docs/phoneme_model.md: create a new phoneme model document.

docs/phoneme_model.md

@@ -0,0 +1,45 @@
+# Phoneme Model
+
+- [References](#references)
+
+----------
+
+Evan Kirshenbaum's feature set used in his ASCII transcription of the
+International Phonetic Alphabet (IPA)<sup>\[<a href="#ref1">1</a>\],
+\[<a href="#ref2">2</a>\]</sup> describes the phonemes in a way consistent with
+how the phonemes are organised in the IPA code chart. That is the approach used
+in the [Phonemes](phonemes.md) document to describe the phonemes in a phoneme
+definition file.
+
+Those phoneme features often represent the action of more than one articulatory
+mechanism used to produce speech, or affect the same area. Internally, espeak-ng
+makes use of the articulatory model, not the IPA descriptions. This document
+describes how the feature-based IPA model is mapped to the articulatory model.
+
+People working on adding new voices or languages do not need to read this
+document, but should instead read the [Phonemes](phonemes.md) document. This is
+intended for people working on the espeak-ng codebase, or people interested in
+how espeak-ng works internally.
+
+__NOTE:__ This model is in the process of being implemented. As such, the
+current implementation does not reflect this document.
+
+## References
+
+1. <a name="ref1"></a> Kirshenbaum, Evan,
+   [Representing IPA phonetics in ASCII](http://www.kirshenbaum.net/IPA/faq.html) (HTML). 1993.
+
+2. <a name="ref2"></a> Kirshenbaum, Evan,
+   [Representing IPA phonetics in ASCII](http://www.kirshenbaum.net/IPA/ascii-ipa.pdf) (PDF). 2001.
+
+3. <a name="ref3"></a> International Phonetic Association,
+   [The International Phonetic Alphabet and the IPA Chart](https://www.internationalphoneticassociation.org/content/ipa-chart). 2015.
+   Creative Commons Attribution-Sharealike 3.0 Unported License (CC-BY-SA).
+
+4. <a name="ref4"></a> Wikipedia.
+   [International Phonetic Alphabet](https://en.wikipedia.org/wiki/International_Phonetic_Alphabet). 2017.
+   Creative Commons Attribution-Sharealike 3.0 Unported License (CC-BY-SA).
+
+5. <a name="ref7"></a> Wikipedia.
+   [Extensions to the International Phonetic Alphabet](https://en.wikipedia.org/wiki/Extensions_to_the_International_Phonetic_Alphabet). 2017,
+   Creative Commons Attribution-Sharealike 3.0 Unported License (CC-BY-SA).

docs/phonemes.md

@@ -41,11 +41,6 @@ As well as using ASCII characters for specific IPA phonemes, this transcription
 provides a set of 3-letter feature abbreviations allowing a phoneme to be
 described as a sequence of features.
 
-Kirshenbaum uses features that align with the way the IPA phoneme charts are
-organised. This document uses the Kirshenbaum features as a base to work from,
-but extends them to describe the underlying phonetic processes in a way that
-is consistent between languages and voices.
-
 This document is grouped into three sections. The first section displays the IPA
 charts using the feature names instead of their names, showing the IPA phoneme
 at that position in the chart. This makes it easier to look up the features for