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ANSWERS: 3
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One is controlled by Frequency and Amplitude while the other is controlled by Sattelite Signal.
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HD is a digital signal sent along side the regular analog AM or FM, which is why it is also known as IBOC, or "In Band, On Channel" digital radio. An HD radio will first pick up the analog signal, then once the buffer is full, switch over to digital. On AM, the sound change is tremendous compared with typical lo-fi AM radios (see note below); on FM the change is not quite as dramatic, but many FM stations offer more than one channel of programming on their single frequency. XM and Sirius, on the other had, and totally 100 percent digital and sent on different frequencies than AM or FM. XM especially also relies on repeaters, but both XM and Sirius are sent via satellite, while HD is theoretically local. (Note: many AM fans will rightly state that HD does not sound as good as well-done analog AM stereo, and they are right. But analog AM stereo was so often not done right that HD sounds better to almost everyone who hears it)
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HD is local digital, XM is satellite digital. You can maintain the same XM station coast to coast. The HD locals stations only go as far as regular radio stations.
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