Aliasing distortion

Today (in 1997), most commercial sampling playback implementations use only two point linear interpolation. This is described as two point because only two input samples are involved in the interpolation calculation. While this method works reasonably well, implementors have to be careful not to use waveforms with significant high frequency content (energy above ), or aliasing distortion will be noticeable. 

Aliasing Distortion arising from the representation of a signal by its samples due to too low a sampling rate relative to the signal bandwidth. 

Figure 1.3 An analog-to-digital converter needs at least two samples per cycle in order to represent a sound. Otherwise the sampling process creates aliasing distortion...

The ADC needs at least two samples per waveform cycle in order to represent the frequency of the sound if not, then the frequency information is lost. Digital recording systems place a low-pass filter before the ADC in order to ensure that only signals below the Nyquist frequency enter the converter. Otherwise, the conversion process creates foldback frequencies, thus causing a phenomenon known as aliasing distortion (Figure 1.3). 

Both AM and RM can use signals other than sinusoids, applying the same principles discussed above. In any case, great care must be taken in order to avoid aliasing distortion (i.e. generation of frequencies above 50 per cent of the sampling rate) because the highest frequencies of the two sounds will be additive. 

In addition, vibration data collected with a microprocessor-based analyzer is filtered and conditioned to eliminate non-recurring events and their associated vibration profiles. Anti-aliasing filters are incorporated into the analyzers specifically to remove spurious signals such as impacts. While the intent behind the use of anti-aliasing filters is valid, however, their use can distort a machine s vibration profile. 

While our method works well with most situations, it is limited when (oo) exhibits a low-frequency tail. Such a situation is characteristic of percolation, electrode polarization, or other low-frequency process, where the reciprocal of the characteristic relaxation time for the process is just below our frequency window. In this case, aliasing effects distort the transform result. 

Fortunately, in proton spectroscopy it is generally possible to detect the presence of an aliased peak because of its esoteric phase which remains distorted when all others are correct. In heteronuclear spectra that display only a single resonance such phase characteristics caimot provide this information as there is no other signal with which to make the comparison. In such cases it is necessary to widen the spectral window and record any movement of the... 

Once the interferograms have been preprocessed to reduce the noise and the distortions, one can proceed to the extraction of the spectra. In order to avoid aliasing, the spectral band of operation of the detectors is set from 25 to 212cm . ... 

A finite grid cannot represent arbitrarily large fc-vectors. Instead, they are folded back into the first Brillouin zone and distort there the true spectrum. This effect is usually referred to as aliasing . See, e.g.. Sect. 12.1 in (39. ... 

Figure 6 Aliasing due to an improper sweep-width setting (a) normal spectrum recorded with a sweep width encompassing all resonances (b) spectrum with aliased peaks (asterisk) taken with a sweep width narrower than the sample s chemical-shift range. Note the change in position of resonances at either end of the spectrum and the uncorrectable baseline distortions observed for peaks in the middle. The sample is poly(isobutyl methacrylate) in CDCI3.

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