Speex

Abstract: In recent years wideband speech coding has attracted quite a lot research. However, there have been only a few papers describing the quality difference between narrow and wideband coded speech. In this paper we describe the testing methodology and how the sample processing was done in order to evaluate these two conditions in a single test. Results from a six different listening tests are shown and explained in detail. In addition to standard AMR wideband and narrowband codecs we have tested two so called open source codecs namely iLBC and Speex. We have no knowledge that these codecs have been tested earlier in a formal listening test.

Abstract: Speex is an open-source voice codec suitable for use in Voice over IP (VoIP) type applications. This document describes the payload format for Speex generated bit streams within an RTP packet. Also included here are the necessary details for the use of Speex with the Session Description Protocol (SDP). Editors Note All references to RFC XXXX are to be replaced by references to the RFC number of this memo, when published.

Abstract: One key aspect of the CELP algorithm is that it shapes the coding noise using a simple, yet effective, weighting filter. In this paper, we improve the noise shaping of CELP using a more modern psychoacoustic model. This has the significant advantage of improving the quality of an existing codec without the need to change the bit-stream. More specifically, we improve the Speex CELP codec by using the psychoacoustic model used in the Vorbis audio codec. The results show a significant increase in quality, especially at high bit-rates, where the improvement is equivalent to a 20% reduction in bit-rate. The technique itself is not specific to Speex and could be applied to other CELP codecs.

Abstract: We proposed to use support vector machines (SVMs) to recognize speakers from signal transcoded with different speech codecs. Experiments with SVM-based text-independent speaker classification using a linear GMM supervector kernel were presented for six different codecs and uncoded speech. Both matched (the same codec for creating speaker models and for testing) and mismatched conditions were investigated. SVMs proved to provide high accuracy of speaker recognition, however requiring higher number of Gaussian mixtures than in the baseline GMM-UBM system. In mismatched conditions the Speex codec was shown to perform best for creating robust speaker models.