Imagine music streaming from a satellite thousands of miles away, uninterrupted by static and commercials. That’s the promise of satellite radio, which became available to consumers in fall 2001 with the launch of XM and Sirius. Since then, changes in technology and legal and regulatory frameworks have connected satellite capabilities to a wide range of consumer and business products, including “direct to home” satellite TV, Global Positioning System services, and cellular telephony.
Satellite radio is a subscription-based service, and most channels are advertisement-free. The lion’s share of listening takes place when people are on the move in cars, and many car manufacturers offer satellite radio as an option for their models. It is typically included in the purchase price for 6 months or a year, and this serves as an effective sampling tool, with most subscribers continuing their subscription long after the trial period elapses.
The Sirius and XM satellites beam the same audio signals to receivers on the ground, which pick up one or more channels depending on their location and coverage zone. This system architecture, called satellite diversity, has worked remarkably well in preventing drop outs of satellite radio audio when vehicles pass under highway overpasses or enter canyons of natural or manmade origin.
Each incoming satellite signal is received as an IF (intermediate frequency) signal by the receiver, where it is digitized, demodulated, error-corrected, and de-interleaved using specialized circuits. A 16MB PSRAM buffers four seconds of each channel, and the resulting digital audio stream is then output over a Serial Peripheral Interface for processing by a baseband processor that combines the signal with an external analog radio transmitter and produces an analog audio output over a speaker.
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