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
Famous quotes containing the words frequency, track and/or circuits:
“One is apt to be discouraged by the frequency with which Mr. Hardy has persuaded himself that a macabre subject is a poem in itself; that, if there be enough of death and the tomb in one’s theme, it needs no translation into art, the bold statement of it being sufficient.”
—Rebecca West (1892–1983)
“The war is dreadful. It is the business of the artist to follow it home to the heart of the individual fighters—not to talk in armies and nations and numbers—but to track it home.”
—D.H. (David Herbert)
“The Buddha, the Godhead, resides quite as comfortably in the circuits of a digital computer or the gears of a cycle transmission as he does at the top of a mountain or in the petals of a flower.”
—Robert M. Pirsig (b. 1928)