rails of each block. There are four different carrier frequencies available in TVM-430 and they are used alternately in pairs on both tracks of the TGV line. On one track, blocks use alternately 1,700 Hz and 2,300 Hz, while on the other track blocks use alternately 2,000 Hz and 2,600 Hz. Upon these carrier frequencies can be modulated 27 separate audio frequencies, any combination of which can be present at one time; the earlier TVM-300 used eighteen separate frequencies, only one of which could be present at any time. Each block has a receiver at the opposite end to the transmitter, and the loss of the track circuit signal (owing to shorting by train wheels or due to a failure) is interpreted as an indication that the block is occupied. Signalling block boundaries are equipped with electrical separation joints that prevent adjacent blocks from interfering with each other, whilst still letting the traction return current (at 50 Hz) pass through. These tracks circuits are referred as a UM71 C track circuit. The signals which are present in the rail are detected by antennas mounted underneath the front airdam of TGV trains, approximately 1 metre (3 ft) forward of the front axle. These antennas work by inductively coupling to the AC signal shunted between the rails by the first axle. There are four redundant antennas per train, two at each end. Only the two at the "front" of the train (in the direction of travel) are used. The signal from the track circuit is filtered, conditioned, and decoded on board the train by two redundant digital signal processors. Radio interface Continuous signal For the continuous signal an analogue bandpass filter is applied to isolate the range of TVM frequencies between 1,600–2,640 Hz. Within this, each of the four 1,700/2,000/2,300/2,600 Hz carrier-frequencies are tested for the presence of a frequency-modulated 25.68 Hz signal. The 25.68 Hz signal is necessary for the track-circuit function and for the activation of relays within the train TVM430 equipment. The data bits themselves are frequency modulated at 0.64 Hz intervals between 0.88 Hz and 17.52 Hz. Intermittent signal At fixed locations cable loops are used between the tracks. A reference signal of 62.5 kHz is transmitted, along with two 125 kHz carrier frequencies, out-of-phase with each other. A 4,800-baud message is transmitted using phase jumps. Network code The network code is stored in bits 25–27 giving a