IMAG — Image Magnification
Definition
IMAG (image magnification) is the live relay of camera images of the stage onto large screens, so that an audience far from the stage can see faces and details. Cameras feed a switcher, and the programme output goes to projectors or LED walls beside or above the stage. The main constraint is latency: the audience hears the sound from the PA and sees the performer directly, so every frame of delay on the screens makes the image lag behind the sound. ITU-R BT.1359 places the detectability threshold at about 45 ms when sound leads the image, which is fewer than three frames at 60 fps and a little over two frames at 50 fps.
In the field
Lip sync is checked from audience seats rather than from FOH, because the acoustic delay depends on the distance to the stage: sound travels at about 343 m/s, roughly 3 ms per metre, so distant seats receive it later and part of the video delay is offset. When the screen visibly lags the sound, the video path is shortened first, with fewer scalers, frame buffers and processors in series and low-latency modes. Delaying the PA to match the image is also used.
Sources
- ITU-R BT.1359-1 : seuil de détection d'environ 45 ms lorsque le son précède l'image · itu.int
- EBU R37-2007 : tolérance de synchronisation son/image en diffusion · tech.ebu.ch
- Wikipédia, IMAG : reprise caméra sur grand écran, latence (pointeur secondaire) · en.wikipedia.org
- Wikipédia, Speed of sound : environ 343 m/s dans l'air à 20 °C (pointeur secondaire) · en.wikipedia.org
- Biamp, The Impact of Camera Latency on Lip Sync in Live Environments : latence caméra et synchronisation labiale en direct, retard du son pour l'aligner sur la vidéo · blog.biamp.com