Definitions:
Phoneme – Distinct sounds in a language
Grapheme – Distinct unit written in a language
Lexicon – Dictionary that maps phoneme to graph(i.e. Arpabet)
Architecture design choices of the above for ASR :
1. Target audio straight to Grapheme(without lexicon) – so basically extract features to Graphemes(words) – used in small datasets.
2. Go through intermediate step from audio to phoneme to grapheme (less comparisons).