True Peak (dBTP)
Definition
True peak is the maximum level of the continuous waveform reconstructed from digital samples, expressed in dBTP. It can exceed the highest sample value, because the reconstructed signal can peak between two samples (inter-sample peaks). ITU-R BS.1770 specifies its measurement by oversampling the signal by a factor of 4. EBU R128 sets a maximum of −1 dBTP and ATSC A/85 a maximum of −2 dBTP. A file whose sample peaks stay just below 0 dBFS can therefore contain true peaks above 0 dBTP, which can clip a digital-to-analogue converter or a downstream codec.
In the field
Final delivery limiters are usually set to a true-peak ceiling of −1 dBTP rather than 0 dBFS. Lossy encoding (AAC, MP3) can raise peaks, so AES TD1004 recommends a maximum of −1 dBTP for streamed audio, with a safety limiter placed before the encoder.
Sources
- ITU-R BS.1770-5 (2023), annexe 2 : crête vraie mesurée par suréchantillonnage 4× · itu.int
- EBU R 128 : crête vraie maximale −1 dBTP · tech.ebu.ch
- Auphonic, EBU R128 et ATSC A/85, ATSC A/85 : −24 LKFS, CALM Act · auphonic.com
- AES TD1004.1.15-10 : crête maximale −1 dBTP et limiteur avant encodage avec perte · aes.org
- NUGEN Audio, « Loudness vs True Peak » : crêtes inter-échantillons au-dessus de la crête d'échantillon · nugenaudio.com
- ATSC A/85:2013, « Techniques for Establishing and Maintaining Audio Loudness for Digital Television » : crête vraie maximale −2 dBTP · atsc.org