Subjective ratings of instantaneous and gradual transitions from narrowband to wideband active speech

Abstract

In advanced heterogeneous telecommunication networks, network resources can dynamically dictate the type of speech coding that is used. An increase in resources allows for lower coding distortion or it might also be used to provide wideband speech instead of narrowband speech. Existing studies have demonstrated that wideband speech is preferred to narrowband speech, but they have also demonstrated that an abrupt transition from narrowband to wideband is perceived as an impairment, even though it is a transition to a higher quality signal. We describe our recent work that resulted in subjective scores for abrupt and gradual transitions from narrowband to wideband at the midpoint of a six-second segment of active speech. On average, signals that start narrowband and end wideband are rated slightly lower than constant narrowband signals and results are nearly the same for abrupt and gradual (2.5 second) transitions. Scores from 20 listeners show a wide range of individual opinions so we conclude that studies of bandwidth transitions may be quite sensitive to the listener population sample.Index Terms-Narrowband speech, speech coding, subjective testing, wideband speech BACKGROUND AND MOTIVATIONTelecommunication networks are simultaneously becoming less homogeneous and more adaptive. This makes it more likely that the network resources available to support a given call can change during the call, especially if one or more call participants are mobile. If network resources increase and additional data capacity is available then it is possible to switch to higher bit rate speech coding. One might keep the encoded speech bandwidth fixed and use the extra bits to reduce coding distortion. Another possibility is to switch from narrowband (NB) to wideband (WB) speech coding.Unless otherwise indicated, we use what we consider to be the canonical definitions of speech passbands, based on the original NB and WB digital speech coders. Thus NB indicates a speech passband of 300 to 3400 Hz consistent with the minimum −3 dB bandwidth for G.711 PCM specified in [1] and WB refers to a speech passband of 50 to 7000 Hz as given in [2].Several studies, including [3], have shown that WB is preferred to NB in controlled subjective experiments. Note that a transition from NB to WB entails both a low frequency extension (LFE) and a high frequency extension (HFE). The study in [3] shows that the HFE alone does not enhance perceived speech quality but the LFE alone does. If the LFE is in place then the HFE further enhances speech quality.Consistent with this finding, more recent NB/WB speech coders include most or all of the LFE in the NB mode, and switching to WB entails mainly HFE. The specifications for the G.729.1 speech coding algorithm indicate a nominal NB bandwidth of 50 to 4000 Hz and a nominal WB bandwidth of 50 to 7000 Hz [4]. Measurements of the AMR speech coders show NB −3 dB bandwidth from 85 Hz up to 2800 to 3600 Hz (the upper limit depends on AMR mode) [5], and WB bandwidth from 50 Hz up to 5700 to 6600 Hz (the upper limit d...

When should a speech coding quality increase be allowed within a talk-spurt?

“…For example, wideband (WB) speech (50 to 7000 Hz nominal passband) has a documented higher perceived quality than narrowband (NB) speech (300 to 3400 Hz nominal passband) [1]-[3], but a transition from NB to WB speech coding is perceived as an impairment [4]- [6]. If the transition happens early enough in a speech recording, the value of the WB portion can exceed the harm of the transition, for a net improvement (relative to NB only) in overall speech quality.…”

“…But if the transition happens later in a speech recording, the shorter duration of the WB portion means that its value does not overcome the harm of the transition. This was the case for NB-to-WB transitions at the 45 second point in a 60 second recording [4], [5] or at the three-second point of a six-second recording [6]. In [6] we also experimented with gradual transitions (up to 2.5 seconds long) but found they did not mitigate the harm of the transition.…”