larry@uunet.uu.net (Larry Lippman) (04/12/90)
In article <6195@accuvax.nwu.edu> Leichter-Jerry@CS.YALE.EDU@venus.ycc. yale.edu writes: > It's claimed that the reason Ma Bell was so slow to replace the little > incandescent bulbs in multi-line phones with LED's was a security > problem. It seems that voices on the line modulate the power > available to the indicators. The reluctance of the old incandescents > was high enough that no useful information could be gotten from > them, but it was alleged that the LED's provided a nice clear signal > which could be read, say, with a decent telescope and a little > equipment, from the building across the street. Just when I thought I'd heard them all... :-) In a 1A, 1A1 or 1A2 key telephone system, which is the only apparatus having incandescent lamps that fits your description, there is absolutely *no* connection between any voice path and the 10 VAC circuit which operates the lamps. Replacing the 51-type lamp with an LED isn't going to make any difference. The only device which *could* modulate lamp power is the 3-type speakerphone, where the lamp is powered from the same unregulated, rectified DC which feeds the amplifier circuitry. However, only gross variations on level with excessive receive gain to the point of distortion (not likely) could cause enough supply voltage fluctuation to be visible on the lamp. Even under these extremely rare circumstances, I doubt that any intelligence could be demodulated using a photometric detector aimed at the ON lamp - even if the ON lamp were an LED. By the time LED's became common, the 4-type speakerphone had already replaced its 3-type predecessor. Incidently, modulating lamps in a manner undetectable to the human eye has been used as a transmission medium for eavesdropping devices. However, there is no factual basis or connection to the particular urban legend which you relate. <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {boulder|decvax|rutgers|watmath}!acsu.buffalo.edu!kitty!larry <> TEL 716/688-1231 || 716/773-1700 {utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 || 716/773-2488 "Have you hugged your cat today?"