Data processing: speech signal processing – linguistics – language – Audio signal bandwidth compression or expansion
Reexamination Certificate
1998-05-26
2001-02-06
Hudspeth, David R. (Department: 2741)
Data processing: speech signal processing, linguistics, language
Audio signal bandwidth compression or expansion
Reexamination Certificate
active
06185539
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention refers to a method for coding an audio signal which has been digitized at a low sampling rate. In particular, the invention refers to a coding method which is only slightly modified relative to the Standard ISO/IEC 13878-3 (MPEG2 layer 3) and which enables audio signals which are digitized at a lower sampling rate than the sampling rate according to the Standard ISO/IEC 13818-3 to be transmitted at a low bit rate.
2. Description of the Related Art
The existing Standard ISO/IEC 13818-3 published in May 15, 1995 defines with layer 3 a coding method for signals with sampling frequencies between 24 kHz and 16 kHz and makes possible bit rates of down to 8 kbit/s. In particular at this very low bit rate, which is very attractive for a transmission in computer networks e.g., the use of still smaller sampling frequencies would be desirable. The cited Standard ISO/IEC 13818-3 does not provide these, however.
SUMMARY OF THE INVENTION
Starting from this prior art it is therefore the object of the present invention to develop further the cited method for coding audio signals in such a way that, with the smallest possible deviation from the Standard ISO/IEC 13818-3, sampling can be performed at sampling rates which do not conform to the Standard ISO/IEC 13818-3; furthermore, decoding with existing decoders should be possible without much being needed in the way of adaptation.
The present invention provides a method for coding an audio signal digitized at a low sampling rate to obtain time domain audio samples, comprising the steps of producing a frequency domain representation of the time domain audio samples, the frequency domain representation including a total number of successive frequency lines; subdividing the total number of successive frequency lines into a plurality of scale factor bands each scale factor band having a number of successive frequency lines wherein a scale factor is assigned to each scale factor band, the assigned scale factor being used for coding the frequency lines in the respective scale factor band; forming a plurality of regions, each region including a plurality of successive scale factor bands wherein the scale factors assigned to the plurality of scale factor bands in a region are each coded with the same number of bits, which is determined according to the largest scale factor of the region, and wherein a region including the scale factor bands having frequency lines that correspond to the higher frequency range frequency lines among the frequency lines in all regions is the highest region; and setting to a value of zero the scale factors that are assigned to the scale factor bands in at least the highest region to obtain zero-valued scale factors, the value of zero corresponding to a multiplication factor of 1; coding the frequency lines of at least the highest region with the zero-valued scale factors; and refraining from coding the zero-valued scale factors themselves.
In general the present invention provides coding of audio signals which have been digitized at a sampling rate which is lower than the sampling rate according to the Standard ISO-MPEG2 layer 3.
In general in the case of the subject matter of the present invention, as also in the case of the known Standard ISO/IEC 13818-3, the successive frequency lines of the digitized audio signal which are assigned to a scale factor band are coded with the same scale factor, this being transmitted together with the coded scale factor band (see table B.8 of ISO/IEC 13818-3).
In further conformity With the known method according to the cited Standard ISO/IEC 13818-3, successive scale factor bands form a region within which all the scale factors are each coded with the same number of bits, which is determined according to the largest scale factor of this region (see section 2.5.2.13 of ISO/IEC 13818-3).
In the Standard ISO-MPEG2 layer 3, all the scale factor bands of all the regions are assigned scale factors. Only the last band, wherein lie those frequency lines remaining after the desired assignment of the frequency lines, does not have a scale factor when coding (see section 2.5.2.11, subparagraph ‘scalefac 1[gr][tc][sfb], scalefac s[gr][tc][sfb][window], is pos[sfb]’ of ISO/IEC 13818-3).
In contrast to the Standard ISO/IEC 13818-3, the present invention is so conceived that at least the frequency lines of the highest region of scale factor bands are coded with the scale factor 0, so that for at least the highest region no scale factor is coded and transmitted. The bits which are saved through the missing scale factor or scale factors are used for the finer quantization, compared to the Standard ISO/IEC 13818-3, of the frequency lines in the rest of the spectrum.
According to a further important aspect of the present invention, the grouping of the frequency lines into scale factor bands is modified relative to the cited Standard ISO/IEC 13818-3 in such a way that the scale factor bandwidths within the highest region are reduced relative to the scale factor bandwidths of the highest region according to the Standard ISO-MPEG2 layer 3.
REFERENCES:
patent: 0 457 391 (1991-05-01), None
patent: 0 525 774 A2 (1992-07-01), None
patent: 0 612 159 A2 (1994-02-01), None
ISO/IEL “Standard 11172-3” ISO pp. 73 and 741.
ISO/IEL “Standard 13818-3” ISO pages all.
ISO/IEL “STandard 13818-3 amendment” ISO pages all.
Brandenburg Karlheinz
Buchta Rainer
Dietz Martin
Gerhauser Heinz
Kunz Oliver
Dougherty & Clements LLP
Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung
Hudspeth David R.
Zintel Harold
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