This is not a direct answer to your question but let me explain about difference between voicing/devoicing vowels and prolonging vowels.
There are several ways to pronounce です or the likes.
- des (1 syllable
non-moraic 3 morae, /de/ is longer than /s/, sounds chopped foreign)
- des: (1 syllables 2 morae, /de/ and /s:/ are even, sounds eastern-dialect-ish)
- desu (2 syllables 2 morae, /de/ and /su/ are even, sounds western-dialect-ish)
- desu: (2 syllable 3 morae, /su:/ is longer than /de/, sounds feminine)
Japanese ears perceive no.1 as でぇす, no.2 and 3 as the same thing and normal pronunciation of です while no.4 as ですぅ, which is different from the rest and perhaps feminine.
However, English speakers perceive no.1 and no.2 as the same word "des" while no.3 and 4 as another same word "desu". If they fail to differentiate no.3 and 4, they may mix features unique to each of them.