Audio Frequency Track Circuits
While pulse-coded track circuits have on/off cycles measured in pulses per minute, as their name implies, audio-frequency track circuits have signal frequencies in the range between 2 kHz and 20 kHz. Because of the relatively high frequency, the signal quickly attenuates and, while pulse codes can travel for several miles, audio frequency codes can only travel between a few hundred and a few thousand feet. However, this has an advantage in that by carefully matching the carrier frequency to the block length, the need for insulated rail joints can be largely eliminated. In rapid transit and light rail systems, where high traffic density mandates short signal blocks, the lack of a need for insulated rail joints (and impedance bonds) can result in significant cost savings.
Like standard track circuits, the audio frequency track circuits provide positive train detection and they provide the transmission of signal aspects which can change between block boundaries, similar to pulse code cab signals. Also like the pulse code system, the vehicle-borne equipment reads the code embedded in the audio frequency carrier and then passes this on to the train control system to alert the operator and/or reduce train speed as necessary.
Read more about this topic: Cab Signalling
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