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Digital radios: A possible death trap
Author: RBK
Date: 12-15-2011 - 16:16
It is common for railroad users in congested areas to walk on one another by transmitting at the same time. Examples include a detector sounding off or switching moves taking place while the dispatcher is trying to talk. Distortion caused by the collision in analog mode tells other users listening that somebody has doubled and to ask them to repeat. This collision won't be heard in digital mode! In fact you won't hear anything at all because the interspersion of multiple data streams is discarded by a digital receiver. Consider the following scenario which happened to a conductor at my terminal. These transmissions occurred over the top of each other.
Detector radio: UP Detector, milepost 555.5, track 1, no defects, total axles 412, temperature 78 degrees, length 5678, detector out.
Conductor radio: Help help help! I need an ambulance. Somebody help me!
The train dispatcher never heard the conductor's plea for help. The FM capture effect came into play here because the conductor's radio was drowned out by the detector. This wasn't true for the engineer on the head end. He made out the words "help, help, help" despite the severe distortion caused by the mixing of signals. He then radioed the dispatcher to send paramedics and ran back to his conductor over a mile away.
They were lucky the FM capture effect wasn't as pronounced at the head end. They were also lucky the engineer understood the garbled plea for help. Had NXDN digital radios been in service that day, it is very unlikely the engineer would have heard the conductor at all. The receiver on the locomotive would have discarded the digitized audio stream coming from the conductor's radio and probably the detector once the two collided with each other. Remember, digital radios don't have the processing power of the human brain. They aren't able to mentally tune out distractions or choose the most urgent transmission relevant to the user.
RF engineers and the radio manufacturers are painfully clueless about the nature of railroad communications. This is vastly different than a city or county police/fire agency using digital radios in a confined area. Police dispatchers doesn't have to compete with a track foreman issuing Form B instructions, a local performing switching on the mainline, detectors transmitting at unpredictable times, or yardmasters issuing yarding instructions.
I believe it will be a matter of time before these digital radios kill somebody.
Detector radio: UP Detector, milepost 555.5, track 1, no defects, total axles 412, temperature 78 degrees, length 5678, detector out.
Conductor radio: Help help help! I need an ambulance. Somebody help me!
The train dispatcher never heard the conductor's plea for help. The FM capture effect came into play here because the conductor's radio was drowned out by the detector. This wasn't true for the engineer on the head end. He made out the words "help, help, help" despite the severe distortion caused by the mixing of signals. He then radioed the dispatcher to send paramedics and ran back to his conductor over a mile away.
They were lucky the FM capture effect wasn't as pronounced at the head end. They were also lucky the engineer understood the garbled plea for help. Had NXDN digital radios been in service that day, it is very unlikely the engineer would have heard the conductor at all. The receiver on the locomotive would have discarded the digitized audio stream coming from the conductor's radio and probably the detector once the two collided with each other. Remember, digital radios don't have the processing power of the human brain. They aren't able to mentally tune out distractions or choose the most urgent transmission relevant to the user.
RF engineers and the radio manufacturers are painfully clueless about the nature of railroad communications. This is vastly different than a city or county police/fire agency using digital radios in a confined area. Police dispatchers doesn't have to compete with a track foreman issuing Form B instructions, a local performing switching on the mainline, detectors transmitting at unpredictable times, or yardmasters issuing yarding instructions.
I believe it will be a matter of time before these digital radios kill somebody.
