Donwtrend and normalisation

Abstract

This thesis reports on three studies of downtrend and normalisation in Hong Kong Can¬ tonese, with the goal of finding out what kind of frame of reference can be used to identify and equate the same Cantonese tone in different positions-in-utterance.

Downtrend as a general tendency of pitch to lower throughout an utterance is known to affect how an accent is realised. An accent is realised with a lower fO value late in an utterance than early in the utterance. One approach is to treat downtrend as a phonetic frame of reference so that different fO values of the same accent can be equated in terms of the same phonetic specifications . One issue that this approach needs to deal with is to characterise the phonetic frame of reference, and to decide which features in the f0 contour can be attributed to the phonetic specifications of accents.

Studies in tone normalisation showed that phonetic specifications of tones are consistent and invariant across speakers after normalisation. Variations of f0 range and level was factored out. By applying normalisation to downtrend data, it is expected to separate out the phonetic specifications of tones from any modification of the frame of reference.

The results showed that downtrend resulted from modifications of local fO range and the phonetic specifications of tones were consistent and invariant relative to the localised fO range. The localised fO range as a frame of reference was found to be perceptually valid and significant in tone identification.