Though DVI was soon superseded by HDMI in view of the capabilities of the latter to carry both digital video and digital audio over the same interconnect, yet it is still a very popular interface with an estimated 50 million DVI enabled devices that are expected to double within the coming two years.

At the same time, HDMI - released in December 2002 and backed by major electronic manufactures - is soon becoming the preferred interconnect in the home theater - partly as it helps to minimize the maze of cables behind home entertainment centers.

 

Image quality wise, DVI and HDMI are one and the same thing - using the same TMDS protocol to transmit digital data from source to sink over an appropriate DVI or HDMI cable. This also explains why all that is required to connect a DVI-D with an HDMI enabled device is the use of a DVI to HDMI cable adaptor terminated with the appropriate end connectors.

In the mist of all the development in this fast changing digital world, the dilemma still remains - which is capable of producing a better image - the latest digital interconnects, or analog, DVI and HDMI, or component video and the various flavors of RBG?

The simple truth is that there is no straightforward answer to this rather complex question. Digital does not always implies better image quality in case of a video connection. 

It is not possible to predict whether DVI and HDMI cables will perform better than an analog video connection; there are significant differences in the way digital and analog signals are handled that are heavily dependent upon the characteristics of the source device and the display rather than on the type of video connection in use ...but why is this so?

DVI, HDMI, Component Video & RGB

What are the differences?

Prior to continuing with our discussion, it would be appropriate to have an understanding of the main differences and similarities between these different video standards.

First of all, DVI, Component Video and RGB, are all pure video interconnects; HDMI is a hybrid in that it carries both audio and video. When it comes to video, all four standards are capable of supporting a wide range of video resolutions from standard definition to High-Definition video. In other words, they all have the bandwidth to support the highest image resolutions possible today.

The principal important difference is that DVI and HDMI deliver the signal in a digital format - using a bit-stream of ones and zeros to convey picture information. Instead, Component Video and RGB are analog standards that deliver the signal in the form of continuously varying voltages.

However, one very important common characteristic between these four video standards is that all four deliver the red, green, and blue color information on separate data channels or 'wires' over the respective video interface; this applies irrespective of whether we are talking about a digital DVI or HDMI cable connection, or an analog RGB or Component Video interconnect.

 

So how is this done?

DVI and HDMI Standards: As already expressed in our HDMI Cable Guide, these digital standards deliver the red, green, blue and synch information over three separate data channels using the same unique TMDS encoding protocol.

TMDS stands for transition minimized differential signaling; it conveys data by transitioning between 'on' and 'off' states while utilizing an advanced encoding algorithm to minimize the transitions necessary to transport data between the DVI or HDMI 'source' e.g. an HDMI-enabled digital satellite TV set-top box, and a DVI or HDMI-enabled 'sink' e.g. a digital television.

The process requires that video information is transmitted as a series of 24-bit pixels - 8 bits each for each of the primary colors. These three '8-bit packets' are encoded using TMDS into three 10-bit words and then transmitted over three data channels using separate differential pairs on the DVI or HDMI cable.

Component Video: Component video splits up color information over three components in a somewhat similar manner to DVI and HDMI - however, the actual content of the three video components is different from the three data channels in a DVI or HDMI cable interconnect.

While in a DVI or HDMI data link, each of the three channels represent separate but complete red, green and blue information, component video uses 'color-difference' information carried over three different signal-carrying wires representing:

• Luminance - also referred to  as 'Y', or 'green' channel; this is the total brightness of the image along with the sync pulses

• Red Minus Luminance - referred to as 'Pr' or 'V' channel

• Blue Minus Luminance - referred to as 'Pb' or 'U' channel.

This also explains why component video is also known as 'Y/Pb/Pr' or 'YUV'. From these signals, the display device separates out the sync information and reconstitutes the red, green and blue components of the picture.

RGB and its variants: RGsB, RGBS, and RGBHV: To a certain extent, RGB represents the origins of component video.

