ferraus filter

Scientific filters: Butterworth, Chebyshev, Bessel, elliptic

Synopsis

ferraus filter <input> [options] [-o <output>]

Designs an analog prototype, transforms it to the requested band, maps it to digital with the prewarped bilinear transform, and runs it as a cascade of second-order sections. --freq is the -3 dB point for butterworth and bessel, the passband edge (end of the ripple) for chebyshev1 and elliptic, and the stopband edge for chebyshev2. Band filters take low..high and have twice --order poles. The metrics report the realised response and group delay at a few frequencies.

This command is also an fx stage and a pipeline op: ferraus fx in.wav -o out.wav --fx "filter:key=value".

Examples

Very steep elliptic low-pass

ferraus filter in.wav --design elliptic --type lowpass --freq 4kHz --order 6 -o lp.wav

Phase-friendly Bessel high-pass

ferraus filter in.wav --design bessel --type highpass --freq 40Hz -o hp.wav

Telephone band

ferraus filter in.wav --design chebyshev1 --type bandpass --freq 300Hz..3.4kHz --ripple 1dB -o phone.wav

Options

Flag	Type	Default	Meaning
`--design`	butterworth \| chebyshev1 \| chebyshev2 \| bessel \| elliptic	`butterworth`	Filter family
`--type`	lowpass \| highpass \| bandpass \| bandstop	`lowpass`	Response type
`--freq`	text	`auto`	Cutoff (1kHz) or band (300Hz..3.4kHz); auto = 8 kHz LP, 80 Hz HP, 300 Hz-3.4 kHz BP, 50-70 Hz BS
`--order`	integer	`4`	Prototype order (steepness) Range: 1..24.
`--ripple`	gain	`0.5dB`	Passband ripple in dB (chebyshev1, elliptic)
`--stopband`	gain	`60dB`	Stopband attenuation in dB (chebyshev2, elliptic)
`--mix`	percent	`100%`	Dry/wet balance: 0% returns the input unchanged, 100% is fully processed

Global options

Every command also accepts the global options (-o, --json, --dry-run, --overwrite, --progress, ...). See Global options.

Exit codes

Code	Meaning
0	Success
1	Processing failed
2	Usage or parameter error
3	Input missing or unreadable
4	Unsupported format or codec
8	Cancelled
70	Internal error (bug)

Common mistakes

Forgetting that outputs are never overwritten: pass --overwrite or a new -o path when re-running.
Gains need a unit when not in dB: 0.5x is a factor, -6 and -6dB are decibels.

For models

MCP tool: ferraus_filter
Schema: ferraus schema filter
Pipeline: { op: filter, args: { ... } } (flag names with _ instead of -)
fx stage: yes

{"command": "filter", "defaults": {"design": "butterworth", "type": "lowpass", "freq": "auto", "order": "4", "ripple": "0.5dB", "stopband": "60dB", "mix": "100%"}}