Intelligibility Improvement of Analog Communication Systems Using an Amplitude Control Technique

Mobile satellite relay voice communications systems must provide high quality and reliable voice communications using relatively simple and inexpensive mobile equipment and easily achievable effective isotropic radiated power. High noise level reception will occur with analog voice communication systems operating over a power-limited link. At high noise levels, the low-level speech phonemes become masked by noise, which results in reduced speech intelligibility. In addition, a listener subjected over an extended period of time to this high noise level can become discomforted to the extent that his ability to understand what is being communicated is significantly reduced. An amplitude control technique has been employed for use with analog voice communication systems, which improves low-level phoneme reception and eliminates the received noise between words and syllables. Tests were conducted at the NASA Goddard Space Flight Center on a narrow-band frequency-modulation simplex voice communication channel employing the amplitude control technique. Presented for both the modified rhyme word tests and the phonetically balanced word tests are a series of graphical plots of the tests' score distribution, mean, and standard deviation as a function of received carrier-to-noise power density ratio. At low received carrier-to-noise power density ratios, a significant improvement in the intelligibility was obtained. A voice intelligibility improvement of more than 2 dB was obtained for the modified rhyme test words, and a voice intelligibility improvement in excess of 4 dB was obtained for the phonetically balanced word tests.