Latency
Definition
Latency is the delay between the moment a signal enters a system and the moment it leaves it. In air, sound travels at about 343 m/s at 20 °C, roughly 1 ms for every 34 cm, and its speed rises by about 0.6 m/s per degree Celsius. When two copies of the same signal are summed with a time offset, comb filtering appears, and an offset of 1 ms places the first cancellation at 500 Hz. In digital systems, latency accumulates at each conversion, processing stage and network hop. Dante hardware uses a latency setting of 1 ms by default.
In the field
A study presented at the 2007 AES convention (Lester and Boley, Shure) found that latency tolerance varies with the instrument and is lower with in-ear monitors than with floor wedges. Singers are especially sensitive, because they also hear their own voice through bone conduction and the delayed monitor signal combines with it. When FOH and monitors share a console, the processing inserted on monitor paths therefore affects what the performers feel.
Related lessons
Sources
- Lester et Boley, « The Effects of Latency on Live Sound Monitoring », AES 123e convention, 2007, papier 7198 : tolérance variable selon l'instrument, moindre en ears qu'en wedges · aes.org
- Church Production, « How digital audio latency can affect musicians » : synthèse de l'étude AES de Lester et Boley · churchproduction.com
- Speed of sound, formule c ≈ 331,3 + 0,606 × T (air sec) : environ 343 m/s à 20 °C · redcrab-software.com
- Audinate, Dante Controller User Guide : latence par défaut de 1 ms · dev.audinate.com
- Shure, « How to Avoid Comb Filtering with Podium Mics » : filtrage en peigne dû au décalage entre deux captations · shure.com
- ProSoundWeb, « A Matter Of Timing: Clarifying Latency And Putting It Into Context » : voix perçue par conduction osseuse, filtrage en peigne avec le retour retardé, surtout en ears · prosoundweb.com