| Anti-aliasing Filter |
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Information AboutAnti-aliasing Filter |
| CATEGORIES ABOUT ANTI-ALIASING FILTER | |
| digital signal processing | |
| linear filters | |
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Since the theorem states that the power of frequencies outside the bandwidth must be zero then the anti-aliasing filter would have to be a perfect filter to completely satisfy the theorem. This is true because a bandwidth-limited filter must have infinite time to filter. So every realizable anti-aliasing filter will permit some Aliasing to occur. The amount of aliasing that does occur depends on how good the filter is and what the Sampling Rate is. APPLICABILITY OF OVERSAMPLING The purpose in Oversampling is to relax the requirements on the anti-aliasing filter or to further reduce the aliasing. Since anti-aliasing filters are Analog , oversampling allows for the filter to be cheaper because the requirements are not as stringent, as well as allowing the anti-aliasing filter to have a smoother frequency response, and thus a less complex phase response. FREQUENCY SPECTRUM SHAPE Most often, an anti-aliasing filter is a Low-pass Filter . However, this is not a requirement. The Shannon-Nyquist Sampling Theorem states that the sampling rate must be greater than twice the bandwidth, not maximum frequency, of the signal. For the types of signals that are bandwidth limited, but not centered at zero, a Band-pass Filter would be used as an anti-aliasing filter. For example, this could be done with a Single-sideband Modulated or Frequency Modulated signal. If one desired to sample an FM Radio broadcast on Channel 200 , then an appropriate anti-alias filter would be centered on 87.9 MHz with 200 kHz bandwidth (or Pass-band of 87.8 MHz to 88.0 MHz), and the sampling rate would be no less than 400 kHz. (In this case not the audio of the broadcast is sampled, but the actual transmission signal itself, which is not very common.) SIGNAL OVERLOAD It is very important to avoid input signal overload when using an anti-alising filter. If the signal is strong enough, it can cause clipping at the analog-to-digital converter, even after filtering. Because the signal distortion created by the clipping of the filtered waveform occurs after the anti-aliasing filter, it will have components spread throughout the frequency spectrum, including those out-of-band parts of the spectrum which cause aliasing. In Digital Audio , the resulting aliased distorted signal of "digital clipping" has a characteristic sound that can be easily recognised. SEE ALSO |
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