DVI/HDMI vs VGA in digital signage

DVI/HDMI vs VGA in digital signage

As DVI and HDMI connections become more and more widely used in digital signage, we are often asked: which is better, DVI (or HDMI) or VGA? There is no clear answer to this question.

First, let’s get some basic knowledge of the elements. DVI and HDMI are exactly the same as one another regarding image quality and resolution. The main differences between HDMI and DVI are that HDMI carries audio and video signal; further more HDMI uses different types of connectors. Technology wise both HDMI and DVI uses the same encoding technology, and for that reason DVI source can be connected to an HDMI connector on display, or vice versa, with a DVI/HDMI cable, with no use of signal converter

What are the difference between DVI, HDMI and VGA?

DVI/HDMI and VGA Video are all video signals which support a variety of resolutions, each one of them deliver the signal from source to display in different ways. The main difference is that DVI/HDMI delivers the signal in a digital format. VGA is an analog format, which deliver the signal, not as a digital stream, but as a set of varying voltages representing the red, green and blue components of the signal.

DVI/HDMI and VGA deliver signals as red, green, and blue color components, together with sync information. The DVI/HDMI standard delivers these along three data channels in a format called T.M.D.S., which stands for “Transmission Minimized Differential Signaling”. This basically involves a blue, red and green sync that are added, and separate the channels

VGA is delivered, almost similarly, with the color information split up three ways. However, VGA uses a color difference type signal, which consists of Luminance, the green red or blue channel, representing the total brightness of the image. The sync pulses for both horizontal and vertical are delivered on the Y channel.

Both HDMI/DVI and VGA signal types are fundamentally quite similar; they break up the image in similar ways, and deliver the same type of information to the display. How they differ, as we’ll see, will depend to a great extent upon the particular characteristics of the source and display devices, and can depend upon cabling as well.

What is better Digital or Analog?

Digital signal transfer, it is assumed, is error-free, while analog VGA signals are always subject to some amount of degradation and information loss. There is an element of truth to this argument, but it tends to back fire in real-world testing. 
First, there is no reason to get signal degradation of an analog VGA signal in digital signage installation where the distance between the player and the screen is short. Digital signage installation in a large retail or education facility for example can present a challenge for analog cabling. But, it is a flawed assumption to suppose that digital signal handling is always error-free. DVI and HDMI signals aren’t subject to signal error correction like downloading a file; once information is lost, it’s lost for good. That is not a consideration with well-made cable over short distances, but can easily become a factor at long distance.

So what should I use for best Image Quality?

DVI or HDMI signal formats is pure digital. There are some arguments that by taking a digital signage player and connecting it straight into digital connector as a DVI or HDMI signal, and then delivering that digital signal straight to the digital signage display, there is a sort of a perfect no loss of information signal. If the display itself is a digital display like LCD, the signal never has to go through digital to analog conversion and therefore is less altered on the way.

The Role of Cable in picture quality

Picture quality, in general, should not be a significant factor in the DVI/HDMI in short distance bellow 50 ft, as long as the cables are of high quality. There are, however, issues when you centralize your player and you send the signal to a display 200ft a way. In this kind of environment analog or digital signal will have problem without using DVI/HDMI/VGA over cat 5 extender. Without using these extenders signal quality issues can come into play.
DVI and HDMI connections, uses SDI technology (serial digital video) which was designed originally to run on coax where impedance can be controlled and compensate. DVI/HDMI signals are run balanced, using twisted-pair cable technology which control impedance to about +/- 10%, and for this reason, are subject to the digital cliff phenomenon. Up to some distance, a DVI or HDMI cable will perform just fine and will not compromise the ability of the display device to reconstruct the original bit stream without information lost. 
As we run longer distances, the ability to reconstruct the bit stream decreases. As a result unrecoverable bit errors start to occur. As distance increases more information is lost to a point that the display becomes unable to reconstitute enough information to render an image. A DVI/HDMI cable that works perfectly at 40 feet may not work at 80 feet. It is very hard to determine the exact distance a DVI or HDMI cable will fail, most quality HDMI /DVI cable will work fine up to 50ft.

The Answer: It Depends

So, which is better, DVI/ HDMI or VGA? The answers unsatisfying, but the truth is that it depends. It depends upon your source player, type of cable, type of display and the distance, and there’s no good way, in principle, to say in advance whether the digital or the analog connection will draw a better picture. You may find that some digital signage player looks better through its DVI or HDMI output, while a different player using VGA output better image through its RGB cable, on the same display. Some installers reports ghosting/blurriness with text using analog and when switching to digital interface made things much crisper. On the other hand, other installers who used DVI/HDMI long cable found that the text was blurry, colors were off and the image didn’t scale to fit correctly and got no issues at all with VGA. In this case, if you are using cable longer than 50ft you should use VGA over Cat5/6 extenders that will control picture quality over distance over 50ft like Minicom.