A prior art method for coding phonetic features was formally set up during the U.S. census in 1858. A certain abstraction of phonetic features was considered to be required, in particular because at that point of time, a fairly high fraction of immigrants in first or second generation lived there who named their children with some assimilated names, i.e., first names or last names. For example, the name Schmidt was assimilated to Smith, or Johannson to Johnson. As it was foreseeable that the original way for writing these names would be lost anyhow, the evaluation of the census was stored supplementary with those phonetical features without using any electronic data processing.
This method is known by the name SOUNDEX and is applied until now in many technical fields, for example, in reservation systems in which lists of names are searched according to phonetical features only since customer contact is often by telephone call and the typist may not know the proper spelling of the customer's name. The basic algorithm is the same as it was in 1858 and comprises the rules and steps as follows:
1. Every code according to SOUNDEX contains four characters: a letter followed by three digits whereby in case of missing digits the code is filled up with the digit ‘0’.
2. The first letter of the character string which is transformed by the SOUNDEX code, for example a name, is taken without any change as the first code element into the SOUNDEX code.
3. The letters a, e, i, o, u, y, w, h are not coded. All remaining characters are coded as follows:                b, f, p, v are coded to 1        c, g, j, k, q, s, q, z are coded as 2        d, t are coded as 3        l is coded as 4        m, n is coded as 5        r is coded as 6.        
4. Two subsequent occurrences, i.e., instances, of the same SOUNDEX code is avoided by only taking the first of them.
Based on the above mentioned rules the following examples result:
EulerE460GaussG200HilbertH416HeilbronnH416SchmidtS530SmitS530
A more detailed description of the prior art SOUNDEX coding method is provided by D. Knuth in ‘The Art of Computer Science, Vol. III’, Addison-Wesley, 2nd ed., S. 391–392.
The disadvantages of said prior art SOUNDEX coding method can be summarized as follows:
1. The SOUNDEX code is limited to 1+3=4 characters. As a typically character string has a bigger length, e.g., a length between 6 and 8 characters not all characters can be coded. Thus, as described above the character strings ‘Hilbert’ and ‘Heilbronn’ have the same SOUNDEX code although they sound quiet different, they have a different length and they can thus be described as ‘not related with each other’. In this respect the SOUNDEX method is too ambiguous.
2. The rules of the SOUNDEX method are not able to transfer longer character groups into their phonetical equivalence. The SOUNDEX method is limited to avoiding the vowels and to the comprehension of consonants having the same phonetic code. The SOUNDEX code is not adapted to particular letter combinations as are for example the English ‘ight’ as in ‘fight’ which has the same phonetic features as ‘ite’.
3. Today, phonetic features are often stored together with the original data in a data base. Accordingly, the SOUNDEX code properties have to be checked for the persistency and comparability. Such a check yields that the SOUNDEX code has to be stored as a character string as it comprises at least one character and a character compare procedure has to be started against the code whenever the SOUNDEX code is applied in a database search. Compared to a bit string compare procedure this represents a significant loss of performance.