Improving speaker recognisability in phonetic vocoders

Abstract

Phonetic vocoding is one of the methods for coding speech below 1000 bit/s. The transmitter stage includes a phone recogniser whose index is transmitted together with prosodic information such as duration, energy and pitch variation. This type of coder does not transmit spectral speaker characteristics and speaker recognisability thus becomes a major problem. In our previous work, we adapted a speaker modification strategy to minimise this problem, modifying a codebook to match the spectral characteristics of the input speaker. This is done at the cost of transmitting the LSP averages computed for vowel and glide phones. This paper presents new codebook generation strategies, with gender dependence and interpolation frames, that lead to better speaker recognisability and speech quality. Relatively to our previous work, some effort was also devoted to deriving more efficient quantization methods for the speakerspecific information, that considerably reduced the average bit rate, without quality degradation. For the CD-ROM version, a set of audio files is also included.

Phonetic vocoder assessment

“…Hence, speaker recognisability is one of the main issues in this class of coders. Our approach to minimise this drawback was to include some speaker adaptation capability in the coder [4] [5]. The present paper describes our recent work on this coder, involving on one hand its formal assessment in clean laboratory conditions and, on the other hand, its performance in non-clean environments with a much wider speaker population.…”

“…No MOS tests were performed. In fact, these tests involve trained listeners, which are confronted with processed sentences and requested to judge the quality in a five-point scale (1)(2)(3)(4)(5). However, listeners normally avoid marking the quality in the scale extremes.…”