Levels & dynamics

True peak

Playback does not simply jump between stored points: a waveform is reconstructed between them. It may exceed the ceiling even when no stored point does.

Reading the result

Above 0 dBTP, the estimated reconstruction exceeds full scale. This describes headroom, not proof of audible distortion on every device.

Illustrative synthetic signal. Drawing dimensions do not represent detector thresholds or durations; these are specified in the method below.

Go deeperMethod, limits, verification and sources

True peak estimates the maximum of the waveform reconstructed between stored PCM samples. A sample peak below 0 dBFS can still reconstruct above full scale, which is called an inter-sample over. The result is reported in dBTP, where 0 dBTP is full scale.

Calculation#

Each decoded channel is oversampled by a factor of four using a 48-tap, Blackman-windowed sinc low-pass FIR. The streaming polyphase implementation has 12 taps per phase and evaluates four outputs for each input sample. At end of stream, eleven zero frames drain the FIR history so the final audio samples receive the complete reconstruction filter. The peak is the maximum of the oversampled magnitudes and the original stored-sample peak, across every channel:

dBTP = 20 × log10(oversampled peak)

Silence has no finite logarithmic peak and is represented internally by negative infinity. The measurement is deliberately separate from the clipping counter, which uses only stored samples.

Display interpretation#

The app colours the result as a delivery-headroom cue:

dBTP	Display meaning
At or below −1	Green headroom cue
Above −1 through 0	Neutral
Above 0 through +1	Warning cue
Above +1	High-over cue

These colours help compare files; they are not a statement that every platform or codec has identical headroom requirements.

Limits#

The meter uses the BS.1770-style 4× oversampling approach and its own fixed 48-tap filter. It is not presented as formal conformance testing for every BS.1770 implementation, codec or DAC reconstruction filter. Different filters, sample-rate handling and end-of-stream treatment can differ slightly. True peak is a level estimate, not a clipping diagnosis or an audible-distortion score.

Tests and manual fixture#

cargo test -p flaccompagnon-core analysis::truepeak

The unit suite includes a quarter-rate sine sampled away from its crest: its stored peak is below 0.70 while the 4× meter recovers a peak above 0.95. An independent zero-stuffed convolution checks overshoot from the final two samples, and an impulse regression checks that reconstruction never underreports the stored sample peak. For a practical comparison, import a loud clipped fixture from Clipping and compare the clip event count with its dBTP value.

Reference: ITU-R BS.1770-5.

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