Broadcast and CD playback de-emphasis
Vinyl has the RIAA curve (chapter 15). FM radio and early compact discs use different pre-emphasis schemes — treble boosted on record, treble cut on playback — and if you digitize the wrong side of that bargain, the file sounds dull (de-emphasis never applied) or harsh and thin (de-emphasis applied twice).
FM — 50 µs and 75 µs
FM broadcasters boost highs before transmission so hiss is less audible; your receiver cuts them back on playback. Archives of off-air FM captures, or files from tuners that output “flat” RF-discriminator audio, often still carry that treble tilt.
The time constant names the curve:
- 50 µs — common in Europe and much of the world.
- 75 µs — common in the Americas.
cathar deemphasis fm_capture.wav --curve fm50 --out flat.wav
cathar deemphasis us_fm.wav --curve fm75 --out flat.wav
Listen after one pass. If it gets brighter instead of more natural, you may have started from already-flat audio — don’t de-emphasize twice.
Early CD — 50/15 µs optional pre-emphasis
Most CDs you meet are flat — no pre-emphasis. A minority of early titles (roughly the Red Book optional pre-emphasis flag era) were mastered with a 50/15 µs emphasis curve. If a CD rip sounds oddly dull or muffled compared to a reference, this is one thing to check.
cathar deemphasis odd_cd.wav --curve cd --out flat.wav
RIAA vs deemphasis vs riaa
| Curve | Typical source | Cathar command |
|---|---|---|
| RIAA | Vinyl digitized without a phono stage | riaa |
| FM 50 / 75 µs | Off-air FM, some tuner captures | deemphasis --curve fm50 / fm75 |
| CD 50/15 µs | Rare pre-emphasized CD masters | deemphasis --curve cd |
riaa is vinyl-specific (and supports --elliptical for stereo rumble).
deemphasis is the broadcast/CD family. Pick the curve that matches how the
recording was emphasized before capture, then apply the matching playback
de-emphasis once.
How the big tools do it
- Audacity and most DAWs expose generic EQ; you can dial an FM de-emphasis shelf by hand, but there’s no one standard menu item.
- Dedicated archival tools sometimes bundle FM and tape NR curves alongside vinyl workflows.
- Cathar keeps each curve as a named, inspectable first-order filter — one command, one job, scriptable like everything else in the toolkit.