Impact of the GSM AMR Codec on Automatic Vowel Formant Measurement in Praat and VoiceSauce

Automatic formant measurement is generally reliable but can be affected by various factors, such as telephone transmission. As forensic speaker identification often involves comparison of direct (face-to-face) speech with a telephone recording, it is necessary to examine what effect telephony has on the speech signal. This study focuses on the impact of the AMR codec - this codec being the standard in mobile telephony - on formants. In comparison with previous studies, our study analyses the impact of both versions of the codec (narrowband and wideband) at all possible bit rates and on a large amount of data. Furthermore, the effect was examined in two processing tools - Praat and VoiceSauce. Our results revealed considerable shifts of formants when compressed by the codec and indicate that the extent of the shifts differs not only for individual formants but also for the two genders, vowel qualities and the software used.