Vinyl digitization — RIAA and elliptical mono

If you digitize a vinyl record with a USB turntable or a phono preamp into your computer, the file you get is not flat, neutral audio. The groove was cut with a deliberate tilt baked in — the RIAA curve — and the low end of a stereo LP often carries rumble that is out of phase between the left and right channels. Cathar’s riaa command fixes the first problem; optional elliptical mono helps with the second.

Why vinyl sounds wrong without RIAA

Vinyl has a physical problem: low frequencies need wide groove wiggles, which eat up space and limit playing time. The recording industry agreed on a trick: boost the bass and cut the treble when cutting the master, so the groove stays narrow. Every home turntable and phono preamp is supposed to do the opposite on playback — cut the bass back down and restore the highs — so what you hear is flat again.

That playback correction is RIAA de-emphasis. If you skip it (or your capture chain already applied it and you apply it again), the result is obviously wrong: boomy bass, dull highs.

  what was cut into the groove (pre-emphasis):   bass UP, treble DOWN
  what playback must do (de-emphasis):           bass DOWN, treble UP  ← cathar riaa
  what you want at the end:                      flat, natural balance

cathar riaa recording.wav --out flat.wav applies the standard playback curve. Cathar normalises so 1 kHz stays at reference level — the usual anchor point for RIAA.

Already corrected? If your chain includes a phono stage that already applies RIAA, running riaa again will over-correct. Listen first: if it already sounds balanced, skip this step.

Elliptical mono — taming stereo rumble

On stereo LPs, very low frequencies (rumble, warp, groove noise) are often mostly out of phase between left and right — they pull the stylus sideways without carrying much musical stereo information. Elliptical mono sums only the low band to mono while leaving everything above a crossover frequency in full stereo.

  below ~200 Hz:   L and R lows → one shared mono low  (rumble stops fighting the image)
  above crossover: untouched stereo

For stereo files:

cathar riaa stereo_capture.wav --out flat.wav --elliptical 200

Mono files get RIAA only; elliptical needs two channels.

Where this sits in a vinyl chain

A typical cathar workflow after capture:

1. riaa — correct the playback curve (and optionally --elliptical).
2. dewow — speed drift / pitch wobble if the turntable or belt isn’t steady (chapter 18).
3. azimuth — align the right channel to the left if the stereo image sounds thin or smeared (chapter 18).
4. declick — loud impulse pops and dust ticks (chapter 6).
5. decrackle — dense surface crackle between the big pops (chapter 19).
6. denoise / noiseprint — surface hiss (chapter 4).
7. dehum — mains buzz if the turntable motor leaks 50/60 Hz (chapter 5).
8. normalize — delivery loudness (chapter 10).

For outright drop-outs or mutes on a transfer, inpaint can fill short gaps (chapter 19).

How the big tools do it

• Audacity has no built-in RIAA; users rely on the turntable’s phono stage or third-party EQ curves.
• iZotope RX and dedicated vinyl tools (e.g. open Vinyl Restoration Suite) bundle RIAA, declick, denoise, and sometimes wow/flutter in one GUI.
• Cathar keeps each stage separate and inspectable — one command per transform, scriptable and deterministic.

For archival work, RIAA is not optional maths; it’s the step that turns “raw groove data” into “music that sounds like the record you remember.”