Abstract Grapheme-based modeling has an advantage over phone-based modeling in automatic speech recognition for under-resourced languages when a good dictionary is not available. Recently we proposed a new method for parameter estimation of context-dependent hidden Markov model (HMM) called eigentriphone modeling. Eigentriphone modeling outperforms conventional tied-state HMM by eliminating the quantization errors among the tied states. The eigentriphone modeling framework is very flexible and can be applied to any group of modeling unit provided that they may be represented by vectors of the same dimension. In this paper, we would like to port the eigentriphone modeling method from a phone-based system to a grapheme-based system; the new method will be called eigentrigrapheme modeling. Experiments on four official South African under-resourced languages (Afrikaans, South African English, Sesotho, siSwati) show that the new eigentrigrapheme modeling method reduces the word error rates of conventional tied-state trigrapheme modeling by an average of 4.08% relative
