TVM-430 system. In older versions of TVM, the target speed was updated only at every block boundary, resulting in a "staircase" style speed profile which is not representative of the continuous speed changes effected by the driver. However, with the additional information of block length and profile, TVM-430 is able to generate a continuously varying target speed through calculations performed in the onboard signalling computer, thus giving a much more realistic speed profile of continuous acceleration or deceleration for the driver to follow. In addition to the continuous speed control afforded by TVM-430, single instructions can be passed to the train by inductive loops located between the rails, which couple to a corresponding sensor under the train. Using the same frequency encoding principle, 28 bits of information can be recovered from a beacon, at speeds up to 400 kilometres per hour (250 mph). They come in two lengths depending on the line speed, 7 metres and 4.5 metres. These are called BSP () Intermittent Transmission Loops (ITL). They consist of two half loops, which together transmit the message via a 125 kHz frequency, phase shifted with a 62.5 kHz carrier frequency. The information passed along concerns a variety of events or actions required: Entry or exit from a high-speed line Arming or disarming the TVM-430 system Closing air conditioning vents before entering a tunnel Raising or lowering the pantographs Switching to a different electrical supply voltage Oversight A "black box" similar to an aircraft flight data recorder, passively watches over the entire process, monitoring a variety of parameters and recording the events. In TVM-430-equipped trainsets, older paper-strip recording equipment has been replaced by the ATESS digital recording system. Every action taken by the driver (throttle, brakes, pantographs) as well as signalling aspects (for TVM-430, KVB, and conventional signals) are recorded on magnetic tape for later analysis using a desktop computer. Another system, known as VACMA, oversees the driver's alertness. It consists of a foot pedal control that the driver needs to hold down for the TGV to move. This pedal must be released then re-pressed once every 60 seconds if no other activity takes place. There is one period of time before a buzzer sounds, and another period of time before the automatic emergency brakes come on. The control can be released for a very short period of time before a buzzer sounds, and another very short period of time