Jonathan gets the prize, tho it wasn't due to areas where the signal was weak or as the pagers aged. It was just that there were tone sets which when done in certain orders of RFD stations caused other stations to tone out.
The updated plan was put into effect on the transition from Federal Informer boxes (which were non-narrowbandable) used for trip-lights to CDM1250s, (which are narrowbandable). This was also a good time to make the trip-light circuitry in each station as uniform as possible, from the radio out to whatever is used as a relay to turn on the trip-lights. On one occasion while replacing an Informer at one station, the lights tripped when 2 other stations were toned in sequence. "Lucy!! You got some 'splaining to do!!!"
For example, when Stn 16 & 17 were toned out in that order, Stn 24's lites would come on. Stn 16's old tones were 688.3 & 767.4, while #17 had 688.3 and 810.2Hz. Stn 24's tones were 767.4 Hz, and 688.3 Hz. So a tone out would be like this :
(688.3 for 1 second)(767.4, 3 seconds)(688.3, 1 second)(810.2, 3 seconds)
So at Stn 24, the radio just waited, and when it heard the 2nd tone for Stn 16 (767.4 Hz), it heard it for AT LEAST 1 second, it was effectively 'cued up' for the 1st tone for Stn 17; (688.3 Hz). As soon as that tone came across, Stn 24's radio said Oh, my, here's a call I'm gonna trip my lights and turn on da speakers! Yippee! Stn 24's crew on duty would go "Hunnhh?" sometimes call 911 and ask 'Did you call us?' Nope.
It happened rarely enough that it seemed simply random, so everybody ignored it. Until 2 CEMS techs were at Stn 15 one day, and 2 other stations toned out but 15's lites came on. (as well as the lights in our heads...).
Once the existing tone plan was studied with an eye toward falsing, several other combinations were found.
But, I see a hand in the audience: "Isn't the tone timing supposed to be 1 second of the 1st tone, and 3 seconds of the 2nd tone?" Yes, you're right, that is the industry standard. On the other hand, in order to (ahemclearsthroat) accept other product's or manufacturer's variations in tone timing, the CDM1250's and Minitor V pagers will alert on shorter sequences, like 1 second/1 second, or some such. One common thing I saw on the bench during the Mintor 2 years was the Long-B tone group call, which is 'standard' at 8 seconds, pretty much always alerted after only 5 seconds, and continued alerting as long as the tone was being transmitted; that is, usually 3 more seconds.
I suspect, tho have not had the time to research it more fully, that even within a product line there are differences in timing windows between firmware levels within the radios. And that many firmwares don't look for a tone length of X seconds, plus or minus, but instead just want 'At Least' X seconds, don't care if it's even 3 seconds or more...
Class dismissed for now.
And stay tuned!