Spectral Characteristics of Digit-Stimulating Speech Sounds

A digit simulation in TOUCH-TONE calling (Ref. 1, pp. 9-12, 15-16) is, by practical definition, a speech segment capable of causing digit registration in a TOUCH-TONE signaling system. Spectral analyses have been performed on a number of speech segments, each of which was selected solely on the basis of having the above property. Briefly, a valid TOUCH-TONE signal requires the simultaneous presence of two code frequencies for a certain minimum length of time, and with some minimum signal-to-noise ratio. It was therefore theoretically anticipated ("Ref. 1, pp. 10-12) that each of these speech segments would be linked by two other common characteristics: (1) a frequency spectrum having two sharply dominant peaks, and (2) a high degree of periodicity for some minimal length of time. There is good reason to believe that speech segments of this general nature are likely to be troublesome in any signaling system based on the transmission of voiceband tones over speech channels. Due to the inherent rarity and relatively brief duration of the voiceproduced digit simulation, some special procedures were required both in obtaining and analyzing these speech segments. The remainder of this article comprises a description of these procedures, followed by a presentation and discussion of the resulting spectral analyses. 2839