Data compression and expansion of an audio signal

[0002] This is a continuation of application Ser. No. 08/937,435, filed Sep. 25, 1997, now U.S. Pat. No. 6,269,338 issued 31 Jul. 2001.

[0003] The invention relates to the field of data compression and most closely relates to real time compression of a digitally sampled audio data stream.

[0004] The invention relates to a data compression apparatus for data compressing an audio signal, to a data including compression method, a transmitter including the data compression apparatus, a recording apparatus including the data compression apparatus, a record carrier having the data compressed audio signal recorded on it in a track of the record carrier, to a data expansion apparatus for data expanding a data compressed audio signal, to a data expansion method a receiver including the data expansion apparatus and to a reproducing apparatus including the data expansion apparatus.

[0005] Data compression on an audio signal is well known in the art. Reference is made in this respect to EP-A 402,973, document D1 herein. The document describes a subband coder, in which an audio signal is A/D converted with a specific sampling frequency, such as 44.1 kHz, and the resulting samples in the form of eg. 24 bits wide words of the audio signal, are supplied to a subband splitter filter. The subband splitter filter splits the wideband digital audio signal into a plurality of relatively narrow band subband signals. Using a psycho acoustic model, a masked threshold is derived and blocks of samples of the subband signals are subsequently quantised with a specific number of bits per sample for each block of the subband signals, in response to the masked threshold, resulting in a significant data compression of the audio signal to be transmitted. The data compression carried out is based on ‘throwing away’ those components in the audio signal that are inaudible and is thus a lossy compression method. The data compression described in document D1 is a rather intelligent data compression method and requires a substantial number of gates or instructions, when embodied either in hardware or software, so that it is expensive. Moreover, the subsequent expansion apparatus also requires a substantial number of gates or instructions, when implemented in hardware or software-respectively.

[0006] Those skilled in the art are hereby referred to the following listed documents:

[0007] (D2) “A digital decimating filter for analog-to-digital conversion of hi-fi audio signals”, by J. J. van der Kam in Philips Tech. Rev. 42, no. 6/7, April 1986, pp. 230-8

[0008] (D3) “A higher order topology for interpolative modulators for oversampling A/D converters”, by Kirk C. H. Chao et al in IEEE Trans. on Circuits and Systems, Vol. 37, no. 3, March 1990, pp. 309-18

[0009] (D4) “A method for the construction of minimum-redundancy codes”, by D. A. Huffman in Proc. of the IRE, Vol. 40(10), September 1952.

[0010] (D5) “An introduction to arithmetic coding” by G. G. Langdon, IBM J. Res. Develop., Vol. 28(2), March 1984.

[0011] (D6) “A universal algorithm for sequential data compression” by J. Ziv et al, IEEE Trans. on Inform. Theory, Vol. IT-23, 1977.

[0012] The above citations are hereby incorporated in whole by reference.

[0013] The invention aims at providing a data compression apparatus for data compressing an audio signal such that it is more simple and that the corresponding expander apparatus can also be more simple and less expensive.

[0014] The data compression apparatus in accordance with the invention includes

[0015] input apparatus for receiving the audio signal,

[0016] conversion apparatus for carrying out a conversion on the audio signal so as to obtain a 1-bit bitstream signal, the conversion apparatus including sigma-delta modulator apparatus,

[0017] lossless coding apparatus for carrying out a substantially lossless data compression step on the bitstream signal so as to obtain a data compressed bitstream signal, and

[0018] output apparatus for supplying the data compressed bitstream signal. More specifically, when the audio signal is an analog audio signal, the conversion apparatus is in the form of A/D conversion apparatus for carrying out a 1-bit A/D conversion on the analog audio signal so as to obtain the bitstream signal.

[0019] The invention is based on the following recognition. The audio signal can be applied in analog form or in digital form. When A/D converting, in accordance with the invention, an analog audio signal with a 1-bit A/D converter (also named: bitstream converter or sigma-delta modulator), the audio signal to be A/D converted is sampled with a frequency which is generally a multiplicity of the frequency of 44.1 kHz or 48 kHz. The output signal of the 1-bit A/D converter is a binary signal, named bitstream signal. When the audio signal is supplied in digital form, sampled at eg. 44.1 kHz, the samples being expressed in eg. 16 bits per sample, this digital audio signal is oversampled with a frequency which is again a multiplicity of this sampling frequency of 44.1 kHz (or 48 kHz), which results in the 1-bit bitstream signal.

[0020] Converting an audio signal into a 1-bit bitstream signal has a number of advantages. Bitstream conversion is a high quality encoding method, with the possibility of a high quality decoding or a low quality decoding with the further advantage of a simpler decoding circuit. Reference is made in this respect to the publications ‘A digital decimating filter for analog-to-digital conversion of hi-fi audio signals’, by J. J. van der Kam, document D2, and ‘A higher order topology for interpolative modulators for oversampling A/D converters’, by Kirk C. H. Chao et al, document D3 in the list of related documents.

