Selon Garth Wallace :
> In Japanese it really is [toN.ka.ts)u].

Actually, if you want to be really correct phonetically, it's 
[toN.ka.t_sM_0] (/u/ is unrounded, and normally unvoiced at the end of a 
word after an unvoiced consonant). Actually, the last vowel in such a 
word is so voiceless that to untrained ears it sometimes sounds as if 
it's absent.

> BTW, devoicing only happens to /i/ and /u/, so you were right the first time.

Correct, and often with the devoicing those vowels become so dark they 
nearly disappear (the most common cases are the polite copula "desu" 
/de.sM/ and the polite suffix "masu" /ma.sM/, which in normal speech 
often sound more like [de.s=] and [ma.s=] :) ).
Christophe Grandsire-Koevoets.

You need a straight mind to invent a twisted conlang.