Satellite radio uses digital signals from satellites in geostationary orbit to deliver a variety of audio content to subscribers via a receiver that decodes the signal. The service is subscription based, and the two main satellite radio providers are Sirius and XM Radio (which merged in 2007). The system works well in automobiles, and many manufacturers include it as standard equipment. Satellite radio is popular with drivers who value diverse programming and uninterrupted listening, particularly on long drives.
The radio receives the signal from the satellites via an antenna, then converts it to a digital format using a DSP chip. The DSP is a complex integrated circuit that performs a host of operations including demodulation, error correction, de-interleaving and decoding to produce the digital output, which is then converted to an analog signal by a D/A converter. The DSP chips used in satellite radios have evolved over time, and some manufacturers offer proprietary DSP chips for their receivers to ensure the best performance.
The satellite radio signal is transmitted on multiple frequency channels, a technique called multicasting. This allows multiple transmitters to operate on a single frequency network, providing greater range for the signal and lessening the chance of scintillating conditions that occur when driving under highway overpasses or through urban areas. The signal itself is an OFDM QPSK signal, a cousin to WiMax and LTE. It incorporates hierarchical modulation to provide more robust error correction on control channels while sacrificing a bit of bandwidth for the core audio content.
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