[0021] 1-bit D/A converters are used in CD players, as an example, to reconvert the bitstream audio signal into an analog audio signal. The audio signal recorded on a CD disk is however not a data compressed 1-bit bitstream signal.

[0022] It is well known in the art that the resulting bitstream signal of the 1-bit A/D converter is, roughly, a random signal which has a ‘noisy-like’ frequency spectrum. Such types of signals are hard to data compress.

[0023] Surprisingly, however, it was established that using a lossless coder, such as a variable length coder in the form of a Huffman coder or an arithmetic coder, a significant data reduction could be obtained, in spite of the noisy character of the bitstream signal from the 1-bit A/D converter.

[0024] Those skilled in the art will understand the invention and additional objects and advantages of the invention by studying the description of preferred embodiments below with reference to the following drawings which illustrate the features of the appended claims:

[0025] FIG. 1 shows an embodiment of the data compression apparatus.

[0026] FIG. 2ashows the frequency spectrum of the output signal of the 1-bit A/D converter 4 in FIG. 1 and FIG. 2bshows the frequency spectrum of the same output signal in a smaller frequency range.

[0027] FIG. 3 shows the data compression apparatus incorporated in a recording apparatus for recording the data compressed bitstream signal on a record carrier.

[0028] FIG. 4 shows the data compression apparatus incorporated in a transmission apparatus for transmitting the data compressed bitstream signal via a transmission medium.

[0029] FIG. 5 shows an embodiment of the data expansion apparatus.

[0030] FIG. 6 shows the data expansion apparatus incorporated in a reproducing apparatus for reproducing the data compressed bitstream signal from a record carrier.

[0031] FIG. 7 shows the data expansion apparatus incorporated in a receiving apparatus for receiving the data compressed bitstream signal from a transmission medium.

[0032] FIG. 8 shows a further embodiment of the recording apparatus further provided with an error correction encoder and a channel encoder.

[0033] FIG. 9 shows a further embodiment of the reproducing apparatus further provided with a channel decoder and an error correction unit.

[0034] FIG. 10 shows an embodiment of the data compression apparatus in which the lossless coder is in the form of an arithmetic coder.

[0035] FIG. 11 shows an embodiment of the data expansion apparatus in which the lossless decoder is in the form of an arithmetic decoder.

[0036] FIG. 1 shows an embodiment of the data compression apparatus, including an input terminal 1 for receiving the audio signal. In the present example, the audio signal is an analog audio signal. The input terminal 1 is coupled to an input 2 of a 1-bit A/D converter 4, also called: sigma-delta modulator. An output 6 of the 1-bit A/D converter 4 is coupled to an input 8 of a data compression unit 10. An output 12 of the data compression unit 10 is coupled to an output terminal 14.

[0037] The 1-bit A/D converter 4 is adapted to carry out a 1-bit A/D conversion on the audio signal so as to obtain a bitstream signal which is supplied to the output 6. To that purpose, the A/D converter 4 receives a sampling frequency equal to N*−f_svia an input 16. Wherein f_sis a frequency equal to eg. 32 kHz, 44.1 kHz or 48 kHz and N is a large number, such as 64. The audio signal is sampled in the A/D converter 4 with a sampling frequency of eg. 2.8224 MHz (64*44.1 kHz). The bitstream signal appearing at the output 6 of the A/D converter thus has a bitrate of 2.8224 MHz.

[0038] The data compression unit 10 is in the form of a lossless coder. Lossless coders have the advantage that they can data compress the audio signal in such a way that, after data expansion by a lossless decoder, the original audio signal can be reconstructed in a substantially lossless way. Thus, there is substantially no loss of information after compression-expansion. Lossless coders can be in the form of a variable length coder. Variable length coders are well known in the art. Examples of such variable length coders are Huffman coders, arithmetic coders and Lempel-Ziv coders. Reference is made in this respect to the publications ‘A method for the construction of minimum-redundancy codes’ by D. A. Huffman, document D4, ‘An introduction to arithmetic coding’ by G. G. Langdon, document D5, and ‘A universal algorithm for sequential data compression’ by J. Ziv et al, document D6.

[0039] The data compression unit 10 carries out a substantially lossless data compression step on the bitstream signal so as to obtain a data compressed bitstream signal at its output 12, which is supplied to the output terminal 14.

