Lessons · Engineering · Sampling fast enough to be honest
Sampling fast enough to be honest
To capture a signal without inventing content, it must be sampled at more than twice its highest frequency. Sample too slowly and high frequencies do not disappear -- they reappear disguised as low ones, which is aliasing, and nothing downstream can undo it.
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In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is why a recording of a fast vibration can show a slow one that is not there, and why an anti-aliasing filter goes before the converter rather than after. Once the data is wrong it stays wrong.
How to think about it
Decide the highest frequency that genuinely matters, double it, and sample above that with margin. Then filter above the limit BEFORE sampling, because filtering afterwards removes nothing -- the false low frequency is already indistinguishable from a real one.
Worked example
Sample at more than twice the highest frequency.The minimum, not the target.
Too slow and high frequencies alias down.They appear as low frequencies.
Aliasing cannot be undone afterwards.The data no longer contains the difference.
Filter before the converter, not after.This is the only prevention.
Your turn
The highest frequency of interest is 400 Hz. Write the minimum sampling rate in Hz.
2 x 400 = Hz
Solve one, graded on the server
The trap
Filtering after the converter. By then a 900 Hz signal sampled at 1 kHz has become a 100 Hz one and no filter can tell it from a real 100 Hz.