EFFICIENT SPECTRAL ENVELOPE CODING USING VARIABLE TIME/FREQUENCY RESOLUTION AND TIME/FREQUENCY SWITCHING
This application is a Divisional of application Ser. No. 09/763,128 filed on May 15, 2001 now U.S. pat. No. 6,978,236 and for which priority is claimed under 35 U.S.C. § 120. Application Ser. No. 09/763,128 is the national phase ofPCT International Application No. PCT/SE00/00158 filed on Jan. 26, 2000, under 35 U.S.C. § 371, and which designated the United States of America. PCT International Application No. PCT/SE00/00158 claims priority under 35 U.S.C. § 119(a) on Patent Application No. 9903552-9 filed in Sweden on Oct. 1, 1999. The entire contents of each of the above-identified applications are hereby incorporated by reference.
The present invention relates to a new method and apparatus for efficient coding of spectral envelopes in audio coding systems. The method may be used both for natural audio coding and speech coding and is especially suited for coders using SBR [WO 98/57436] or other high frequency reconstruction methods.
BACKGROUND OF THE INVENTION
Audio source coding techniques can be divided into two classes: natural audio coding and speech coding. Natural audio coding is commonly used for music or arbitrary signals at medium bitrates, and generally offers wide audio bandwidth. Speech coders are basically limited to speech reproduction but can on the other hand be used at very low bitrates, albeit with low audio bandwidth. In both classes, the signal is generally separated into two major signal components, the "spectral envelope" and the corresponding "residual" signal. Throughout the following description, the term "spectral envelope" refers to the coarse spectral distribution of the signal in a general sense, e.g. filter coefficients in an linear prediction based coder or a set of time-frequency averages of subband samples in a subband coder. The term "residual" refers to the fine spectral distribution in a general sense, e.g. the LPC error signal or subband samples normalized using the above time-frequency averages. "Envelope data" refers to the quantized and coded spectral envelope, and "residual data" to the quantized and coded residual. At medium and high bitrates, the residual data constitutes the main part of the bitstream. At very low bitrates, the envelope data constitutes a larger part of the bitstream. Hence, it is indeed important to represent the spectral envelope compactly when using lower bitrates.
Prior art audio coders and most speech coders use constant length, relatively short, time segments in the generation of envelope data to achieve good temporal resolution. However, this prevents optimal utilisation of the frequency domain masking known from psycho-acoustics. To improve coding gain through the use of narrow filterbands with steep slopes, and still achieve good temporal resolution during transient passages, modem audio coders employ adaptive window switching, i.e. they switch time segment lengths depending on the signals statistics. Clearly a minimum usage of the short segments is a prerequisite for maximum coding gain. Unfortunately, long transition windows are needed to alter the segment lengths, limiting the switching flexibility.
The spectral envelope is a function of two variables: time and frequency. The encoding can be done by exploiting redundancy in either direction of the time/frequency plane. Generally, coding of the spectral envelope is performed in 5 the frequency direction, using delta coding (DPCM) or vector quantization (VQ).
SUMMARY OF THE INVENTION
The present invention provides a new method, and an apparatus for spectral envelope coding. The coding scheme is designed to meet the special requirements of systems, where the residual signal within certain frequency regions is excluded from the transmitted data. Examples are systems employing HFR (High Frequency Reconstruction), in particular SBR (Spectral Band Replication), or parametric coders. In one implementation, non-uniform time and frequency sampling of the spectral envelope is obtained by adaptively grouping subband samples from a fixed size filterbank, into frequency bands and time segments, each of which generates one envelope sample. This allows instantaneous selection of arbitrary time and frequency resolution within the limits of the filterbank. The system defaults to long time segments and high frequency resolution. In the vicinity of transients, shorter time segments are used, whereby larger frequency steps can be used in order to keep the data size within limits. In order to maximize the benefits of the non-uniform sampling in time, variable length of bitstream frames or granules are used. The variable time/
3Q frequency resolution method is also applicable on envelope encoding based on prediction. Instead of grouping of subband samples, predictor coefficients are generated for time segments of varying lengths according to the system. The invention describes two schemes for signalling of the
35 time and frequency resolution used. The first scheme allows arbitrary selection, by explicit signalling of time segment borders and frequency resolutions. In order to reduce the signalling overhead, four classes of granules are used, offering different cost/flexibility tradeoffs. The second scheme exploits the property of a typical programme material, that transients are separated at least by a time T„ml„, in order to reduce the number of control bits further. Hereby, a transient detector in the encoder, operating on a time interval Trfe<<=T„m,„, equal to the nominal granule length, determines the position of the onset of a possible transient. The position within the interval is encoded and sent to the decoder. The encoder and decoder share rules that specify the time/frequency distribution of the spectral envelope samples, given a certain combination of subsequent control signals, ensuring an unambiguous decoding of the envelope data.
The present invention presents a new and efficient method for scalefactor redundancy coding. A dirac pulse in the time domain transforms to a constant in the frequency domain, and a dirac in the frequency domain, i.e. a single sinusoid, corresponds to a signal with constant magnitude in the time domain. Simplified, on a short term basis, the signal shows less variations in one domain than the other. Hence, using prediction or delta coding, coding efficiency is increased if the spectral envelope is coded in either time- or frequencydirection depending on the signal characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described by way of 65 illustrative examples, not limiting the scope or spirit of the invention, with reference to the accompanying drawings, in which: