Measuring the effects of adaptive multirate (AMR) codecs on formant tracker performance.
Ewald Enzinger · The Journal of the Acoustical Society of America · 2010
Several approaches to forensic speaker comparison rely on formant center-frequency measurements as features due to their rather straightforward interpretation as resonance frequencies of the cavities of the human vocal tract. Formant tracking algorithms, mostly based on linear predictive coding, are commonly used for automatic extraction. Telephone conversations constitute a substantial amount of forensic material, which increasingly involves wireless communication channels instead of landline transmission. The effects and limitations introduced by the adaptive multirate (AMR) set of codecs that is used for speech transmission in GSM and UMTS networks are therefore of special interest in forensic settings. To evaluate the extent of the effects that are caused solely by the codecs, speech recordings were en- and de-coded with the different bitrate levels provided by the AMR narrowband codec. The formant frequencies of vowel segments were extracted using different trackers and settings. The preliminary results suggest partial shifts in frequency depending on codec level and individual speakers, but no consistent trend emerges.