Abstract
IN long–distance telephony when messages or signals have to be transmitted simultaneously over wire networks, an error in dialling may result in reaching a group of numbers not assigned for service. In such cases a special tone may be used to inform the customer of his error, and this special development in switching is widely used in the Bell System networks of the United States. In the Bell Laboratories Record of April, Mr. M. E. Krom communicates a paper giving the development of the ‘no such number’ tone. The new tone varies continuously in frequency, like that of a siren, alternately rising and falling at half–second intervals. The sound is quite different from any other tone used in the Bell system. A.t the lowest pitch the fundamental frequency is 200 cycles per second, and at the highest pitch, 400 cycles. Harmonics up to 6,000 cycles are in both tones, and these give the latter a richness not found in single–frequency waves. The tone is generated by a ‘relaxation’ oscillator consisting of a vacuum tube, condenser and resistance. The tone is amplified by another vacuum tube which raises the level above that of the dial and busy tones. To lengthen the life of the vacuum tubes, the plate circuits are closed only when the tone is required; the filaments are continuously heated, however, to maintain the circuit in readiness for instant service. During field trials it has reduced circuit–holding time on numbers wrongly dialled and resulted in a higher percentage of correct numbers on the second dialling.
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New Tone for Dial Telephone Systems. Nature 148, 193 (1941). https://doi.org/10.1038/148193b0
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DOI: https://doi.org/10.1038/148193b0