Internet Low Bit Rate Codec (iLBC) Status of this Memo This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited.

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A B S T R A C TA recent independent survey found a 2.4 kb/s waveform-interpolation (WI) algorithm to perform better than other state-of-the-art coders. However, this coder had a very high level of computational complexity. The introduction of various techniques, including a time-varying waveform sampling rate and a cubic B-spline waveform representation, has reduced the computational complexity by an order of magnitude. The new implementation allows full-duplex reaktime operation on a single DSP device and on an average workstation (the latter using nonoptimized, compiled C source code). The new coder also contains a number of new features which improve the quality of the reconstructed speech signal. I N T R O D U C T I O NIn the last ten years, the performance of 2.4 kb/s speech coders has increased significantly, resulting in new applications and a need to replace the existing $31015 standard [l]. The waveform interpolation (WI) coding paradigm [2] was recently found to provide state-of-the-art performance at 2.4 kb/s [l]. The coder performed very well in terms of quality, robustness against channel errors and background noise, and speaker recognizability [3]. In all of these tests, the 2.4 kb/s WI coder was found to be statistically equivalent to, or better than, the 4.8 kb/s FS10lG standard [4].To make the WI coder useful for practical applications, it must be implementable on a single DSP. We describe procedures which reduce the computational complexity to a level that permits this. In addition, we describe procedures to improve the reconstructed speech quality.The outline of this paper is as follows: section 2 reviews the basic structure of WI coders; sections 3 and 4 describe our improved WI implementation. Section 5 provides a complexity overview and section G provides conclusions.2. WI P R I N C I P L E S evolving waveform. A two-dimensional signal, g ( t , d), is defined, which displays the representative shape of the time waveform along the 4 axis. The waveform is repeated along the 4 axis with a normalized period of 27r. For voiced speech, the waveform evolves relatively slowly and for unvoiced speech the waveform evolves rapidly. Figure 1 provides a block diagram of the current 2.4 kb/s WI coder, including the sampling rates of the outputs of the blocks. The first step in the coding process is the linearprediction (LP) analysis, which is performed at a rate of Sf (the frame rate; 40 Hz in our implementation). The LP In WI codinp [ 2 ] , the speech sipnal i s represented by an alignment power h determination scale interpolate I synthesize inter olate 7 Figure 1. Basicoperationsin the 2.4 kb/s WI. The samplingrate of the output of each processing unit is shown on the right. For clarity, spectral and power quantization have not been depicted. parameters are quantized using a 30-bit split VQ with linear mapping of block codes [5] to obtain robustness against channel errors. The LP parameters are conventionally interpolated to a subframe update rate of S,f (160 Hz). Filtering the speech signal, s ( t ) ...