11.1. Diphone introduction

The basic idea behind building diphone databases is to explicitly list all possible phone-phone transitions in a language. This makes the incorrect but practical and simplifying assumption that co-articulatory effects never go over more than two phones. The exact definition of phone here is in general nontrivial, and what a "standard" phone set should be is not uncontroversial -- various allophonic variations, such as light and dark /l/, may also be included. Unlike generalized unit selection where multiple occurrences of phones may exists with various distinguishing features, in a diphone database only one occurrence of each diphone is recorded. This makes selection much easier but also makes for a large laborious collection task.

In general, the number of diphones in a language is the square of the number of phones. However, in natural human languages, there are phonotactic constraints -- some phone-phone pairs, even whole classes of phones-phone combinations, may not occur at all. These gaps are common in the world's languages. The exact definition of never exist is also problematic. Humans can often generate those so-called non-existent diphones if they try, and one must always think about phone pairs that cross over word boundaries as well, but even then, certain combinations cannot exist; for example, /hh/ /ng/ in English is probably impossible (we would probably insert a schwa). /ng/ may really only appears after the vowel in a syllable (in coda position); however, in other languages it can appear in syllable-initial position. /hh/ cannot appear at the end of a syllable, though sometimes it may be pronounced when trying to add aspiration to open vowels.

Diphone synthesis, and more generally any concatenative synthesis method, makes an absolutely fixed choice about which units exist, and in circumstances where something else is required, a mapping is necessary. When humans are given a context where an unusual phone is desired, for example in a foreign word, they will (often) attempt to produce it even though it falls outside their basic phonetic vocabulary. The articulatory system is flexible enough to produce (or attempt to produce) unfamiliar phones, as we all share the same underlying physical structures. Concatenative synthesizers, however, have a fixed inventory, and cannot reasonably be made to produce anything outside their pre-defined vocabulary. Formant and articulatory synthesizers have the advantage here. This is a basic trade off, concatenative synthesizers typically produce much more natural synthesis than formants synthesizer but at the cost of being only able to produce those phones defined within their inventory.

Since we wish to build voices for arbitrary text-to-speech systems which may include unusual phones, some mapping, typically at the lexical level, can be used to ensure all the required diphones lie within the recorded inventory. The resulting voice will therefore be limited, and unusual phones will lie outside its range. This in many cases is acceptable though if the voice is specifically to be used for pronouncing Scottish place names it would be advisable to include the /X/ phone as in "loch".

In addition to the base phones, various allophonic variations may also be considered. Flapping, as when the /t/ becoming a /dx/ in the word "butter" is an example of an allophonic variation reduction which occurs naturally in American English, and including flaps in the phone set makes the synthetic speech more natural. Stressed and unstressed vowels in Spanish, consonant cluster /r/ verses lone /r/ in English, inter-syllabic diphones verses intra-syllabic ones -- variations like these are well worth considering. Ideally, all such possible variations should be included in a diphone list, but the more variations you include, the larger the diphone set will be -- remember the general rule that the number of diphones is nearly the square of the number of phones. This affects recording time, labeling time and ultimately the database size. Duplicating all the vowels (e.g. stressed/unstressed versions) will significantly increase the database size.

These inventory questions are open, and depending on the resources you are willing or able to devote, can be extended considerably. It should be clear, however, that such a list is simply a basic set. Alternative synthesis methods and inventories of different unit sizes may produce better results for the amount of work (or data collected). Demi-syllable databases and mixed inventory methods such as Hadifix [portele96] may give better results under some conditions. Still, controlling the inventory and using acoustic measures rather than linguistic knowledge to define the space of possible units in your inventory has also been attempted as in work like Whistler [huang97]. The most extreme view where the unit inventory is not predefined at all but based solely on what is available in general speech databases is CHATR [campbell96].

Although generalized unit selection can produce much better synthesis than diphone techniques, using more units makes selecting appropriate ones more difficult. In the basic strategy presented in this section, selection of the appropriate unit from the diphone inventory is trivial, while in a system like CHATR, selection of the appropriate unit is a significantly difficult problem. (See Chapter 12 on unit selection for more discussion of such techniques). With a harder selection task, it is more likely that mistakes will be made, which in unit selection can give some selections which are much worse worse that diphones, even though other examples may be better.