According to Johansson (1995a),
The degree of phonetic detail given in speech transcription varies from none to a very precise phonetic or phonemic transcription [...] Where there is a great deal of phonetic or phonemic detail, it will be more convenient to design a specialized writing system.
The specialized writing system recommended by the Text Encoding Initiative is the International Phonetic Alphabet. For the electronic exchange of texts, a machine-readable alphabet has to be used, and in this respect SAMPA is considered to be a computer version of part of the IPA system (EAGLES Spoken Language Working Group, 1995). The X-SAMPA extension proposed by Wells provide the parts which were missing from SAMPA, and should then be considered as a system fitted for the purposes of segmental transcription of spoken corpora.
However, possible problems of compatibility between SAMPA and Unicode (more information on Unicode can be found at URL http://www.stonehand.com/unicode/standard.html have been detected in the discussions that took place during the EAGLES Workshop on `Issues in Corpus Work' (Madrid, January 1996), and this issue should be carefully explored.