.
 When symbols in the composite compressed signal, that belonged to the
 information signal applied to the input 1 of the apparatus of FIG. 1, are
 available for decoding in the decoder 42 (that is: ready for data
 expansion), the control unit 64 generates a control signal, such that the
 switch 50 is set into its position a-c, so that the probability value
 corresponding to the symbol, generated by the probability determining unit
 60 can be applied to the control input 44 of the decoder 42. Further, also
 the switch 48 is set into its position a-c, so that the decoded symbol
 occurring at the output 46 is supplied to the output terminal 52.
 When symbols in the composite compressed signal, that belonged to the
 information signal applied to the input 2 of the apparatus of FIG. 1, are
 available for decoding in the decoder 42 (that is: ready for data
 expansion), the control unit 64 generates a control signal, such that the
 switch 50 is set into its position b-c, so that the probability value
 corresponding to the symbol, generated by the probability determining unit
 62 can be applied to the control input 44 of the decoder 42. Further, also
 the switch 48 is set into its position b-c, so that the decoded symbol
 occurring at the output 46 is supplied to the output terminal 54. In this
 way, the original information signals can be regenerated.
 FIG. 4 shows a data expansion apparatus for decoding the data compressed
 signal, received via the transmission or recording medium from the
 apparatus of FIG. 2. The apparatus of FIG. 4 shows a large resemblance
 with the apparatus of FIG. 3, with the difference that that apparatus of
 FIG. 4 further includes quantizers 70 and 72 and a signal combination
 units 74 and 76. The quantizer 70 has an input coupled to the output of
 the prediction filter 56, and an output which is coupled to a first input
 of the signal combination unit 74. The signal combination unit 74 has a
 second input which is coupled to the a-terminal of the switch 48. The
 output of the signal combination unit 74 is coupled to the output terminal
 52 and to the input of the prediction filter 56. The quantizer 72 has an
 input coupled to the output of the prediction filter 58, and an output
 which is coupled to a first input of the signal combination unit 76. The
 signal combination unit 76 has a second input which is coupled to the
 b-terminal of the switch 48. The output of the signal combination unit 76
 is coupled to the output terminal 54 and to the input of the prediction
 filter 58.
 The decoder 42 receives the data compressed residual composite signal via
 the input terminal 40. The decoder 42 carries out an arithmetic decoding
 step on the data compressed residual composite signal under the influence
 of the probability signal p, supplied to the control input 44 so as to
 generate a replica of original residual composite signal which is supplied
 to the c-terminal of the switch 48. Under the influence of the switching
 control signal generated by the control unit 64, the composite signal is
 split into replicas of the original residual signals. Those residual
 signals are supplied to the second inputs of the signal combination units
 74 and 76. Via their first inputs, they receive predicted versions of the
 original information signals, so that by combining a predicted version of
 an information signal and its corresponding residual signal in an additive
 sense in a signal combination unit, the replica of the original
 information signal is obtained.
 The functioning of the prediction filters 56 and 58, the quantizers 70 and
 72 and the probability determining units 60 and 62 is again identical to
 the functioning of the corresponding elements in the apparatus of FIG. 2.
 The arithmetic encoder used in the embodiments of FIG. 1 and 2 is adapted
 to encode an n-level signal (either the n-level composite information
 signal, or the n-level composite residual signal) using a probability
 signal in order to obtain the data compressed signal. Instead of using an
 arithmetic coder, one could have used a well known finite state coder. The
 arithmetic decoder used in the embodiment of FIG. 2 is adapted to decode
 the data compressed information signal using a probability signal in order
 to obtain a replica of the n-level information signal.
 FIG. 5 shows an embodiment of a transmission apparatus which is in the form
 of a recording apparatus. The recording apparatus includes the data
 compression apparatus shown in FIG. 1 or 2. The recording apparatus
 further includes a write unit 106 for writing the data compressed signal
 generated by the apparatus of FIG. 1 or 2 in a track on the record carrier
 108. In the present example, the record carrier 108 is a magnetic record
 carrier, so that the write unit 106 includes at least one magnetic head
 110 for writing the data compressed n-level information signal in the
 record carrier 108. The record carrier may however be an optical record
 carrier 109, such as a CD disk or a DVD disk.
 Transmission via a transmission medium, such as a radio frequency link or a
 record carrier, generally requires an error correction encoding and a
 channel encoding carried out on the data compressed signal to be
 transmitted. FIG. 5 shows such signal processing steps. The recording
 apparatus of FIG. 5 therefore includes an error correction encoder 102,
 well known in the art, and a channel encoder 104, also well known in the
 art.
 FIG. 6 shows the data expansion apparatus of FIG. 3 or 4 incorporated in a
 receiver apparatus, which is in the form of a reproduction apparatus. The
 reproducing apparatus further includes a read unit 112 for reading the
 data compressed composite signal from a track on the record carrier 108.
 In the present example, the record carrier 108 is a magnetic record
 carrier, so that the read unit 112 includes at least one magnetic head 114
 for reading the data compressed n-level information signal from the record
 carrier 108. The record carrier may however be an optical record carrier
 109, such as a CD disk or a DVD disk.
 As has been explained above, transmission via a transmission medium, such
 as a radio frequency link or a record carrier, generally requires an error
 correction encoding and a channel encoding carried out on the data
 compressed n-level information signal to be transmitted, so that a
 corresponding channel decoding and error correction can be carried out
 upon reception. FIG. 6 shows the signal processing steps of channel
 decoding and error correction carried out on the received signal, received
 by the reading apparatus 112. The reproducing apparatus of FIG. 6
 therefore includes a channel decoder 116, well known in the art, and an
 error correction unit 118, also well known in the art, so as to obtain a
 replica of the data compressed n-level information signal.
 Whilst the invention has been described with reference to preferred
 embodiments thereof, it is to be understood that these are not limitative
 examples. Thus, various modifications may become apparent to those skilled
 in the art, without departing from the scope of the invention, as defined
 by the claims.
 Further, the invention lies in each and every novel feature or combination
 of features.
 REFERENCES
 (D1) F. Bruekers et al, &gt;Improved lossless coding of 1-bit audio signals=,
 presented at the 103rd Convention of the AES, Sep. 26-29, 1997, preprint
 4563 (I-6)
 (D2) G.G. Langdon, &gt;An introduction to arithmetic coding=, IBM J. Res.
 Develop., Vol, 28, pp. 135-49, March 1984.
 (D3) P.G. Howard et al., &gt;Arithmetic coding for data compression=, in Proc.
 of the IEEE, 1994.