True peak vs sample peak: why your −0.1 dB master still clips

Your limiter reads −0.1 dBFS. No red lights, no clipping. Then you export an MP3, play it back, and it distorts. The culprit is the gap between sample peak and true peak.

Two different "peaks"

Sample peak is the highest value among the actual digital samples in your file. It's what a normal peak meter shows. Keep it under 0 dBFS and no sample clips.
True peak is the highest point of the continuous analog waveform that gets reconstructed from those samples on playback. And that waveform can rise higher than any single sample.

Digital audio stores discrete points. To play them, a converter draws a smooth curve through the points — and between two samples that curve can overshoot both of them. That overshoot is an inter-sample peak. Your file can read −0.1 dBFS at the sample level while its reconstructed waveform actually crosses 0 dBFS.

Where it bites

Two places turn an inter-sample peak into audible distortion:

D/A converters. When the converter reconstructs the waveform, an inter-sample peak above 0 dBFS clips at the output stage — even though no stored sample was over.
Lossy codecs (MP3, AAC). Encoding to a lossy format changes the waveform slightly, and that can raise peaks further. A master sitting right at 0 dBFS sample peak routinely gains inter-sample overs after encoding — exactly the format your streaming listeners hear.

So "−0.1 dBFS, no clip" on your sample meter is not a guarantee. The thing that clips is the true peak, and a sample meter never showed it to you.

How true peak is measured

ITU-R BS.1770 defines true-peak measurement by oversampling — reconstructing the between-sample curve (typically at 4× or higher) and reading its real maximum. The result is reported in dBTP (decibels true peak). A true-peak meter is doing this oversampling for you; a plain sample-peak meter is not.

The fix, in one line

Measure and limit true peak, not just sample peak, and leave headroom below 0 — commonly −1 dBTP — so inter-sample overshoot and lossy encoding have room to breathe. (The how-to is in How to set a true-peak ceiling.)

Takeaways

Sample peak = tallest stored sample; true peak = the reconstructed waveform's real maximum (inter-sample peaks).
A master under 0 dBFS sample peak can still exceed 0 dBFS true peak — and clip in a converter or after MP3/AAC encoding.
True peak is measured by oversampling per ITU-R BS.1770, reported in dBTP.
Limit true peak and leave headroom (−1 dBTP is the common target).

Part of the Loudness and true peak guide.