The Effect of Bandwidth on Speech Intelligibility

Of all the elements that affect the intelligibility of speech in telephony, bandwidth has been shown to be one of the most critical. This paper discusses the relation between bandwidth and intelligibility in speech, how improved bandwidth can compensate for other deficiencies such as noise and reverberation, and how the importance of wider audio bandwidth is becoming increasingly recognized in contemporary speech communication systems. Introduction Some progress has been made in alleviating telephony's deficiencies in the years since the first transcontinental phone call in 1915, as many sciences have enabled a better understanding of the causes and solutions of these problems. Acoustics, physics, chemistry, and electronics enabled major advances in the design of the telephone instrument, with new designs for mouthpiece and earpiece alone producing a 10dB frequency improvement by 1940. Similar improvements brought closer control to the gain of these elements (early experiments required the talker to tap the carbon microphone to loosen the granules inside). As the telephone evolved, antisidetone circuits were added so the talker could better judge his own loudness. The network added echo suppression, and later, digital echo cancellation, to reduce the far-end echo that became more troublesome as long-distance calls became more routine. However, in the last sixty years, little progress has been made in the amount of audio bandwidth that can be carried by the telephone network. Early telephone connections were not intentionally limited, but were constrained by the characteristics of the transducers and equipment then available. Intelligibility research was commonly conducted with frequencies extending from 4 kHz to 8 kHz and sometimes beyond, but the telephone network was expected to carry signals only to about 3 kHz into the 1930s, and to about 3.5 kHz with the first multiple-channel carrier systems. With standardization, and the codification of digital telephony in G.711, the upper frequency limit of the telephone network is now commonly accepted to be about 3.3 kHz at best. The last pre-divestiture Bell PSTN tests in 1984 showed significant rolloff at 3.2 kHz for short and medium connections, dropping to 2.7 kHz in long distance connections. At the low end of the spectrum, the telephone network carries frequencies no lower than 220Hz, and most commonly only as far down as 280 or 300 Hz. In contrast to this limited telephone performance, we find that the response of the human ear is better matched by FM radio and television spanning 30Hz to 15 kHz, CD audio covering 20 Hz to 20 kHz, professional and audiophile audio from 20 Hz to above 22 kHz, and even AM radio extending up to 5 kHz. To help close the large gap between the human ear capabilities and conventional telephone performance, Polycom has been incorporating new wideband technologies into its telephony products, beginning in 2002 with VTX technology to extend business 2 telephone audio to 7 kHz on standard analog phone lines, and now adding standardscompliant wideband audio to IP telephony products. Bandwidth and Intelligibility Crandall noted in 1917, "It is possible to identify most words in a given context without taking note of the vowels.the consonants are the determining factors in.articulation. " "Take him to the map" has a very different meaning from "take him to the mat," and a handyman may waste a lot of time fixing a "faucet" when the faulty component was actually the "soffet. " Pole, bole, coal, dole, foal, goal, told, hole, molt, mold, noel, bold, yo, roll, colt, sole, dolt, sold, toll, bolt, vole, gold, shoal, and troll all share the same vowel sound, only differing in the consonants with which it is coupled, but the difference can drastically change the meaning of a sentence. Consonant sounds have this critical role in most languages, including French, German, Italian, Polish, Russian, and Japanese. And, of course, consonants occur frequently in speech. "P" and "t," one of the most commonly confused pairs, account for over 10 percent of the phonemes in simple speech. "F" and "s" are 6.8 percent, "m" and "n" another 10.3 percent, and so on. Overall, more than half of all phonemes are consonants. This critical role of consonants in speech presents a serious challenge for the telephone network. The reason for this is that the energy in consonant sounds is carried predominantly in the higher frequencies, often beyond the telephone's bandwidth entirely. While most of the average energy in English speech is in the vowels, which lie below 3 kHz, the most critical elements of speech, the consonants, lie above. The difference between "f" and "s," for example, is found entirely in the frequencies above 3 kHz; indeed, above the 3.3 kHz telephone bandwidth entirely. Note (Figure 1) how the burst of high-frequency sound that distinguishes the "s" in "sailing" from the "f" in "failing" occurs between 4 kHz and 14 kHz. When these frequencies are removed, no cue remains as to what has been said.