Triphone definition

[Zack H]

I was looking at some past papers and I was unsure if I am confusing the definition of a triphone.

I am under the impression that a triphone is a subphone unit that makes up a third of a phone. However, I don’t remember if there is a particular name for a sequence of three phones, e.g. for n-gram 3 words is considered a trigram, is the same true for phones?

What is the correct term for a sequence of 3 subphones used in HMM ASR?
What is the correct term for a sequence of 3 phones?

[Simon]

The terminology is potentially confusing.

A triphone is a model of one whole phone. It’s not a model of a fraction of a phone, and it’s not a model of a sequence of three phones.

It’s called triphone because it takes a context of three phones into account (previous, current and next). So, it’s a context-dependent model of a phone. Depending on the amount of context we take into account, we use different terms to describe a model of a phone:

• monophone – context-independent
• biphone -depends on either left- or right-context
• triphone – depends on left and right context
• quinphone – depends on 2 phones to the left and 2 to the right

(Note that a biphone model is not a model of a diphone!)

Jurafsky and Martin’s explanations may not be the clearest, especially when they start talking about “sub-phones”. By that, they simply mean the states within an HMM.

[Author]

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