And now, by popular demand, (AA says: “I’m popular?”) a few high-speed captures of LED signs in Times Square:
(All videos were captured at 1000fps with a Casio Exilim EX-FS10, rendered here as 30fps video.)
When transitioning from one frame to the next, the M&M’s sign and all components of the ABC sign update all pixels on the sign at once, demonstrated here in less than one of my video capture frames, so we know it’s less than 1ms. The different components of the ABC sign seem to update at different times because of a sync issue, not an LED technology issue. (more…)
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. (more…)