RGB comes in three principal varieties, each requiring a different number of connections. The most common type is RGBHV, with five lines: one for red, one for green, one for blue, one for the horizontal sync and one for the vertical sync. RGBHV is the standard used in VGA and other analog PC computer monitors. RGBS, having four connections, differs from RGBHV in that the vertical and horizontal sync are combined on a single channel, while RGsB or 'sync-on-green', places the sync information on the green channel; note however that this variant of RGB is still not compatible with component video.

 

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It is thus clear that all four video formats are very much similar in nature, breaking up the image in similar ways - though using different forms - to deliver the same type of information to the display.

In other words, as we will soon see in this article, the difference in performance that may arise between these analog and digital standards, is in reality not related to the nature of the video interface used to convey the video information between source and display in as much as to the particular characteristics of the source and display devices, and the quality of the connecting cables and connectors used in the process.

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Is Digital really Better?

There seems to be the misconception among many that digital is always better, but... is it really so?

Many assume that while analog is always subject to an element of degradation, digital transfer of information is error free - the information is represented by just ones and zeros, hence it is easier to ensure that the information received at the display is an exact replica of the information originating at the source.

Surprisingly as it may be, the reality is somewhat different. Technically, there is no reason why there should be any perceivable degradation of an analog RGB or video component signal over substantial distances. The maximum cable runs in home theater installations do not present a challenge for high quality analog cables.

Instead, this needs not be the case with digital interconnects; e.g., HDMI cables are typically subject to a maximum of 15 meters - a distance which can be easily exceeded especially in dedicated home theater installations using a ceiling mounted video projector.

Further more, DVI and HDMI do not use error correction; hence once information is lost, it's lost for good. While this do not factor in over short distances with well-made cables, yet it will surely come into play when working with long DVI and HDMI cable runs.

 

What Determines Image Quality in a Video Signal?

Have you ever thought that from the point of origination of a video signal e.g. DVD, analog or digital TV, etc., till it reaches the point to be displayed as an image onto the respective display device, irrespective of whether this is an analog CRT-based screen or a digital display such as an LCD or Plasma TV, the signal goes through a never ending list of processes and conversions?

And this applies even if we were to start with a digital source to display an image over a digital screen. Every single process in the chain would eventually leave its mark in the end result.

For example, very few digital displays operate at the native resolutions of common source material; this would necessitate the use of scaling techniques to match the source with the screen native resolution. Scaling may introduces a number of video artifacts that are surely independent of the component video or HDMI cable connection used in the process to transfer the video information between source and display device.

But there are many other signal processes to go along the whole signal chain, from the reading of data from an optical media and the transferring of such into electrical signals, to the presentation of colors and the rendering of picture information on the display device.

What's more, many argue in favor of DVI or HDMI cables on the sole basis that if you combine these digital interconnects with a digital source such as a DVD and a digital display, e.g. DLP projector or plasma TV, in order to get an all pure digital setup, then you will get the very best in picture quality. Their reasoning here is that if you remain digital all the way without having to go from digital to analog and back to digital again, there will not be any loss or alteration of picture information.

But there is a very serious flaw in this simplistic approach. Many seems to forget that digital signals along the many different processes are encoded in different ways, and these have to be converted back and forth between these process till the signal is ready to appear as an image on the display device.

Consequently, conversions are always going on. In addition, 'digital-to-digital' conversion is no more a guarantee of signal quality than 'digital-to-analog'.

In other words, it is not possible to state which one is better or worse - this quality issue is highly variable. It is greatly dependent upon the characteristics of the electronic circuitry involved inside the respective devices. This explains why say an HDMI cable may operate perfectly well with one source/display combination, at say 40 feet, but then fails to operate with another source/display combination at even shorter cable runs.

The main problem here is that you never know what exactly is going on inside consumer equipment, and how the different signals are being processed, scaled, decoded, converted, etc. This applies irrespective of whether we are dealing with pure analog, analog and digital, or with pure digital.

