tcdweb/README.md

tcdweb - Transcode Detector (Web Version)
=========================================

![Screenshot](tcdwebshot.png)

**tcdweb** is a browser-based audio authenticity analyzer that determines whether
an audio file is a genuine native encode or a *transcode* (lossy → lossless
re-encode). It can also detect *upscaling* (a lossy file re-encoded at a higher
bitrate by the same lossy codec, e.g. 128 → 320 kbps MP3).

This is the **web version** of [tcd](https://codeberg.org/armin/tcd), the
original CLI tool. At this point **tcdweb is a complete, independent fork** with
its own codebase, its own analysis engine, and its own UI. It runs entirely in
the browser via the **Web Audio API** - no server-side processing, no file
uploads, no FFmpeg dependency, and no command line needed.

---

Live-Demo
---------

There is a live demo of tcdweb available at: [bsd.pm/tcdweb](https://bsd.pm/tcdweb).

Obligatory AI notice
--------------------

tcdweb was developed with the assistance of AI.

---

What it does
------------

Drag or load an audio file, and tcdweb will:

1. Decode it client-side using the browser's built-in decoder
2. Compute the frequency spectrum via a Hann-windowed FFT (4096-point, 50% overlap)
3. Extract five key metrics (see below)
4. Apply a two-layer verdict system to classify the file
5. Display a detailed decision log, confidence score, and interactive charts

It includes a **realtime FFT analyzer** (with 8 window functions, configurable
peak-hold decay, and cutoff max falloff), a **waveform viewer with playback**,
**BPM detection**, **metadata tag parsing** (ID3v1/v2, FLAC, APE), and a full
data table with every computed value.

---

Key metrics
-----------

- **Cutoff**: highest frequency with measurable energy (as Hz and % of Nyquist).
  Lossy codecs chop off high frequencies - the lower the cutoff, the more
  aggressive the compression.
- **Steepness (Transition Bandwidth)**: how abruptly the spectrum drops at the
  cutoff point. Lossy encoders produce sharp brick-wall filters (low steepness).
- **Noise Floor**: average noise level in the top quartile of the spectrum.
  Lossy quantization raises the noise floor.
- **Roughness (Coefficient of Variation)**: how jagged/bumpy the spectrum is
  before the cutoff. Lossy encoding introduces quantization noise that creates
  spectral scalloping.
- **Band Ratio**: energy ratio between 16–20 kHz and 12–16 kHz. Transcodes show
  an unnatural dip in the top octave.

---

Supported formats
-----------------

| Lossy | Lossless |
|---|---|
| MP3 | FLAC |
| AAC / M4A | WAV / AIFF |
| Ogg Vorbis | ALAC |
| Opus | WavPack (WV) |
| WMA | APE |
| AC3 / EAC3 | DSF / DFF |
| MP2 / MP1 | |

Support depends on the browser's built-in decoder (all modern browsers support
the most common formats).

---

Verdicts
--------

| Verdict | Input codec | Meaning |
|---|---|---|
| **NATIVE** | lossy | Single encode at the stated bitrate - genuine |
| **UPSCALED** | lossy | Re-encoded from a lower bitrate (e.g. 128 → 320 kbps) |
| **GENUINE** | lossless | No evidence of lossy origin |
| **TRANSCODE** | lossless | Originated from a lossy source, decoded to lossless |
| **SILENT** | any | No detectable audio content |

---

Detection logic
---------------

The verdict is determined in two layers:

**Layer 1 (Primary)** - Cutoff + Steepness. A low cutoff combined with a sharp
drop (low steepness) is a definitive sign of a lossy encoder's lowpass filter.

**Layer 2 (Secondary)** - Roughness + Band Ratio. Applied when the cutoff is
high enough to pass Layer 1. Catches transcodes where the cutoff is near
Nyquist but the spectrum still shows quantization artifacts.

Lossy files are checked for upscaling by comparing the cutoff ratio to
bitrate-specific expectations.

---

Confidence score (0–100%)
-------------------------

A continuous score based on how far the metrics deviate from the thresholds.
Higher confidence means stronger evidence supporting the verdict.

---

How it differs from the original tcd CLI
-----------------------------------------

| Feature | tcd (CLI) | tcdweb |
|---|---|---|
| Platform | C program, command line | Pure JavaScript, browser |
| Decoder | FFmpeg (system dep) | Web Audio API (built-in) |
| Analysis | 4096 FFT, same algorithm | 4096 FFT, same algorithm |
| Realtime spectrum | Terminal TUI | Canvas-based interactive chart |
| Playback | No | Yes, with waveform and seek |
| BPM detection | No | Yes (autocorrelation) |
| Metadata parsing | No | ID3v1/v2, FLAC, APE |
| Stereo mode / VBR info | No | Yes (MP3 header parsing) |
| File size limit | None | Browser memory (~2 GB typical) |

---

Limitations
-----------

- **Browser audio decoding** limits which formats work (depends on `AudioContext.decodeAudioData`).
- **Very short files** (< 4096 samples) cannot be analyzed.
- **High-bitrate lossy encodes** (320 kbps MP3, 256 kbps AAC) may not be
  distinguishable from lossless by cutoff alone.
- **Already-filtered material** (e.g. deliberate 15 kHz LPF during mastering)
  may produce false positives.

---

References
----------

- Nyquist–Shannon sampling theorem - Wikipedia
  https://en.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem
- Equal-loudness contour (Fletcher–Munson curves) - Wikipedia
  https://en.wikipedia.org/wiki/Equal-loudness_contour
- LAME MP3 encoder psychoacoustic model - Hydrogenaudio Knowledge Base
  https://wiki.hydrogenaudio.org/index.php?title=LAME
- "Audio Authentication Using Spectral Analysis" - University of Michigan
  open-access thesis, 2025
  https://doi.org/10.7302/28306