The one thing that I want to know is how they transmit their data. For example, in my cars with XM, when I drive under an overpass, I don't hear a pause in the music. But if I sit long enough under the overpass, the music cuts off. So it appears that XM is transmitting the data at a higher bit-rate than the music requires. Then there must be a carousel system so that if the radio loses the signal it can get the data about to be played from the buffer.