# Signal

The `Signal` class implements a piecewise automation curve for scalar parameters such as BPM. It stores automation points and evaluates the curve at any audio time, supporting step, linear, and exponential ramps.

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## Purpose

Musical performances often require smooth tempo changes. `Signal` provides the mathematical foundation for these changes by scheduling automation points and interpolating between them. The `TickSource` reads the BPM curve through this `Signal` so that tempo ramps produce a continuously varying tick rate.

## Features

- `setValueAtTime` for step changes
- `linearRampToValueAtTime` for linear interpolation
- `exponentialRampToValueAtTime` for exponential curves
- `getValueAtTime` for curve evaluation at arbitrary times
- `cancelScheduledValues` for clearing future points
- Rejects non-positive and non-finite values with `AUDIO_TRANSPORT_INVALID_BPM`

## Usage: Basic

```ts
import { Signal } from "@suaveplan/audio-transport";

const bpm = new Signal({ initialValue: 120 });
bpm.linearRampToValueAtTime(240, 1);
console.log(bpm.getValueAtTime(0.5)); // 180
```

## Usage: Advanced

```ts
const bpm = new Signal({ initialValue: 100 });
bpm.setValueAtTime(120, 0.5);
bpm.exponentialRampToValueAtTime(240, 1.5);
console.log(bpm.getValueAtTime(1)); // ~169.7
```

## API Reference

### Constructor

`new Signal(options?)`

### Properties

- `initialValue` — value before any points
- `value` — convenience getter returning value at time 0
- `points` — readonly array of automation points

### Methods

- `setValueAtTime(value, time)` — step change
- `linearRampToValueAtTime(value, time)` — linear ramp
- `exponentialRampToValueAtTime(value, time)` — exponential ramp
- `getValueAtTime(time)` — evaluate curve
- `cancelScheduledValues(time)` — remove points at or after time

## Implementation Notes

Points are stored sorted by time. Exponential ramps require strictly positive start and target values; otherwise `AUDIO_TRANSPORT_INVALID_BPM` is thrown. The curve evaluation handles interpolation before the first point, between points, and after the last point.