Audio Analysis: Finding Fake Lossless

Losselot uses dual analysis to detect transcodes: binary metadata inspection and FFT-based spectral analysis.

The Quick Version

Lossy codecs like MP3 remove high frequencies. This creates permanent scars:

Source Quality	Typical Cutoff	What’s Missing
128kbps MP3	~16kHz	Cymbals, air, sparkle
192kbps MP3	~18kHz	Some high-end detail
320kbps MP3	~20kHz	Only ultrasonic
True Lossless	~22kHz	Nothing

Converting an MP3 to FLAC doesn’t restore lost frequencies. The scars remain.

Spectral Analysis

The Spectrogram

Spectrogram Example

A spectrogram shows frequency (vertical) over time (horizontal). Brightness = energy.

What to look for:

Horizontal cutoff lines - Where frequencies suddenly stop
Dark bands at top - Missing high frequencies
Consistent cutoff - Same frequency across entire track

The Frequency Response

Frequency Response

The frequency response curve shows exactly where audio cuts off. A sharp drop around 17-20kHz with steep rolloff is the telltale sign of lossy compression.

Binary Analysis (MP3)

For MP3 files, Losselot reads encoder metadata embedded in the file.

LAME Headers

The LAME encoder stores a “lowpass” value that reveals original encoding settings:

Header: LAME3.99r
Lowpass: 16000 Hz
Bitrate: 320 kbps

⚠️ MISMATCH: 320kbps should have ~20kHz lowpass

The smoking gun: A “320kbps” MP3 with a lowpass of 16kHz was definitely transcoded from a 128kbps source.

Re-encoding Detection

Multiple encoder signatures in one file indicate re-encoding:

Found: LAME3.99r (at offset 0x100)
Found: LAME3.100 (at offset 0x2a0)
Found: Lavf58.29.100 (at offset 0x340)

⚠️ ENCODING CHAIN: LAME → LAME → FFmpeg

Each lossy pass causes cumulative damage.

CFCC: Lo-Fi Detection

Not all high-frequency rolloff indicates lossy compression. Tape recordings and lo-fi productions have natural rolloff.

The Problem

MP3 @ 160kbps:          Tape Recording:
       |                        \
       |                         \
       |_____                     \___
      16kHz                      varies
   (brick wall)              (gradual rolloff)

Both have missing high frequencies - but for different reasons.

The Solution

Cross-Frequency Coherence Coefficient (CFCC) measures correlation between adjacent frequency bands:

MP3: Sudden decorrelation at cutoff (cliff)
Tape: Gradual decorrelation following dynamics

How It Works

Divide spectrum into adjacent bands
Measure correlation between each pair
Look for sudden correlation drops

Band Pair    | Correlation | Change
-------------|-------------|--------
14-15 kHz    | 0.92        | -
15-16 kHz    | 0.89        | -0.03
16-17 kHz    | 0.31        | -0.58  ← CLIFF DETECTED
17-18 kHz    | 0.28        | -0.03

A correlation drop > 0.25 between adjacent bands signals a codec cliff.

Scoring

Signal	Score Impact
`cfcc_cliff` detected	+25
`decorrelation_spike`	+15
`lofi_safe_natural_rolloff`	-15

The negative score for natural rolloff prevents false positives on legitimate lo-fi recordings.

Mixed-Source Detection

Some productions use both lossy and lossless sources:

22kHz  |    ██     ██  ██      |  ← Lossless elements
       |    ██     ██  ██      |
16kHz  |████████████████████████|  ← MP3 samples
       |████████████████████████|
0 Hz   |████████████████████████|
       0:00              4:00

The “pillars” of high-frequency content punching through indicate:

Base track from MP3 samples
Some elements (cymbals, synths) from lossless sources

This is not a fake - the file honestly contains what it contains. But it reveals the production used pre-lossy material.

Verdicts

Verdict	Score	Meaning
CLEAN	0-34	Looks like genuine lossless
SUSPECT	35-64	Something’s off - investigate
TRANSCODE	65-100	Almost certainly from lossy source

Detection Flags

Spectral Flags

severe_hf_damage - Major frequency loss
hf_cutoff_detected - Clear lossy cutoff pattern
weak_ultrasonic_content - Not enough content above 20kHz
dead_ultrasonic_band - Virtually nothing above 20kHz
steep_hf_rolloff - Unnaturally sharp cutoff
cfcc_cliff - CFCC detected codec cliff
lofi_safe_natural_rolloff - Natural rolloff (reduces score)

Re-encoding Flags

multi_encoder_sigs - Multiple encoders detected
encoding_chain(LAME → FFmpeg) - Specific chain identified
lame_reencoded_x2 - Double LAME encoding
lowpass_bitrate_mismatch - Lowpass doesn’t match bitrate

Try It

# Analyze a single file
./losselot suspicious-file.flac

# Analyze a folder
./losselot ~/Music/

# Interactive web UI
./losselot serve ~/Music/ --port 3000

# Quick scan (binary only, no FFT)
./losselot --no-spectral ~/Music/

← Back to Home

Next: Decision Graph →