In a previous post, I analyzed a video synchronization test from a recent video installation, and suggested that although the synchronization between screens was not perfect, it was certainly satisfactory, and as good as might be expected given the design of the system. Now, lets see why.
When a multi-screen video system is in proper sync, each video frame begins at exactly the same time. A system that draws at 60fps will draw each frame in approximately 16.67 milliseconds. During those 16ms, an LCD display will update all of the pixels on screen from top to bottom. We will call the moving line across which the pixels are updated the Raster Line. In this slow-motion test video, you can see video frames alternating between full black and full white, updated from top to bottom. The screens are mounted in portrait orientation, which is why the updates happen right to left:
Many of the screens seem to be updating together, but some do not. This is because the system does not include dedicated hardware to ensure that the signals are in sync, so many of the displays begin their updates at different time. The system is frame-synchronized, meaning that all displays begin a raster sync within one raster-pass of each other. It just isn’t raster-synchronized.
If the displays were indeed raster-synchronized, we might represent their signals like so:
In an otherwise black video, Display 1 flashes a single frame (Frame 1 / F1) of white, followed by a flash of white on Display 2 in Frame 2. In slow-motion, we would see the raster line, represented here as diagonal lines, move across bother displays in sync, like two Tangueros walking together across a dance floor. The red line represents the particular pass of the raster line at which both displays transition from Frame 1 to Frame 2. In all of these illustrations, the red line indicates a pass of the raster line, or a switch between frames, that in a frame-synchronized system would occur at exactly the same time.
It is important to keep in mind that in this system, video is provided at 30fps, so each frame of video is essentially drawn twice in two consecutive passes of the raster line. This cannot be seen on screen as no change occurs in any of the pixels, but we can see it in our illustration as the diagonal line separating F1a and F1b, the two rendered passes of video Frame 1.
In our system, of course, the raster lines are not synchronized between displays. So even when we intend for both displays to flash white at the same time, it is possible that one display might begin a raster pass at the very end of another’s pass of the same frame:
We can certainly hope that the raster lines of any two average displays are nearly synchronized, but in observance of Murphey’s Law, we must always assume the worst case, as indicated above in Displays 1 and 2. Here, we can see that although the frames might be in sync, with all rasters commencing within 17ms of each other, we will expect to see the raster on Display 2 commence just at the end of that of Display 1. This kind of behavior can be seen in our test video on the third and fourth columns, with the raster seeming to pass smoothly from one display to the other. In truth, any case in which two rasters start more than 16.67ms apart from each other demonstrates an imperfect frame sync, but for simplicity we will just say that the worst case in one in which they commence 17ms apart, as illustrated above in Displays 1 and 2.
So, what might this look like in a case where we see flashes in two consecutive video frames on two separate displays in a worst-case scenario?
In the case of Displays 1 and 4, we have a point in time, Time X, at which both displays are entirely black. In the case of Displays 2 and 3, at Time X we see both displays entirely white. We still consider them to be in sync, and we should not consider these anomalies to indicate a failure of the synchronization mechanism.
There is a minor issue in the test video that does bear mentioning. The flashes move through four rows of white in each column. There are extremely brief moments during which you might notice a touch of white that is visible in the first and third row. This should be obvious after repeated viewing. I leave it as an exercise for the reader to demonstrate why, even in a worst case scenario, this should not be observed if the frame synchronization mechanism is working properly. (<sarcasm>Yeah, right.</sarcasm>) So why am I not concerned? Because it is the nature of LCD display pixels to take some time to switch from full white to full black. Typical response times for the particular displays in this system are specified as 9ms, which means that after the raster line passes and updates a pixel from black to white, it may take an average of 9ms (more than half a raster pass) for that pixel to fully change. I say “Average” because white-to-black and black-to-white transitions are usually slightly different, and the spec will mention only the average of the two. If the response time was an ideal zero ms, our raster line would be crisp and clear in our slow-motion capture, but in reality it is not. The raster line is blurry because after it passes, the pixels take time to change between black and white. We can expect that some pixels might remain white for a brief time after we expect them to go dark, resulting in this subtle observable discrepancy.
What does all this mean? It means that in a slow-motion test video of 24 synchronized displays, we observe nothing to suggest that the synchronization mechanism isn’t performing as well as we could hope for. To the viewer, the synchronization is true, and we deem the project a success.