system had a maximum coverage of 50 km, with this system they can reach 140 km in a straight line. With this system they can do an entire phase, like the Tour de France, of course, they have to carefully plan the helicopter refuelling. Each relay helicopter can repeat up to four signals and each of them have two carriers which can be configured in many ways. Two signals can be sent per each carrier or four per each to obtain a total redundancy. This provides the system with a lot of flexibility and it’s very important, because these events could last longer than what was anticipated. In a cycling race, for example, is normally to have a breakaway group and the bunch several kilometres behind. In this case, one motorcycles and one helicopter remain with one group and the rest go with the other group. The helicopter with the Wescam usually has some freedom and can move according to the producer directives. To be able to choose at any time the
TMBi - 36
type of modulation and the number of signals repeated on each helicopter, makes possible the adaptation of the system to each moment.
IMAGE
QUALITY
Another important point of this system is the image quality it has. They have tried to minimise the coding and decoding process of the video signal and that is way all the system works with ASI signals. The video signal is coded in MPEG-4 in the motorcycle and it’s not decoded until it reaches the mobile unit on the finish line and then used in the production. Except at this moment, the rest of the system works with Trasport Stream. To be clear, the signal flux was checked in several tests on which they measured with an equipment the quality of the compressed signals. They did several tests comparing the signals bit by bit and pixel by pixel. This generated a measurement similar to PSNR, which offers impartial information. Moreover, some test were done to
evaluate the subjective quality. With all this information they created several curves to see the signal loss after the different process of coding and decoding. From the third decoding, the quality loss was significant. The results indicated that it was best to code just one time and this is the reason why the image is coded on the motorcycle and a second time to be sent to Torrepaña. This also explains the use of ASI. It’s has also been important the election of the bitrate with which the signals are coded in the video motorcycles. “We did several tests with 9, 10, 11, 12 megabit, etc. The reason why, is because in not the same one case than another. We observed the advantages and the values we obtained at the end. If all motorcycles coded at 30 Mbits the data flux at the end was of 120 MBits, which is very big. We have to reach a medium point and that is why we did different tests with several diagrams, until we found what we liked. We measured the PSNR, bet-