Perceptual coding of narrow-band audio signals at low rates

This paper describes a coding paradigm using coding tools based on the characteristics of the human hearing system so as to accommodate a wide range of narrow-band audio inputs without annoying artifacts at low rates (down to 8 kb/s). The narrow-band perceptual audio coder (NPAC) employs a variety of algorithms to account for the perceptually irrelevant parts of the input signal in addition to statistical redundancies. The new algorithms used in the NPAC coder include a perceptual error measure in training the codebooks and selecting the best codewords which takes into account the audible parts of the quantization noise, a perception-based bit-allocation algorithm and a new predictive scheme to vector quantize the scale factors. The NPAC coder delivers acceptable quality without annoying artifacts for most narrow-band audio signals at around 1 bit/sample. Informal subjective tests have shown that the NPAC coder outperforms a commercial low-rate music coder operating at 8 kb/s.     

Continuous-wave Z-scan measurement of photorefractive SBN:60

A cw-probe Z-scan technique was employed to measure the photoinduced index change in a photorefractive SBN:60 crystal. For this experiment a three-detector data-acquisition system was used to account for temporal changes in the laser. The effects of various beam parameters such as intensity, polarization, and wavelength were studied. A theoretical simulation of the Z scan based on a band-transport model of photorefractive-index variation was also developed. This model provides reasonable agreement with the experimental results.