Effects of telephone transmission on the performance of formant-trajectory-based forensic voice comparison-Female voices

In forensic-voice-comparison casework a common scenario is that the suspect's voice is recorded directly using a microphone in an interview room but the offender's voice is recorded via a telephone system. Acoustic-phonetic approaches to forensic voice comparison often include analysis of vowel formants, and the second formant is often assumed to be relatively robust to telephone-transmission effects. This study assesses the effects of telephone transmission on the performance of formant-trajectory-based forensic-voice- comparison systems. The effectiveness of both human-supervised and fully-automatic formant tracking is investigated. Human-supervised formant tracking is generally considered to be more accurate and reliable but requires a substantial investment of human labor. Measurements were made of the formant trajectories of /iau/ tokens in a database of recordings of 60 female speakers of Chinese using one human-supervised and five fully-automatic formant trackers. Measurements were made under high-quality, landline-to-landline, mobile-to-mobile, and mobile-to-landline conditions. High-quality recordings were treated as suspect samples and telephone-transmitted recordings as offender samples. Discrete cosine transforms (DCT) were fitted to the formant trajectories and likelihood ratios were calculated on the basis of the DCT coefficients. For each telephone-transmission condition the formant-trajectory system was fused with a baseline mel-frequency cepstral-coefficient (MFCC) system, and performance was assessed relative to the baseline system. The systems based on human-supervised formant measurement always outperformed the systems based on fully-automatic formant measurement; however, in conditions involving mobile telephones neither the former nor the latter type of system provided meaningful improvement over the baseline system, and even in the other conditions the high cost in skilled labor for human-supervised formant-trajectory measurement is probably not warranted given the relatively good performance that can be obtained using other less-costly procedures.