And as if to complicate matters, there is also the problem of equipment setting and system calibration. Improper settings would not help in producing the best results while different settings on different inputs will lead to different system response to the same signal when applied across different inputs.

 

Cable and Connector Quality: What are the implications on signal conveyance performance?

We have already discussed in detail the implications of cable built quality and cable length on the overall performance in the conveyance of information between source and display or speakers, in the following articles:

• Quality, Cable Length, and Performance Implications

• HT Connections - An Introduction to Cable Anatomy

We therefore would not go into the relevant detials here.  Rather, what we will be discussing are the implications of cable quality vis-à-vis the difference in performance between analog and digital signals. These differences would not normally emerge as long as we are dealing with high quality cables at short to moderate cable runs. Differences between DVI and HDMI cables  vs RGB and component video however, would soon show up as we lower cable quality or increase the cable length.

The battle here would surely turn out in favor of analog interconnects. RGB and component video are extremely robust signal types; you can easily run 100 feet - possible even 200 feet - of good quality component video cable without any need for boosters, relays or other special equipment, without encountering any signal quality issues. Surely, at the longer lengths, cable quality is a prime consideration - and tight control over cable impedance is necessary (ideally, within ±2% of the specified cable impedance), to prevent signal reflections along the cable which are the primary cause of ghosting.

The situation with DVI and HDMI cables is somewhat different. As already expressed under the respective DVI and HDMI cable guides, these digital standards make use of twisted copper pair instead of coaxial cable to transport information. The problem with twisted pair is that it is not possible to maintain the same tight control over impedance as instead is possible with coaxial cables. The best twisted pair cables control impedance to about +/- 10%. As already stated earlier on, poor impedance control leads to signal reflections along a DVI or HDMI cable between source and sink, interfering with the bitstream originating from the source.

Meanwhile, whenever a digital signal is passed through a cable, the leading and trailing edges of the transitions in voltage representing the digital data, are rounded off. This rounding increases drastically with distance up to a point where it may be hard for the receiver to reconstruct the original bitstream.

In digital standards where no error correction is used, the end result is simply the loss of information; there is no way to restore the lost information resulting from the rounding off, or interference resulting from signal reflections due to improper impedance matching.

As long as the level of errors is contained, DVI and HDMI cables would still perform in that the rounding of edges and reflections will not compromise the display ability to re-constitute the image - however, these errors would often show as 'pixel-dropouts', also referred to as 'sparkles'. Sparkles would normally start to appear within a few feet beyond the maximum length at which the DVI or HDMI cable is designed to operate correctly.

Now, try to increase the length of say an HDMI cable beyond the point at which 'sparkles' first start to appear, and you would soon end up with no image at all. We have reached what is often referred to as the 'digital cliff', because of the abruptness that this failure process takes place. So much information is lost that the display becomes unable to reconstitute enough information to even render an image.

 

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In practice, it is very hard to predict when a DVI or HDMI signal will fail. As stated earlier on, you may possibly get away with a 40 feet run for an HDMI cable with no problems at all for a particular source/display combination; however, the same HDMI cable may still fail at shorter lengths with a different source/display setup.

So what if you need a 200 feet run to extend your HDTV? Straight DVI and HDMI cables would not operate at such lengths. Unfortunately, there is no cheap solution here;  you will have to invest at least a few hundred dollars more in a high quality DVI or HDMI cable extender, or preferably make a future-proof move towards fiber optic technology.

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Do YOU still think that digital is always better?

The truth is that there is no simple answer. There is no simple way to determine in advance whether RGB or component video will render a better picture than a DVI or an HDMI cable. It all depends upon your source and display combination. What may operate well using an HDMI cable between a source and display setup, may well produce a better picture when connecting another source say through component video over the same display device. You need to plug-in your cables and give the whole setup a try using the different connectivity options available on your gear.

One final advice: It is always important to test your cables with your source/display combination; remember, these are expensive stuff - a good quality 20 feet HDMI cable may well cost over a hundred dollars - hence, check also the return policy with the merchant before making a purchase.