Human Factors Evaluation of Vocoders for Air Traffic Control ... - FAA

Relative Frequency
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Vocoder A
Vocoder B
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Taxonomy Code
Figure 19. Relative frequencies across categories for audio preference test selection rationale.
3.4 Exit Survey
Table 32 shows the results of the exit survey. The mean and standard deviation are with respect
to a 10-point scale with the anchors as shown. As illustrated, the participants were generally not
knowledgeable in vocoder technology prior to the study, which is a favorable indication of an
unbiased group. The enthusiasm level was somewhat high for the group as was the participants’
value of the time spent performing the evaluation. These aspects add validity to the data
obtained.
4. Discussion
4.1 Analysis of Subjective Ratings
Analysis of the subjective ratings for both intelligibility and acceptability revealed three-way
interactions of the independent variables of equipment, background noise, and sex of speaker.
Interactions are basically statistical results that mean that the impacts of the various independent
variables do not add up neatly. This made the analysis more challenging but within capability.
It is unlikely that one explanation could account for all the interactions. However, one possible
source for the interactions could lie in the voice and background noise behaving as a system
when processed through the speech compression algorithm of the vocoders rather than two
independent sound sources. This nonlinear type of behavior could explain the interactions seen
regarding the participant ratings of both intelligibility and acceptability of the vocoder processed
messages. These interaction effects are less prominent when the equipment variable is normal
analog radio, which was expected to behave linearly with respect to the input signal.
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