how XM radio works

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How does XM Satellite Radio work?

XM provides digital programming directly from three high-powered satellites in geostationary orbit above the equator. XM-1 ("Roll") and XM-2 ("Rock") are co-located at 115 west longitude and XM-3 ("Rhythm") is located 85 west longitude in addition to a network of ground-based repeaters. The combination of the three satellites and a series of ground-based repeaters ensure uninterrupted coverage throughout all of continental North America.

A song at XM Radio starts its broadcast life by being recorded into a specific format on a storage medium. It is encoded at a high bit rate (approximately 384kb/s) so as to preserve quality, and cataloged and organized in a similar way that we use ID3 tags for all digital files. The DJ at each XM station then selects the track or album of tracks to play for the next 15 to 30 minutes, previews the music, and allows the control system to encode the original source music using the aacPlus Codec.

Each XM channel is handled by an encoder. Each encoder transmits the necessary digital files for each particular station to a multiplexer, where they are combined with the files from all additional XM channels. Once all of XM's channels are sent to the multiplexer, the massive digital stream is modulated onto an RF carrier, and transmitted on XM's section of the S-Band frequency (2332.5 to 2345.0 MHz) to one of XM's three satellites.

From one of the three satellites, the encoded signals are sent back to earth, and the antenna connected to your XM receiver picks up the frequency. Since your receiver will be the only thing decoding this signal, you can rest assured that compared to the many digital transmissions that re-broadcast, re-store and re-encode the signal, the XM signal will never be sliced again.

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