In this section, we’ll consider the most common questions you may have (and of course we’ll provide the answer, too). Answers are intentionally brief – perhaps just a reminder of one or two steps required to perform some operation. For this reason, we’ll also point you to explanatory information elsewhere in this manual whenever that would be useful.
Testing, One Two
Professionals do not leap into new environments blindly. They prepare, plan, plan some more, and then (most importantly) they test. This allows them to tackle the really tough jobs with confidence.
IMAG and Latency
What’s IMAG? It’s a compression of the expression Image MAGnification. Typically in modern IMAG applications, video cameras supply live imagery to projection systems, magnifying speakers and performers so that audience members further back in large venues can still see what’s going on.
IMAG is a very tricky task at the best of times, one that calls for excellent planning, and where possible, testing. Those designing an IMAG installation have, not just one, but two inter-related broadcasts to consider – in the form of the audio and video streams.
Relativity and the Speed of Light
Wouldn’t it be nice if audio and video travelled from their respective broadcast devices at the same speed? Then, wherever you were seated in the audience, the sound from hypothetically perfect speakers and the video image from huge video displays co-located at the front of the auditorium would reach your ears and your retinas at precisely the same moment!
This is not the case, however. Sound travels quite slowly – so slow, in fact, that even in relatively small venues it reaches those in the rear of the audience noticeably later than those in the front.
In loose terms, for a mid-size auditorium 600 feet long, it takes around a half-second for the audio to reach those in the back. For this reason, audio engineers often position speakers throughout the house, then introduce carefully considered delays by electronic means – to ensure late sound from front speakers does not arrive after sound from the nearest speaker to those further back.
Light, on the other hand, travels so much faster that for all intents transmission can be considered instantaneous in the same setting. So a person in the rear sees the image on a screen at the front long before sound from a co-located speaker arrives.
If transmission of the video signal from the camera lens right through to the projection screen were instantaneous (it’s not, mind you), we’d likely need to find a way to delay it. Viewed in this light, a certain amount of latency is actually a good thing!
Latency and Your Audience
Latency. What’s that you ask? In this context, latency refers to the time it takes for the video signal to transit one part of the IMAG path, from camera lens to final display. Latency is usually expressed either in milliseconds or in video frames (typically either 25 or 29.97 to the second).
As we said above, really a little video latency is not a bad thing as viewed from a surprisingly short distance back into the audience. This is just as well, since for all practical purposes a little latency is also unavoidable.
Even so, as long as audio and video are in sync at your seat, only a rather significant degree of latency is objectionable, unless you happen to be very near the stage. For those in the front rows, a few extra frames of latency may be rather disconcerting. (It’s true that IMAG was conceived primarily for the benefit of those further back, but if the latency is too obvious for those nearest the front it can be disconcerting distraction).
For this reason, it’s desirable to keep video latency to an agreeable minimum – but put away any notion of zero latency. Not only would this require bending the laws of physics, it would be a bad idea.
Even before considering minimizing latency in the device chain, acknowledging that there is always going to be some latency calls for some creative thinking with regard to practical staging. For example, if you design your IMAG layout in such a fashion that those in the front row are unlikely to be able to see the screen(s) without lifting their eyes from the onstage talent, they are extremely unlikely to notice a small amount of latency.
Latency and Your System
Vizrt systems excel at IMAG, but inevitably constitute just one (albeit critical) link in a chain of devices. It is common for each device to contribute a little to the combined total latency for the system.
Your Vizrt system’s portion of the total latency is well within acceptable standards for devices of this sort – roughly between 1 and 2.5 frames (the actual amount can vary slightly within this range, depending on several factors).
For example, suppose a video frame supplied by a camera arrives at an input one millisecond after a different frame has been sent to the output. Obviously the new arrival must wait its turn; it can’t be transmitted until the correct duration for its predecessor has elapsed. Thus the newly arrived frame must wait almost one entire frame, until its turn comes.
Tip: You can actually assess the latency of your pipeline fairly easily. Run timecode directly to 1) a monitor and at the same time 2) through the switcher to a second (identical) monitor. Take a photograph that encompasses both monitors, and compare the timecode shown.
Other Sources of Latency
More often than not, significant latency is added by other devices in the IMAG chain that come after the switcher. Projectors are a common contributor, but at times the cameras themselves are a factor.
Here are some helpful points to consider when designing and connecting your system:
If you use Multiview output from the switcher to the projector – if at all possible, match the resolution sent from the switcher to the native resolution of the projector. (On some projectors this allows the unit to avoid using its own internal scaling, which is often a significant factor in unwanted latency.)
When possible, try supplying the projector with analog video. This can eliminate a lot of the complexity from the process (of course, this is not always possible.)
Certain projectors provide a low latency mode to disable features of the unit that carry a heavy toll in latency. Enabling this mode can make positioning the projector slightly more challenging (as you may sacrifice some ability to position and scale the image using projector menu functions), but the latency reduction can be very worthwhile.
Some cameras include features that add more latency than you would expect. For instance, image stabilization (by definition) adds one field of latency and sometimes more. Disable anything of that sort that you can.