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An envelope detector is an electronic circuit that takes a high-frequency signal as input and provides an output which is the envelope of the original signal. The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal falls. The diode in series rectifies the incoming signal, allowing current flow only when the positive input terminal is at a higher potential than the negative input terminal. Most practical envelope detectors use either half-wave or full-wave rectification of the signal to convert the AC audio input into a pulsed DC signal. Filtering is then used to smooth the final result. This filtering is rarely perfect and some "ripple" is likely to remain on the envelope follower output, particularly for low frequency inputs such as notes from a bass guitar. More filtering gives a smoother result, but decreases the responsiveness; thus, real-world designs must be optimized for the application. == Definition of the envelope == Any AM or FM signal can be written in the following form : In the case of AM, φ(''t'') (the phase component of the signal) is constant and can be ignored. In AM, the carrier frequency is also constant. Thus, all the information in the AM signal is in ''R''(''t''). ''R''(''t'') is called the envelope of the signal. Hence an AM signal is given by the function : with ''m''(''t'') representing the original audio frequency message, ''C'' the carrier amplitude and ''R''(''t'') equal to ''C'' + ''m''(''t''). So, if the envelope of the AM signal can be extracted, the original message can be recovered. In the case of FM, the transmitted signal has a constant envelope ''R''(''t'') = R and can be ignored. However, many FM receivers measure the envelope anyway for received signal strength indication. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「envelope detector」の詳細全文を読む スポンサード リンク
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