Pronunciation Dictionaries and Regular Expressions

07 Jul 2011

Over the past few months I have been compiling a pronunciation dictionary for English. This is a big task. So why start it?

I have been using the eSpeak speech synthesizer to listen to documents and web pages. Every now and then (particularly names and compound words) it would pronounce the word wrong. I had been keeping a list of words that it mispronounced, sending them to the eSpeak developer and then starting over. This has several disadvantages:

  1. how do you know if a word has regressed in pronunciation?
  2. how do you deal with accent and dialect variations?

In addition to this, I want to build my own text to phoneme engine. This requires having the data in place to construct the conversion rules. So I started a pronunciation dictionary for the way I pronounce various words in the cainteoir-english project under the data/reece directory. One of the things I am keen on is keeping each pronunciation dictionary tied to a specific person: this is in order to avoid mixing accent and dialect variations between different speakers.

I have been maintaining a script (tests/ in the cainteoir-engine project) that compares the pronunciation in the pronunciation dictionary against the pronunciation by espeak and indicating success or failure. This is utilized in the cainteoir-english project through the make check command that shows mispronounced words by espeak (according to the pronunciation dictionary). For example:

../cainteoir-engine/tests/ data/reece | grep fail
faux /fˈəʊ/ got: /fˈɔːks/ ... fail
mojito /məçˈiːtəʊ/ got: /məʊhˈiːtəʊ/ ... fail
oort /ˈɔːt/ got: /ˈuːət/ ... fail
poor /pˈɔː/ got: /pˈʊə/ ... fail
prepare /pɹəpˈeə/ got: /pɹɪpˈeə/ ... fail
Elise /əlˈiːz/ got: /ɪlˈiːz/ ... fail
Eoghan /ˈəʊən/ got: /ɪˈɒɡən/ ... fail
Eoin /ˈəʊən/ got: /ɪˈɔɪn/ ... fail

The tests/ has another feature where it can produce an exception dictionary usable by eSpeak containing the mispronounced words and the pronunciation in the pronunciation dictionary. This is used in my fork of espeak on github to generate the dictsource/en_extra file and build the en voice using the make en command. For example, running make en yields:

../cainteoir-engine/tests/ ../cainteoir-english/data/reece | grep -F fail
2143 passed 0 failed 2143 total
wc -l dictsource/en_extra
607 dictsource/en_extra

Or about a 72% accuracy rate. NOTE: This is not a completely accurate representation of the accuracy of the eSpeak en voice since the words are weighted toward those that eSpeak mispronounces such as Eoghan and Eoin and also covers dialect differences like poor.

This means that any word that espeak mispronounces through its rules that are in the pronunciation dictionary are added to the en_extra file, keeping the accuracy for those words. By having this driven automatically through a script means that manually adding words to en_list is not needed (provided the word is in the pronunciation dictionary).

Given the rules only (i.e. no extra words listed in en_list) the number of entries in the en_extra file gives a clear indication of the accuracy of the rules in en_rules for the given accent/dialect of the speaker of the pronunciation dictionary used.

Using Regular Expressions

As the pronunciation dictionary has been expanding – particularly when dealing with names – there have been a growing number of words that are pronounced the same. That it, it is only the spelling that is different.

Using a regular expression-style notation allows the words to be grouped together, reducing the number of entries in the pronunciation dictionary. I have chosen a regular expression notation as it is used in other areas (no need to create yet another syntax to learn) and is powerful in what can be expressed.

This also has an interesting side effect – it forms the initial step in constructing pronunciation rules, suggesting where common word sequences occur. For example, names often end with the pattern (i|ie|y) like the name Jacki/Jackie/Jacky.

Using this information, I have constructed a set of replacement patterns that can be used in the words. These form letter-based replacements such as <y>; as a placeholder for (ee|ey|i|ie|y) in names like Dav<y>, and suffix-based replacements such as +ANNA as a placeholder for a(na|nna|nnah) in names like Jo+ANNA.

This is still a work in progress as the implementation I am using is simplistic and can only handle a limited set of expressions. I also want to express groupings to cover prefixes and suffixes used in words to cover things like {kilo|mega|giga|terra|peta}byte and heal{ed|er|ers|ing|s} with the associated pronunciation for each sub-group.

See the Rule Engine page for my notes on the future direction for this functionality.

Reece H. Dunn