Finn Tore Johansen

A real-time wideband CELP coder for a videophone application

In this paper we present an algorithm for the coding of wideband (7 kHz) speech signals at 16 kbps using code excited linear prediction (CELP), primarily intended for use in audio-visual coding systems (e.g., videotelephony) in the ISDN network, or other telecommunication equipment using loudspeaker sound. The algorithm is a full-band approach and much effort has been put into reducing the computational complexity. This has led to a real-time implementation of the present algorithm on a single TMS320C31 DSP (encoder+decoder). Through a formal subjective evaluation we have demonstrated a performance comparable to the CCITT Rec. G.722 subband coder at 64 kbps.<<ETX>>

Wideband speech coding at 16 kbit/s for a videophone application

Abstract This paper presents results on wideband 7 kHz speech coding at 16 kbit/s where the proposed CELP algorithm is implementable on a single floating point DSP. As a basic coder structure, the long-term predictor is implemented as an adaptive codebook, while a sparse Gaussian codebook with non-overlapping vectors is used for the stochastic excitation. In order to meet the complexity requirements, several methods for efficient codebook search are adopted. With these methods, it is shown that the computational effort for the basic coder structure can be reduced to 12.4 MIPS with a 7 bit stochastic codebook. A two-stage hierarchical search through the adaptive codebook is investigated. This search method reduces the computational effort further although at the cost of a small degradation in coder performance. The coder is evaluated in an absolute category rating (MOS) test using both a hi-fi handset and a loudspeaker, and compared to the CCITT standard coder G.722 at 48–64 kbit/s. The speech quality with the basic CELP structure is judged to be comparable to the G.722 coder at 48 kbit/s.

A non bit-exact approach for implementation verification of the CCITT LD-CELP speech coder

Abstract This paper presents the design methodology behind the floating point verification procedure for the Low-Delay Code-Excited Linear Prediction speech coder, recently selected by the CCITT as Recommendation G.728. This procedure is based on a non bit-exact specification, which is different from previous CCITT speech coder verification procedures. This approach gives additional freedom for the implementor of the algorithm, and will allow for more efficient implementations on various kinds of hardware. However, this flexibility also means that different implementations will respond slightly different to verification test signals. To cope with this, explicit objective measurements of such deviations are used in the verification process. These measurements are simple weighted and unweighted signal-to-noise ratios (WSNR and SNR). In addition to the objective measurements, certain restrictions had to be placed on the test sequence design. In spite of the input restrictions, a set of test sequences giving reasonably good coverage of the LD-CELP algorithm and state space has been found. Evaluation experiments are reported, showing that these sequences have a satisfactory error detecting capability. A final discussion concludes that the chosen verification approach is indeed feasible as an implementor s tool.

A Wideband CELP Coder at 16 kbit/s for Real Time Applications

Since its introduction in 1984, Code Excited Linear Predictive (CELP) [1] coding has received considerable attention for high quality speech coding at low bit-rates. Although most of the research has been focused on coding of narrowband (200-3400 kHz) speech, some recent studies on CELP coding of wideband (50-7000 kHz) speech have been reported [2], [3], [4].