crates/veilvoice-audio/src/meter.rs

The scale a level meter is drawn on.

Why this is here rather than in a front end

crate::live::LiveStats reports a peak, and both front ends draw it. They drew it differently: both were linear, and the desktop one printed a decibel number beside a bar filled linearly, so the number said -12 dB and the bar showed a quarter. Two meters disagreeing about the same reading is worse than one bad meter, because it makes the reader doubt the number.

The scale belongs with the measurement. This module owns the arithmetic; the front ends own how it looks.

Why not linear

Loudness is not linear, and a meter that is has almost no useful range. Ordinary speech recorded at a sensible level peaks around -12 dBFS, which is 0.25 linear: a quarter of the bar, which reads as near-silence. The only way to fill a linear bar is to be clipping. Every real meter is logarithmic for exactly this reason.

What it measures, and what it does not

Sample peak, since the last read. It is not a loudness meter: RMS, LUFS and anything else that correlates with how loud a thing sounds needs a window and a weighting curve, and answers a different question. This one answers "am I being recorded, and am I clipping".

It also cannot see an inter-sample peak, which is a waveform that passes above full scale between two samples and clips in a converter without any single sample exceeding 1.0. Catching those needs oversampling. A front end may say CLIP when a sample reaches full scale, and must not imply it caught the ones it cannot see.

In plain words

This decides how a level meter is drawn, so that the bar and the number beside it always agree.

They did not, once. Both the window and the terminal drew the bar filling evenly with the signal, while the number beside it was in decibels, which do not rise evenly at all. So the number could read -12 dB while the bar looked a quarter full, and a reader who noticed stopped trusting both.

One piece of arithmetic, in one place, used by both. Now they cannot disagree.

WHAT THIS FILE CONTAINS

166 lines defining 3 functions (3 public), 0 types and 2 constants. Everything below is read out of the source, so it cannot disagree with the code.

What happens when it runs. These are the ways in: public, and nothing else in this file calls them, so they are what an outside caller reaches first.

• position line 88 · Where a level sits along a bar: 0.0 at the floor, 1.0 at full scale.
  reaches dbfs
• clipping line 93 · Whether a reading counts as clipping.
  reaches dbfs

WHAT CALLS WHAT

The functions this file defines, and the calls between them. An edge means the callee's name appears, called, inside the caller's body. This is a syntactic reading, not a type-resolved one.

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flowchart TD
    n_dbfs["dbfs<br/>line 80"]
    n_position(["position<br/>line 88"])
    n_clipping(["clipping<br/>line 93"])
    n_clipping --> n_dbfs
    n_position --> n_dbfs
    click n_dbfs href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-audio/src/meter.rs#L80" "open the source"
    click n_position href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-audio/src/meter.rs#L88" "open the source"
    click n_clipping href "https://github.com/tilas01/veilvoice/blob/main/crates/veilvoice-audio/src/meter.rs#L93" "open the source"
    classDef entry fill:#1f2335,stroke:#7aa2f7,color:#c0caf5
    class n_position,n_clipping entry
    classDef api fill:#1f2335,stroke:#7dcfff,color:#c0caf5
    class n_dbfs api

ITEMS