Aliasing and Antialiasing filter
- 2. Aliasing is termed as appearance of high frequency signals as low frequency signal that distracts the desired signal. Aliasing can occur in signals sampled in time, for instance digital audio, and is referred to as temporal aliasing. Aliasing can also occur in spatially sampled signals, for instance patterns in digital images. Aliasing in spatially sampled signals is called spatial aliasing.
- 3. •Aliasing is basically a low pass filter with a cut off frequency equal to one-half the sampling frequency used by the sampler while sampling. Thus fo>=2fc Where, fo- Sampling Frequency fc- Cut-off Frequency This relationship is also known as Nyquist Sampling Criterion.
- 5. An anti-aliasing filter (AAF) is a filter used before a signal sampler to restrict the bandwidth of a signal to approximately or completely satisfy the sampling theorem over the band of interest. Since the theorem states that unambiguous reconstruction of the signal from its samples is possible when the power of frequencies above the Nyquist frequency is zero, a real anti-aliasing filter trades off between bandwidth and aliasing. A realizable anti-aliasing filter will typically either permit some aliasing to occur or else attenuate some in-band frequencies close to the Nyquist limit. For this reason, many practical systems sample higher than required to ensure that all frequencies of interest can be reconstructed, a practice called oversampling.
- 7. An ideal anti-alias filter passes all the appropriate input frequencies (below f1)and cuts off all the undesired frequencies(above f2). However ,such a filter is not physically realizable. The filter passes all the frequ- encies<f1, and cuts-off all frequencies >f2. The region between f1 and f2 is known as Transition Band, where signals are attenuated gradually. The signals in the transition band can still cause aliasing.