How Transcode Detection Works
Lossy codecs like MP3 throw away high frequencies. When someone re-encodes an MP3 into FLAC, the FLAC is technically lossless but only contains what the MP3 had — a fake lossless file. On a spectrogram this shows as a hard, flat lowpass cutoff with empty black space above it, instead of the natural energy that a real CD rip carries up to about 22 kHz.
Rough cutoff fingerprints: MP3 320 kbps ≈ 20.5 kHz, V0 ≈ 19.5 kHz, V2 ≈ 18.5 kHz, 192 kbps ≈ 19 kHz, 128 kbps ≈ 16 kHz. Genuine CD-sourced lossless reaches ~20–22 kHz with a soft rolloff.
Fake Hi-Res
A real 24-bit / 96 kHz file should contain audio above 22 kHz. If a "hi-res" file hits a wall at 22 kHz (or lower), it was upsampled from a CD — or worse, from a lossy source. This tool flags that too.
Beating "Audio Optimizers"
Some tools (like Platinum Notes) paste faint high-frequency noise above the real cutoff to fool checkers that just look for "any energy up high". This analyzer instead finds the sharp energy step and measures the flat, non-musical texture of the added noise — so it still spots the true cutoff hidden underneath.
Fake 24-bit
A file can claim 24-bit while its samples are really only 16-bit, padded with zeros. The analyzer checks whether the sample values are quantized to 16-bit steps and flags padded "fake 24-bit" files.
Checking a Whole Folder
Use the folder mode to scan an entire library at once and get a sortable table of results. Export a CSV report or an .m3u playlist of just the flagged files. On Chrome and Edge you can also move the flagged files into a /_suspect subfolder (reversible) or delete them — everything stays on your device.
Supported Formats
MP3, FLAC, WAV, M4A/AAC, and OGG/Opus decode in all modern browsers. The file is decoded at its native sample rate so the frequency ceiling stays accurate, and analysis happens entirely on your device.
How to Read a Spectrogram
Experienced listeners judge by eye rather than trusting any automatic verdict — automated detectors (including the well-known desktop ones) are notorious for flagging genuine tracks and missing real fakes. Here's what to look for:
- Sharp brick-wall cutoff — a hard, flat horizontal edge with empty black above it is the classic lossy fingerprint. A genuine recording fades out gradually.
- MP3 "blocks" — the definitive tell. Zoom into a ~1 second window: lossy encoding leaves a faint blocky, gridded texture in the high frequencies. You can only see it at high zoom, which is why this tool lets you drag-select a region to inspect it up close.
- Horizontal lines are NOT lossy artifacts — they're usually hum or electrical interference from analog equipment. Don't mistake a thin high line for genuine full-band content.
- Noise is broadband — real high-frequency energy (or dither) spreads across a range of frequencies, never a single line.
- Use a linear frequency scale (this tool does by default) and compare both channels — some fakes differ between left and right.
Important: This Is Evidence, Not Proof
Some genuine recordings are naturally band-limited — old analog masters, certain acoustic material, or particular mastering choices. A gradual low cutoff is reported as "inconclusive" for exactly this reason. The automated result is a screening hint; the spectrogram image is the real arbiter, so read it with your own eyes.