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CEA-909
CEA-909 is the ANSI standard for 8VSB/ATSC smart antennas. The basic concept is that the smart antenna either physically rotates toward the signal, or is stationary, but has elements pointed in different directions and uses only those elements pointed toward the signal. This is accomplished by feedback from the control device, such as a digital-to-analog converter box, telling the smart antenna when the signal is stronger or weaker.
Analog televisions generally give instant feedback as the signal gets better or worse as you move the antenna. Digital television antennas can be difficult to aim correctly because of the cliff effect and because of delays in decoding and displaying the signal. Smart antennas remove the burden of positioning the antenna for digital TVs and can make the tuning process easier than it was with analog television.
== Information from TVP9900 document ==
There are two basic modes of operation for a CEA-909 antenna, mode A and mode B. Mode A allows for communication only from the tuner to the antenna and Mode B allows for bidirectional communication. A 14-bit pulse-width modulation data stream is sent to the antenna and if the antenna is mode B a 10-bit data stream is sent back. The timeout period for a response from a mode B antenna is 100 ms. The system uses a single wire for communication, possibly similar to I²C.
The 10 bits sent back from the antenna are largely unused. Only the first two bits are used with at least one of those being used for a transmission error flag; the remaining 8 bits are reserved. The contents of the 14 bits sent to the antenna are not revealed.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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