[0040] FIG. 2ashows a frequency spectrum of the bitstream signal present at the output 6 of the A/D converter 4, for an input signal in the form of a 5 kHz sinusoid, sampled with a sampling frequency of 2.8224 MHz. The spectrum thus shows frequencies between 0 Hz and 1.4 MHz. FIG. 2bshows part of the spectrum shown in FIG. 2a, namely that part between 0 Hz and 100 kHz, so as to more clearly show the 5 kHz sinusoid contained in the bitstream signal. Clearly visible is the noise-like character of the bitstream signal, especially in the higher frequency region, which seems to imply that carrying out data compression on the signal will not result in a substantial amount of data reduction.

[0041] Contrary to this, investigations have made clear that a significant data reduction can be obtained. In the following table, the results of the data compression realized by three lossless coders are given for three different music fragments:

[0042] where δ is the compression ratio, defined as the ratio of the bitrate of the input signal of the coder to the bitrate of the output signal of the coder.

[0043] FIG. 3 shows an embodiment of a recording apparatus including the data compression apparatus shown in FIG. 1. The recording apparatus further includes a write unit 30 for writing the data compressed bitstream signal in a track on the record carrier 32. In the present example, the record carrier 32 is a magnetic record carrier, so that the write unit 30 includes at least one magnetic head 34 for writing the data compressed bitstream signal in the record carrier 32. The record carrier may however be an optical record carrier, such as a CD disk or a DVD disk.

[0044] FIG. 4 shows an embodiment of a transmitter for transmitting an audio signal via a transmission medium TRM, including the data compression apparatus as shown in FIG. 1. The transmitter further includes a transmission unit 40 for applying the data compressed bitstream signal to the transmission medium TRM. The transmission unit 40 could include an antenna 42.

[0045] 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 bitstream signal to be transmitted. FIG. 8 shows such signal processing steps carried out on the data compressed bitstream signal for the recording arrangement of FIG. 3. The recording arrangement of FIG. 8 therefore includes an error correction encoder 80, well known in the art, and a channel encoder 82, also well known in the art.

[0046] FIG. 5 shows an embodiment of the data expansion apparatus. The apparatus has an input terminal 50 for receiving the data compressed audio signal which is in the form of the data compressed bitstream signal, as supplied by the data compression apparatus of FIG. 1. The input terminal 50 is coupled to an input 52 of a data expansion unit 54, which has an output 56 which is coupled to an input 58 of a 1-bit D/A converter 60. An output 62 of the converter 60 is coupled to an output terminal 64.

[0047] The data expansion unit 54 is a lossless decoder, such as a variable length decoder in the form of eg. a Huffman decoder or an arithmetic decoder. It will be clear that the decoder in the data expansion apparatus of FIG. 5 should be the inverse of the encoder used in the data compression apparatus of FIG. 1, in order to realize a substantially lossless encoding-decoding step. The data expansion unit 54 expands the data compressed bitstream so as to obtain a replica of the original bitstream, which is supplied to the input 58 of the D/A converter 60. The converter 60 converts the bitstream into an analog audio signal which is supplied to the terminal 64.

[0048] FIG. 6 shows the data expansion apparatus of FIG. 5 incorporated in a reproduction apparatus. The reproducing apparatus further includes a read unit 70 for reading the data compressed bitstream signal from a track on the record carrier 32. In the present example, the record carrier 32 is a magnetic record carrier, so that the read unit 70 includes at least one magnetic head 72 for reading the data compressed bitstream signal from the record carrier 32. The record carrier may however be an optical record carrier, such as a CD disk or a DVD disk.

[0049] FIG. 7 shows an embodiment of a receiver for receiving an audio signal via a transmission medium TRM, including the data expansion apparatus as shown in FIG. 5. The receiver further includes a receiving unit 75 for receiving the data compressed bitstream signal from the transmission medium TRM. The receiving unit 75 could include an antenna 77.

[0050] 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 bitstream signal to be transmitted, so that, a corresponding channel decoding and error correction can be carried out upon reception. FIG. 9 shows the signal processing steps of channel decoding and error correction carried out on the received signal, received by the reading apparatus 70 for the reproducing arrangement of FIG. 6. The reproducing arrangement of FIG. 9 therefore include a channel decoder 90, well known in the art, and an error correction unit 92, also well known in the art, so as to obtain a replica of the data compressed bitstream signal.

[0051] Another data compression apparatus is shown in FIG. 10. In the data compression apparatus of FIG. 10, the bitstream signal is supplied to an input 8 of a lossless coder, which is in the form of an entropy coder, such as an arithmetic coder 154. Further the bitstream signal is also an input of a prediction filter unit 152. An output of the prediction filter unit 152 is coupled to an input of a probability determining unit 156. The arithmetic coder 154 encodes the bitstream signal into a data compressed bitstream signal in response to probability values p supplied to its input 192. The probability determining unit 156 determines a probability value indicating the probability that a bit in the bitstream signal supplied by the converter unit 4 has a predetermined logical value, such as ‘1’. This probability value, denoted p in FIG. 10, is supplied to the arithmetic coder 154 so as to enable the data compression of the bitstream signal in the arithmetic coder 154. The determining unit 156 determines this probability value from the output signal of the prediction filter 152. The arithmetic coder 154 can data compress the bitstream signal on a frame-by-frame basis.

