---
source_files:
  - DataPRO/FftSharp.Test/PowerSpectralDensityShould.cs
generated_at: "2026-04-16T03:50:52.298896+00:00"
model: "Qwen/Qwen3-Coder-Next-FP8"
schema_version: 1
sha256: "bdc792ce4fb3e148"
---

# FftSharp.Test

## Documentation Page: `FftSharp.Test.PowerSpectralDensityShould`

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### 1. **Purpose**

This test class (`PowerSpectralDensityShould`) validates the correctness of the `FftSharp.Transform.PSD_Welch` method — specifically its implementation of the Welch method for power spectral density (PSD) estimation — by comparing its output against a precomputed reference dataset (`PSD.csv`). It ensures that the computed PSD matches expected frequency bins (`FFTfreq`) and power values within a defined numerical tolerance, using real-world test data (`Raw.csv` and `PSD.csv`). The class serves as a regression guard to prevent unintended changes to the PSD computation behavior.

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### 2. **Public Interface**

This is a **test fixture** (NUnit `[TestFixture]`) and does **not expose production APIs**. It contains only private helper methods and test methods. The *actual* production APIs under test are referenced but not defined here:

- **`FftSharp.Transform.FFTfreq(int sampleRate, int n)`**  
  *Inferred usage*: Generates the array of frequency bins (in Hz) for an FFT of length `n` at given `sampleRate`. Returns `double[]`.

- **`FftSharp.Transform.PSD_Welch(double[] data, int sampleRate, WindowType windowType, int windowWidth, double overlappingPct, WindowAveragingType averagingType)`**  
  *Inferred usage*: Computes the power spectral density of `data` using Welch’s method. Returns `double[]` of PSD values (in units of `g²/Hz`, per test assertions).  
  - `data`: Input time-domain signal.  
  - `sampleRate`: Sampling rate in Hz.  
  - `windowType`: Window function type (e.g., `WindowType.Rectangular`).  
  - `windowWidth`: Length of each segment (in samples).  
  - `overlappingPct`: Overlap percentage between segments (0–100).  
  - `averagingType`: Averaging strategy (e.g., `WindowAveragingType.Averaging`).  

> **Note**: The types `WindowType` and `WindowAveragingType` are referenced but not defined in this file; their definitions must be found in the `FftSharp` source.

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### 3. **Invariants**

- **Data lengths**:
  - `Raw.csv` must contain exactly **304,478** samples per column (`RawValueLength = 304478`).
  - `PSD.csv` must contain exactly **2,049** samples per column (`PSDValueLength = 2049`).
- **Frequency bin count**: For a signal of length `N`, the PSD output length is `N/2 + 1` (i.e., `2049` for `windowWidth = 4096`), consistent with real-input FFT symmetry.
- **Numerical tolerance**: Comparison uses a dynamic epsilon based on the magnitude of the expected value:  
  `delta = 10^(floor(log10(val1[i])))`, adjusted by ±1 in exponent.  
  Values < `1e-10` (both expected and actual) are ignored (noise floor).
- **Test data format**: CSV files use comma separators, with two columns per file (no header), parsed as `double`.

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### 4. **Dependencies**

- **External libraries**:
  - `NUnit.Framework` (for `[TestFixture]`, `[Test]`, `Assert.Multiple`, etc.)
  - `System`, `System.IO`, `System.Linq`, `System.Collections.Generic` (standard .NET)
- **Internal dependencies** (inferred from usage):
  - `FftSharp.Transform` namespace (contains `FFTfreq`, `PSD_Welch`)
  - `FftSharp.WindowType` enum (e.g., `Rectangular`)
  - `FftSharp.WindowAveragingType` enum (e.g., `Averaging`)
- **Test data files**:
  - `Raw.csv`: Raw time-domain input (304,478 samples per channel).
  - `PSD.csv`: Reference PSD output (2,049 Hz and `g²/Hz` values).
  - Files are loaded relative to `TestContext.CurrentContext.TestDirectory`.

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### 5. **Gotchas**

- **Relative tolerance logic**: The `CloseEnough` method uses a *relative* tolerance scaled by the order of magnitude of the expected value, not a fixed epsilon. This may fail for values near zero or extremely small magnitudes (though those are skipped if both values < `1e-10`).
- **Hardcoded lengths**: `RawValueLength` and `PSDValueLength` are constants. If test data files change size, tests will break silently unless updated.
- **No validation of `PSD_Welch` parameters**: Tests assume valid inputs (e.g., `windowWidth ≤ data.Length`, `overlappingPct ∈ [0,100]`). No tests cover edge cases (e.g., zero overlap vs. 50% overlap, different window types).
- **Assumed units**: The test compares against `g²/Hz`, implying the input data is in `g` (acceleration), but this is not enforced programmatically.
- **File path separator**: Uses hardcoded `"\\"` — may fail on non-Windows systems unless `TestContext` resolves paths correctly.

> **None identified from source alone** beyond the above — but the reliance on external test data and hardcoded constants is a maintenance concern.