Beware of the ‘telephone effect’: the influence of telephone transmission on the measurement of formant frequencies

Abstract

Speech scientists often have to work with speech signals that have been transmitted over the telephone. Although the acoustic properties of telephone transmission such as the band-pass filter characteristics are well known, little attention has been paid to their effect on the measurement of speech parameters. This study deals with artefacts introduced by the lower cut-off slope of the transmission channel on vowel formants. For theoretical reasons, frequency components may be assumed to be attenuated the lower they are. Therefore F1 of most vowels can be expected to be affected most. Attenuation of the lower components of a formant will necessarily increase the relative weight of the higher components for the determination of a formant and thus cause an artificial upward shift of its centre frequency. An empirical investigation with directly and telephone-transmitted samples from ten male and ten female subjects shows that the predicted effect on F1 does in fact occur for all tested vowels except /a/, whose F1 is too high to be affected by the slope of the band-pass. The consequences of measurement errors arising from such artefacts are discussed with special reference to speaker identification and empirical dialectology.

The beauty in a beast: Minimising the effects of diverse recording quality on vowel formant measurements in sociophonetic real-time studies

“…However, even F2 whose frequencies fall within the transmitted range shows some technically introduced artefacts. In comparison to the values obtained from a recording made simultaneously with a studio microphone, F2 of high vowels tends to measure lower values in mobile recordings (Byrne and Foulkes 2004), though the effect tends to be smaller and has not been consistently documented in other phone transmissions (Künzel 2001). The exact magnitudes of these technically introduced effects also seem to vary substantially across different studies and types of phone transmissions, and range between 14 and as high as 60 percent of the original frequency (Byrne and Foulkes 2004;Künzel 2001).…”

“…A series of studies have been conducted in the context of forensic speaker identification (e.g. Byrne and Foulkes 2004;Künzel 2001); and only a few, mostly preliminary investigations have recently pointed out that technical issues of a recording may obscure the patterns of variation and change in sociophonetics, too (De Decker and Nycz 2011;De Decker 2016;Hansen and Pharao 2006;in progress).…”

“…First of all, the band-pass filtering due to the transmission by phone lines (both mobile and landline) is known to interfere with the calculation of the formants (Byrne and Foulkes 2004;Künzel 2001). Harmonics that lie below the lower cut-off boundary (approximately 300 Hz) and above the upper boundary (approximately 3.2 kHz in mobile phones and 3.5 kHz in landline transmissions) are most affected, since their weighting in the calculation of the formant frequencies is decreased.…”

“…De Decker 2016;De Decker and Nycz 2011;Rozborski 2007). Most of them compared recordings made simultaneously with variable equipment, and estimated the resulting differences in formant frequencies those recordings measured (Byrne and Foulkes 2004;De Decker and Nycz 2011;Hansen and Pharao 2006, in progress;Künzel 2001;Plichta 2004;van Son 2005). While all of the factors identified above may potentially play out in sociophonetic real-time corpora and contribute to the corpus diversity in terms of technical quality, the ultimate goal of sociolinguistic research is to unveil gradual, potentially small-scale sound variations and changes across decades.…”

“…In contrast, only F2 of /i/ (but not F2 of /a/ or /u/) was affected, and lowered by 0.4 Bark in noisier recordings. Interestingly, related forensic investigations into the impact of phone transmissions brought to light quite comparable patterns of F1-raising in all vowels across the board and a differential F2-lowering in dependence upon vowel quality (Byrne and Foulkes 2004;Künzel 2001), although in contrast to our results, the magnitude of the raising effect was usually found larger in low-F1 vowels like /i/ and /u/ than in the in high-F1 vowel/a/ (whose frequency mostly lies within the transmitted range).…”