[0052] The functioning of the apparatus of FIG. 10 is as follows. The prediction filter 152 prediction filters the bitstream signal so as to obtain a multi bit output signal. The multi bit output signal has a plurality of levels within a range of eg. +3 and −3. Further, for each of a plurality of subintervals in the value range of the multi bit output signal, it is determined what the probability is that the corresponding bit in the bitstream signal is eg. a ‘1’ bit. This can be realized by counting the number of ‘ones’ and ‘zeroes’ occurring in the bitstream signal during a specific time interval, when the multi bit output signal falls in one of such ranges. The probabilities thus obtained for the various values in the multi bit output signal are subsequently supplied as the probability signal p to the arithmetic coder 154. The data compressed bitstream signal is supplied by the arithmetic coder 154 to an output line 158, for transmission via a transmission medium TRM or a record carrier.

[0053] FIG. 11 shows a corresponding data expansion apparatus for decoding the data compressed bitstream signal, received via the transmission medium TRM. The data processing apparatus of FIG. 11 includes an entropy decoder 172, which receives the data compressed bitstream signal via an input 174. In the present example, the entropy decoder 172 is in the form of an arithmetic decoder that carries out an arithmetic decoding step on the data compressed bitstream signal under the influence of a probability signal p, supplied to an input 176 so as to generate a replica of the original bitstream signal which is supplied to an output 178. The replica is supplied to an input 58 of the reconverter unit 60.

[0054] Further, a probability supply unit 180 is present for supplying the probability signal p to the arithmetic decoder 172. The probability signal p can be obtained in different ways, dependent on how the probability signal has been derived in the encoder. One way is, to derive the probability signal p in an adaptive way from the output signal of a prediction filter 181. In this embodiment, the prediction filter 181 is equivalent to the prediction filter 152 in the encoder and the probability supply unit 180 is equivalent to the probability determining unit 156 in the encoder of FIG. 10. Another way of generating the probability signal p, is by using side information received via the transmission medium TRM, as will be explained hereafter.

[0055] Side information can be generated by the apparatus of FIG. 10 for transmission to the apparatus of FIG. 1. Such side information can include the filter coefficients for the filter 152 that are determined on a frame by frame basis, which coefficients are transmitted to the corresponding prediction filter included in the unit 180.

[0056] Further, the apparatus of FIG. 10 can generate parameters that describe the conversion of the multi bit output signal of the prediction filter 152 into the probability signal p. Such parameters are also included in the side information and transmitted to the supply unit 180 and the filter 181, so as to enable the regeneration of the probability signal p in the apparatus of FIG. 11 on the basis of the multi bit output signal provided by the prediction filter 181.

[0057] The entropy encoder used in the embodiment of FIG. 10 is adapted to encode the bitstream signal using a probability signal in order to obtain the data compressed bitstream signal. One such entropy encoder is the arithmetic coder described above. One other type of such entropy coder is as an example, the well known finite state coder. The entropy decoder used in the embodiment of FIG. 11 is adapted to decode the data compressed bitstream signal using a probability signal in order to obtain a replica of the bitstream signal. One such entropy decoder is the arithmetic decoder described above. One other type of such entropy decoder is, as an example, the well known finite state decoder.

[0058] 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. When the audio signal is supplied in digital form, such as sampled at 44.1 kHz and the samples being expressed in eg. 16 bits, the conversion apparatus are adapted to oversample the digital audio signal with eg. the frequency of 64*44.1 kHz so as to obtain the 1-bit bitstream signal.

[0059] It should further be noted that the invention also applies to an embodiment in which the bitstream signal, as supplied by the converter 4 has undergone an additional signal processing step resulting in a processed 1-bit bitstream signal that is supplied to the lossless coder 10. Such additional signal processing step could include merging a left and right hand signal component of a stereo audio signal, in 1-bit bitstream form, into a processed 1-bit bitstream signal.

[0060] Further, the invention lies in each and every novel feature or combination of features.

[0061] The invention has been disclosed with reference to specific preferred embodiments, to enable those skilled in the art to make and use the invention, and to describe the best mode contemplated for carrying out the invention. Those skilled in the art may modify or add to these embodiments or provide other embodiments without departing from the spirit of the invention. The scope of the invention is not limited to the embodiments, but lies in each and every novel feature or combination of features described above and in every novel combination of these features. Thus, the scope of the invention is only limited by the following claims: