# Can I ignore the Nyquist criterion?

If I have a signal with frequencies up to 2000 Hz but I'm only interested in the frequency range up to 500 Hz, is it correct to sample at 1000 Hz $(f_\mathrm s \geq 2 \cdot f_\mathrm{max})$? Or do I have to apply an analog lowpass filter with cut-off frequency 500 Hz first?

I read this similar post, but didn't understand it well: Ignoring Nyquist

• You have to do lowpass filtering first, otherwise highpass part of the signal (500 - 2000Hz) will alias down to the lowpass one. And the signal of interest will be lost. To avoid this filter out a part of signal spectrum you're not interested in. – Serj Aug 19 '14 at 2:30
• Ignore is not the correct word here, you can violate the Nyquist criteria if you are ok with the signals that will be aliased (e.g. pre-filtered and highly attenuated). – Christopher Felton Aug 19 '14 at 13:14