What this site does. You bring a table of numbers, it gives you
numbers back. This page runs a Butterworth filter over your time series: it keeps
the frequencies you choose and removes the rest, and it draws the filter's
frequency response so you can see exactly what it does. It runs entirely in your
browser — nothing is uploaded, nothing is stored. If you want the reasoning
rather than the answer, the digital filter
article compares six designs and explains cutoff, order and phase.
A filter needs evenly spaced samples, and t must be time in seconds: the sample
rate fs = 1 / Δt sets every frequency on this page. A cutoff must stay below the
Nyquist frequency, fs / 2.
- Paste your data into the Input grid — straight from a
spreadsheet, or as CSV/TSV text — or import a CSV/TSV file.
Column t (s), then up to three y columns. Up to 5,000 rows.
- Rows are sorted by t. A row with no number in t is skipped; a blank y cell is
skipped for that column only - which leaves a gap, so that column is refused
as unevenly spaced.
- Pick a type. Low-pass keeps the slow part, high-pass the fast
part, band-pass one band, band-stop removes one band (a notch, for example
for 50 Hz or 60 Hz hum).
- Set the cutoff in Hz - for a band, the low and the high edge.
The cutoff is where the gain of one pass has fallen to 0.7071 (−3 dB).
- Set the order, 1 to 8. A higher order makes a steeper edge
and more ringing after sudden changes.
- Pick the phase. Zero-phase runs the filter forward and back:
no delay, and the gain is applied twice. Causal runs once, forward: what a
real-time system does, and the output is delayed.
- Real time range — try settings live on the order and cutoff
boxes.
- ▲▼ buttons or the mouse wheel over a
box: the value moves by one step and the result is recomputed at once -
no need to press filter.
- Digit wheel: over a box, the wheel steps the digit just
left of the mouse pointer.
- Limits: the order stays within 1 to 8, a cutoff above
0; a typed value outside is set to the nearest limit.
- Typing or the arrow keys change the box only; press
filter to recompute.
- Press filter. The upper chart shows your data as dots and
each filtered column as a line in the same colour; the lower chart shows the
gain the filter applies at each frequency, with the cutoff marked.
- Read the result. The Output grid lists t, then the filtered
value for each y column. Export saves the Output grid as a
CSV/TSV file.