The effect of narrow-band digital processing and bit error rate on the intelligibility of ICAO spelling alphabet words

Abstract

The diagnostic rhyme test (DRT) is widely used to evaluate digital voice systems. Would-be users often have no reference frame for interpreting DRT scores in terms of performance measures that they can understand, e.g., how many operational words are correctly understood. This research was aimed at providing a better understanding of the effects of very poor quality speech on human communication performance. It is especially important to determine how successful communications are likely to be when the speech quality is severely degraded. This paper compares the recognition of ICAO spelling alphabet words (ALFA, BRAVO, CHARLIE, etc.) to DRT scores for the same conditions. Confusions among the spelling alphabet words are also given. The voice conditions included unprocessed speech, speech processed through the DoD standard linear predictive coding algorithm operating at 2400 bits/s with random bit error rates of 0, 2, 5, 8, and 12 percent, and an 800 bit/s pattern matching algorithm. The results suggest that with distinctive vocabularies like the ICAO spelling alphabet, word intelligibility can be expected to remain very high even when DRT scores fall into the poor range; but once the DRT scores fall below about 75, the intelligibility can be expected to fall off rapidly; and at scores below 50, less than half the words will also be understood.