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| CATEGORIES ABOUT RECONSTRUCTION FILTER | |
| digital signal processing | |
| linear filters | |
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Whilst in theory a DAC gives a series of Impulse s, in practice, the output of a DAC is more typically a series of stair-steps. The Low Pass reconstruction filter smooths the stair step (removes the harmonics above the Nyquist Limit ) to (re)construct the analogue signal corresponding to the digital time sequence. The Sampling Theorem describes why the input of an ADC requires a low-pass analog Electronic Filter , called the Anti-aliasing Filter . For the same reason, the output of a DAC requires a low-pass analog filter, called a reconstruction filter. Ideally, both filters should be Brick-wall Filters , constant phase delay in the pass-band with constant flat frequency response, and zero response from the Nyquist Frequency . This is given by a filter with a ' Sinc ' impulse response. Practical filters have non-flat frequency or phase response in the pass band and incomplete suppression of the signal elsewhere, as a sinc waveform has an infinite response to a signal, in both the positive and negative time directions, which is impossible to perform in real time. In systems that have both, the Anti-aliasing Filter and a reconstruction filter may be of identical design. For example, both the input and the output for audio equipment is sampled at 44.1 kHz. Both Audio Filter s block as much as possible above 22 kHz and pass as much as possible below 20 kHz. Typically both filters are active op-amp filters, with exactly the same selection of resistors and capacitors. Reconstruction filters are also used when "reconstructing" a waveform or an image from a collection of Wavelet coefficients. In Medical Imaging , a common technique is to use a number of 2D X-ray photos or MRI Scan s to "reconstruct" a 3D image. SEE ALSO |
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