Re: ADC without antialias filter? was: Anti-aliasing ADC samples

Marte Schwarz wrote:
hello all,

I have a little topic I couldn't understand completely (or as much I want to ;-)

As a rule of thumb it is well known to use a filter to cut all frequencies above half the sample frequency. But I guess, there are cases that doesn't need this or where the signals would be more real without this filter than with such one. For example ECG-signals. A QRS complex has frequencies in the range of over 100 Hz. But these frequencies where not stationary so one criteria to construct alias is not present.

Wrong. They can still alias, and perhaps not give a tone but a wrong waveform.

---_--_--_----

could become

------__------

which is totally incorrect.

But filtering these Signals to avoid frequencies over half the sampling rate may extend the QRS and in this case changes relevant parameters of the signal. In real world we often have nonstationary signals in which aliasing would not be created by having samplerates below the nyquist criteria. OK, we make errors having not enough sampling rate, sure, but depending on the goal of the signal analysis, may be it is sometimes better to take these errors and avoid others created from (analog) filters.

Where is the error in this statement? Or is it correct? What do you think about?

I think you need to filter. If you are concerned about the waveform shape and do not want ringing: there are special filters for that. But expect that the rolloff is slower.


Thomas
.

Relevant Pages