Intelligibility of ICAO spelling alphabet words as a function of LPC narrow-band processing and bit error rate
Astrid Schmidt-Nielsen · The Journal of the Acoustical Society of America · 1986
The intelligibility of ICAO spelling alphabet words was tested to determine the kinds of confusions that occur under severely degraded digital voice conditions. The test conditions included a 2400 bit/s linear predictive coding (LPC) voice processing algorithm with random bit error rates of 0%, 2%, 5%, 8%, and 12%. The ICAO alphabet (ALFA, BRAVO, CHARLIE,…) was developed for communication under degraded conditions, and the words have been selected with the object of minimizing confusions. Scores on a standard intelligibility test, the Diagnostic Rhyme Test (DRT), were obtained for comparison and to determine a prediction function from DRT scores to spelling alphabet performance. The types of confusions that were obtained with the narrow-band digital speech with bit errors showed a number of substantial differences from the types of errors normally found with spelling alphabet words tested in noise. The most frequent error was that PAPA was heard as ALPHA (but ALPHA was virtually never heard as PAPA). As is to be expected, scores on the spelling alphabet words fell monotonically with falling DRT scores but were considerably higher than the DRT scores for the same conditions up to 5% errors; after that, the spelling alphabet scores dropped off rapidly to levels as low as the DRT scores.