An alarming conversation
Back in the days before cell phones, there were mobile telephones. We had installed a mobile telephone system several years earlier, and it had been operating without problems until it started experiencing station-failure alarms.
The alarm was a tone that the station would send to the system controller whenever it failed to key properly. When the controller received the alarm tone, the station would be disabled until a service tech could make a trip out to the station site to fix the problem and then back to the controller site to reset the system.
The system started experiencing failures on a regular basis, but the tech visiting the site could find no problems; he would simply reset the system, and everything would seem fine. This scenario played out repeatedly, so we were called in to investigate. Interviewing the tech revealed that the problem occurred only late in the day on weekdays.
We found nothing wrong with the equipment, but the alarms persisted. So we needed to know what was going on just prior to the alarm. We used a cassette recorder, a tape loop cassette, and an alarm-triggered switch that would stop the recorder when the alarm sounded.
After two days, we had our answer. A particular user would call his wife on the way home from work. The alarm would occur during their conversation. Listening to the conversation, we discovered that the couple had a new baby, and his wife would hold the baby while talking on the phone with her husband. The baby would start crying, “Waa, Waaa, Waaaa,” until—bang—the alarm detector in the controller responded.
The solution was simple: We added more integration to the alarm detector, with slow attack time and fast decay time. Testing confirmed that the change eliminated the failures. The update was added to the controller, and a field bulletin went out with instructions for making the change. The instructions were titled "Controller CB Filter," which anyone would take to mean Citizens Band Filter.
But we all knew it really was the Crying Baby Filter.