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163 lines
5.4 KiB
Markdown
163 lines
5.4 KiB
Markdown
tcdweb - Transcode Detector (Web Version)
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=========================================
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**tcdweb** is a browser-based audio authenticity analyzer that determines whether
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an audio file is a genuine native encode or a *transcode* (lossy → lossless
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re-encode). It can also detect *upscaling* (a lossy file re-encoded at a higher
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bitrate by the same lossy codec, e.g. 128 → 320 kbps MP3).
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This is the **web version** of [tcd](https://codeberg.org/armin/tcd), the
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original CLI tool. At this point **tcdweb is a complete, independent fork** with
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its own codebase, its own analysis engine, and its own UI. It runs entirely in
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the browser via the **Web Audio API** - no server-side processing, no file
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uploads, no FFmpeg dependency, and no command line needed.
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---
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Live-Demo
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---------
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There is a live demo of tcdweb available at: [bsd.pm/tcdweb](https://bsd.pm/tcdweb).
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Obligatory AI notice
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--------------------
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tcdweb was developed with the assistance of AI.
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---
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What it does
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------------
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Drag or load an audio file, and tcdweb will:
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1. Decode it client-side using the browser's built-in decoder
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2. Compute the frequency spectrum via a Hann-windowed FFT (4096-point, 50% overlap)
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3. Extract five key metrics (see below)
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4. Apply a two-layer verdict system to classify the file
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5. Display a detailed decision log, confidence score, and interactive charts
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It includes a **realtime FFT analyzer** (with 8 window functions, configurable
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peak-hold decay, and cutoff max falloff), a **waveform viewer with playback**,
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**BPM detection**, **metadata tag parsing** (ID3v1/v2, FLAC, APE), and a full
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data table with every computed value.
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---
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Key metrics
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-----------
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- **Cutoff**: highest frequency with measurable energy (as Hz and % of Nyquist).
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Lossy codecs chop off high frequencies - the lower the cutoff, the more
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aggressive the compression.
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- **Steepness (Transition Bandwidth)**: how abruptly the spectrum drops at the
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cutoff point. Lossy encoders produce sharp brick-wall filters (low steepness).
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- **Noise Floor**: average noise level in the top quartile of the spectrum.
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Lossy quantization raises the noise floor.
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- **Roughness (Coefficient of Variation)**: how jagged/bumpy the spectrum is
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before the cutoff. Lossy encoding introduces quantization noise that creates
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spectral scalloping.
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- **Band Ratio**: energy ratio between 16–20 kHz and 12–16 kHz. Transcodes show
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an unnatural dip in the top octave.
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---
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Supported formats
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-----------------
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| Lossy | Lossless |
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|---|---|
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| MP3 | FLAC |
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| AAC / M4A | WAV / AIFF |
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| Ogg Vorbis | ALAC |
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| Opus | WavPack (WV) |
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| WMA | APE |
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| AC3 / EAC3 | DSF / DFF |
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| MP2 / MP1 | |
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Support depends on the browser's built-in decoder (all modern browsers support
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the most common formats).
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---
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Verdicts
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--------
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| Verdict | Input codec | Meaning |
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|---|---|---|
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| **NATIVE** | lossy | Single encode at the stated bitrate - genuine |
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| **UPSCALED** | lossy | Re-encoded from a lower bitrate (e.g. 128 → 320 kbps) |
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| **GENUINE** | lossless | No evidence of lossy origin |
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| **TRANSCODE** | lossless | Originated from a lossy source, decoded to lossless |
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| **SILENT** | any | No detectable audio content |
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---
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Detection logic
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---------------
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The verdict is determined in two layers:
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**Layer 1 (Primary)** - Cutoff + Steepness. A low cutoff combined with a sharp
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drop (low steepness) is a definitive sign of a lossy encoder's lowpass filter.
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**Layer 2 (Secondary)** - Roughness + Band Ratio. Applied when the cutoff is
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high enough to pass Layer 1. Catches transcodes where the cutoff is near
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Nyquist but the spectrum still shows quantization artifacts.
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Lossy files are checked for upscaling by comparing the cutoff ratio to
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bitrate-specific expectations.
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---
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Confidence score (0–100%)
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-------------------------
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A continuous score based on how far the metrics deviate from the thresholds.
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Higher confidence means stronger evidence supporting the verdict.
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---
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How it differs from the original tcd CLI
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-----------------------------------------
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| Feature | tcd (CLI) | tcdweb |
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| Platform | C program, command line | Pure JavaScript, browser |
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| Decoder | FFmpeg (system dep) | Web Audio API (built-in) |
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| Analysis | 4096 FFT, same algorithm | 4096 FFT, same algorithm |
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| Realtime spectrum | Terminal TUI | Canvas-based interactive chart |
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| Playback | No | Yes, with waveform and seek |
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| BPM detection | No | Yes (autocorrelation) |
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| Metadata parsing | No | ID3v1/v2, FLAC, APE |
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| Stereo mode / VBR info | No | Yes (MP3 header parsing) |
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| File size limit | None | Browser memory (~2 GB typical) |
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---
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Limitations
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-----------
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- **Browser audio decoding** limits which formats work (depends on `AudioContext.decodeAudioData`).
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- **Very short files** (< 4096 samples) cannot be analyzed.
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- **High-bitrate lossy encodes** (320 kbps MP3, 256 kbps AAC) may not be
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distinguishable from lossless by cutoff alone.
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- **Already-filtered material** (e.g. deliberate 15 kHz LPF during mastering)
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may produce false positives.
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---
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References
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----------
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- Nyquist–Shannon sampling theorem - Wikipedia
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https://en.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem
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- Equal-loudness contour (Fletcher–Munson curves) - Wikipedia
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https://en.wikipedia.org/wiki/Equal-loudness_contour
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- LAME MP3 encoder psychoacoustic model - Hydrogenaudio Knowledge Base
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https://wiki.hydrogenaudio.org/index.php?title=LAME
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- "Audio Authentication Using Spectral Analysis" - University of Michigan
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open-access thesis, 2025
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https://doi.org/10.7302/28306
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