This dissertation investigates the role of voice quality as a cue to register contrast from both diachronic and synchronic perspectives. It addresses two main questions: (i) What is the underlying cause of vowel quality change in register languages? (ii) How is register represented phonologically? Specifically, it explores these inquiries through a historical study and two acoustic studies conducted on three Austroasiatic languages: Khmer, Eastern Khmu, and Chanthaburi Khmer.Chapter 2 focuses on the absolute and relative chronology of the loss of onset voicing and vowel bifurcation in Khmer, using philological evidence. The aim is to determine whether vowel quality change in Khmer is due to an effect of onset voicing or an effect of voice quality. The findings reveal that both events occurred within a century, making it inconclusive whether vowel quality is derived directly from onset voicing or indirectly from voice quality. Chapter 3 presents an acoustic study of an Eastern Khmu dialect, which possesses onset voicing but lacks tone or register. The study investigates whether onset voicing affects the subsequent vowel. The results demonstrate that only f0 and F1 of the following vowel are influenced by onset voicing. This supports the notion that vowel height or pitch can transphonologize directly from onset voicing without undergoing an intermediate stage where voice quality is contrastive. Chapter 4 focuses on an acoustic study of Chanthaburi Khmer, a Khmer dialect with contrastive register. It examines whether vowel quality contributes to register distinctions in the phonological representations. The findings indicate that temporal alignment between vowel quality and voice quality is observed only for certain speakers. This suggests that the gestural representation of Chanthaburi Khmer registers for some speakers involves either a single gesture associated with all cues to register or multiple coordinated gestures. Conversely, for other speakers, the representation of registers involves multiple gestures without coordination. Chapter 5 discusses the status of vowel quality distinction across registers in both synchrony and diachrony. It presents a gestural representation of register, revised stages of tonogenesis/registrogenesis, and a gestural model of tonogenesis/registrogenesis. Overall, this dissertation demonstrates that vowel quality distinction can directly derive from the contrast in onset voicing. Additionally, three possible gestural representations are proposed: (i) vowel quality and voice quality as cues to register are associated with a single gesture, (ii) they are associated with separate gestures that are coordinated, or (iii) they are associated with separate gestures without coordination.