Sep/091
Output VGA Video Via USB Ports
Here’s something you don’t see everyday. Remember that USB 2 HDMI PC adapter? I can’t remember the company. Anyway, it was supposed to mux audio video signals from your audio card and USB port and output them to HDMI signals.
looks like WITECH came up with another similar USB-video idea. Their Multi-I USB video card provides two standard VGA output connectors and one USB-B input. By hooking up the device at an PC’s USB port you can output video via the two VGA ports at a maximum resolution of 1400×1054. This uses the DisplayLink technology which can convert USB data to video output. You can also interconnect more Multi-I devices (also known as daisy-chaining) so that you can broadcast single input signal to tens of displays at the same time.
It is to bad that the company had to settle for a maximum of 1400×1050 resolution. Most 22” widescreen LCD’s come with a 1680×1050 resolution so this might be a problem. But the truth is this isn’t their fault. It’s a hardware limitation. Let me explain why. We have to start by knowing that the an Usb 2.0 connection provides a theoretical limit of 480Mbits/second. We have 1400×1050 pixels so that means a total of 1470000 pixels.
Each pixels requires 32bits of data (8 bits per color channel – RGB, and 8 bits for alpha). This adds up to 47040000 bits of data. We divide this big number to 1024 twice. The first value will represent the same amount of data in kilobits and the later the same amount in megabits. So we end up with approx. 45Mbits of data for a single frame. But remember that in order to have smooth video output you need at least 24 or 25 frames per second (30 being even better). So if you do the math you find out that at max. resolution, the USB 2.0 port would need to output data at over 1000Mbits/s (1076 actually).
How is that possible? It isn’t. This is obtained by using the proprietary DisplayLink technology which uses an image compression algorithm. The algorithm works by utilizing some amount of CPU processing power, so if you have a slow CPU you can forget about using that 1400×1050 resolution.
A compression algorithm has to achieve a balance in the compression ratio and resource usage. If they where to make the algorithm more aggressive CPU powered would have been compromised. An image can only be compressed up to a certain point without quality loss. So even with a more aggressive algorithm, there are minimal chances for improvement. We’ll juast have to wait for USB 3.0.