|MadSci Network: Engineering|
Telephones, and commercial AM and FM radios are all analogue devices and do not use sample methods. CDs DATs .WAV computer files etc are digital and use sampling to record analogue signals. When an analogue signal (such as speech) is digitised the signal is sampled at a given frequency. That is the "value" of the signal is determined at set intervals. This introduces a possible error when the analogue signal is reconstructed from the sampled data. Basically the higher the frequency of sampling the less error there is in reconstruction. Or alternatively you can think that the higher the sampling rate, the higher the signal frequency that can be successfully reconstructed. You need to sample at least twice as fast as the highest signal frequency there is. So if you wanted to reconstruct a sine wave at 10kHz, you would need to sample at least at 20Khz. Remember a complex signal can be though of as an infinite series of sine waves at different and increasingly higher frequencies (fourier transform analysis). So the faster you sample the "higher" of the simple sine wave frequencies that make up the complex signal can be reconstructed. So very basically CDs DATs .WAV computer files, MP3 etc all have a defined sampling rate. That rate determines the highest sine wave frequency that can be reconstructed faithfully, which also determines the quality of the recording. However higher sampling means you need more storage. It comes down to a trade off. The fact that the human ear can only hear frequencies below 20KHz also plays a role in determining what sampling rate to use. Martin Smith
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