## 2 Problem Formulation  
The process of converting graphemes to phonemes in orthographic text involves converting a string of orthographic characters into a corresponding string of phonetic transcription characters (representing phonemes or allophones) [2]. A 'grapheme' is any of the units of any writing system for any language, a term coined by analogy with the 'phoneme' of a spoken language [26]. Graphemes include alphabetic letters, typographic ligatures, numerical digits, punctuation marks, and other individual symbols of writing systems. Since the orthographic text is the only source of pronunciation information in the process of converting graphemes into phonemes, this process must be based on appropriate formal rules, depicting the correct pronunciation of orthographic strings in a given language [27].  
Phonemes are usually written in specially designed alphabets. The most widely used alphabet is the International Phonetic Alphabet (IPA) [28]. For the Polish language, as with other Slavic languages, a special transcription system, called the Slavistic Phonetic Alphabet (SPA), is most frequently used [29]. The other very commonly used phonetic alphabet is the Speech Assessment Methods Phonetic Alphabet (SAMPA) [30]. SAMPA is a machine-readable phonetic alphabet, using 7-bit printable ASCII characters, based on the IPA. Table 1 presents the phonemic inventory of Polish with examples, in the SPA, IPA, and SAMPA phonetic alphabets and corresponds to the set of phonemes used for the purpose of this study.  
Automatic grapheme-to-phoneme conversion is not a new problem. The first linguist who noted it, and tried to provide a solution for a particular language (Czech), was [PERSON] [31]. Research on solutions to the automatic grapheme-to-phoneme conversion problem have also been initiated for other languages [32; 33; 34].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multicolumn{3}{c}{**Phonetic Alphabet**} & **Example of** \\  & \multicolumn{3}{c}{**Symbols**} & **Occurrence** \\ \cline{2-4}  & **[SPA]** & **[IPA]** & **[SAMPA]** & **in Polish** \\ \hline
1 & [e] & [\(c\)] & [\(e\)] & \multicolumn{1}{c}{sec} \\
2 & [a] & [\(a\)] & [\(a\)] & baba \\
3 & [\(o\)] & [\(>\)] & [\(o\)] & oko \\
4 & [t] & [t] & [\(t\)] & trawa \\
5 & [n] & [n] & [\(n\)] & noc \\
6 & [y] & [i] & [\(1\)] & syty \\
7 & [i] & [\(i\)] & [\(j\)] & jjo \\
8 & [i] & [i] & [\(i\)] & wci \\
9 & [r] & [r] & [r] & rok \\
10 & [s] & [s] & [\(n\)] & sok \\
11 & [v] & [v] & [v] & wada \\
12 & [p] & [p] & [p] & paca \\
13 & [u] & [u] & [\(u\)] & buk \\
14 & [n] & [m] & [\(n\)] & manza \\
15 & [k] & [k] & [k] & kot \\
16 & [a] & [n] & [\(n\)’] & kofi \\
17 & [d] & [d] & [\(d\)] & dudek \\
18 & [l] & [l] & [\(1\)] & lato \\
19 & [y] & [l] & [v] & lysy \\
20 & [s] & [l] & [\(S\)] & szyszka \\
21 & [f] & [f] & [\(f\)] & fala \\
22 & [z] & [z] & [z] & koza \\
23 & [c] & [f] & [\(t\)s] & cacko \\
24 & [b] & [b] & [b] & baba \\
25 & [s] & [s] & [\(g\)] & godlo \\ \hline \hline \end{tabular}
\end{table}
Table 1: The set of Polish phonemes with examples, written in the SPA, IPA, and SAMPA phonetic alphabets, which corresponds to the set of phonemes used for the purpose of this study.  
In Poland, the first linguist who wrote about the possibility of phonetic interpretation of text by machines was [PERSON] in 1969 [35]. The largest contributions to solving the problem of automatic grapheme-to-phoneme conversion for Polish, were the publications of [PERSON] [36; 37]. The first implementation of a grapheme-to-phoneme conversion algorithm for Polish, designed for the machine ODRA 1204, was made in 1971 by [PERSON] [38]. Further attempts to implement automatic grapheme-to-phoneme conversion have also been reported in various publications from that time to the present [39; 40; 41; 42; 43; 44; 45].