Video Latency
Definition
Video latency is the delay between the moment an image is captured and the moment it is displayed. It accumulates at each stage: sensor readout and camera processing, format conversion and scaling, the switcher, compression for IP transport, and processing in the LED processor or the projector. SDI transport adds less than 1 ms, while each frame synchroniser or frame buffer adds up to one frame, about 17 ms at 60 fps and 20 ms at 50 fps. Projectors and displays vary widely according to model and mode. In image magnification (IMAG), the end-to-end total is commonly a few frames, for example 33 to 67 ms, that is 2 to 4 frames at 60 fps. ITU-R BT.1359 places the detectability threshold at about 45 ms when sound leads the picture and 125 ms when it lags, and EBU R37 sets the broadcast tolerance at +40 ms and −60 ms.
In the field
In IMAG the audience hears the sound before it sees the image on screen, which is the direction to which viewers are most sensitive. The first remedy is to shorten the video chain, with fewer conversions and frame buffers and low-latency modes on processors and displays. Delaying the PA to match the screens is also practised, but it moves the sound away from the performer seen directly on stage, so the choice depends on the venue and is checked from the audience seats.
Sources
- ITU-R BT.1359-1 : seuils de détection d'environ +45 ms (son en avance) à −125 ms (son en retard), d'acceptabilité +90 à −185 ms · itu.int
- EBU R37-2007 : tolérance de synchronisation son/image de +40 ms à −60 ms en diffusion · tech.ebu.ch
- Wikipédia, Audio-to-video synchronization : sources de retard dans la chaîne et correction (pointeur secondaire) · en.wikipedia.org
- Wikipédia, IMAG : reprise caméra sur grand écran et contrainte de latence (pointeur secondaire) · en.wikipedia.org