DVI and HDMI Cable Connections
Component Video and RGB
Which is better: Analog or Digital?
HDMI cable interconnects have become the 'de facto' standard in HDTV; according to In-Stat.com, a marketing intelligence research group, HDMI-enabled shipments are expected to surpass 1 billion in 2014!
Yet the debate continues: Which is better: digital or analog, DVI and HDMI or component video and RGB—with its many different flavors? Are there instances when it would make sense opting for one over the other?
In this article, we analyze this hotly-debated issue to show where each of these AV interconnects fits best.
For many digital always deliver a better picture and so... HDMI is definitely the way to go.
Partly, the reason behind this mentality is the soaring presence of HDMI-enabled devices and the ease of availability of relatively inexpensive HDMI cables on the market. Yet the real advantage of HDMI cables over other interconnects, digital and analog, is the capability of HDMI cables to carry both digital video and digital audio over the same interconnect.
DVI is a digital video-only interconnect. But from a picture quality perspective, HDMI and DVI are one and the same thing in that they both use the same TMDS protocol to transmit digital data from source to sink over the appropriate cable. This also explains the ease of interconnecting DVI and HDMI-enabled devices. So the real dilemma remains one of analog vs. digital or rather... HDMI versus Component Video and the many different flavors of RGB; component video is in effect just one of the many different RGB video formats available for high definition video applications.
If in this analog vs. digital controversy, you are after a simple straightforward answer, you are in for a surprise. This is a rather complex issue. The truth is that digital does not always imply better image quality—whether this being digital terrestrial television, digital cable or satellite TV, or a simple video interconnect. What matters most is where each of these different video interconnects fits best.
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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.
As indicated in our introduction, 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 it is 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 further explained 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. an HDTV.
Component Video: Component Video splits up color information over three components in a somewhat similar manner to DVI and HDMI. However, content of the three video components is different from the three data channels used in DVI and HDMI cable interconnects.
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.
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 is RGBHV, with five lines: one for each color, 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.
It is thus clear that these different video interconnects are very much alike, breaking up the image in a similar way—though using different forms—to deliver the same type of information to the display.
In other words, difference in performance that may arise between these analog and digital interconnects, is in reality not related to the nature of the video interface inasmuch as to the particular characteristics of the source and display devices, and the quality of the interconnecting cables and connectors.
As we have stated in our introduction, 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 as it travels over an AV interconnect, digital transfer of information is error free. The reasoning is that with the information being represented by just ones and zeros, it should be easier to ensure that the information received at the display is an exact replica of the information originating at 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 is not necessarily the case with digital interconnects; e.g., HDMI cables are typically subject to a maximum of 15 meters, a distance which is easily exceeded with 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 does not factor in over short distances with good quality cables, it will when working with long DVI and HDMI cable runs.
Have you ever thought that from the point of origination of a video signal e.g. DVD, TV set-top-box, etc., till it is displayed as an image on the display device, a video signal goes through numerous processes and conversions, even when source and display are both digital?
This applies whether we start with an analog or digital source to display the image over an old 'analog' CRT TV or one of the latest 'digital' plasma or LED HDTVs. In other words, irrespective of the nature of the video source and display device, conversions are constantly taking place along the different process in the signal transmission chain. And the hard truth is that every single process in the chain would eventually leave its mark in the end result.
Just consider the simple issue of image resolution. There are hardly any instances when the source material is exactly at the same native resolution of the display device; scaling techniques are used to match the source with the screen native resolution. Yet scaling may introduce a number of visible artifacts that are surely independent of the Component Video or HDMI cable connection used to transfer the video information between source and display. And this is just of the many processes involved. There are many other signal processes along the whole signal chain, from the reading of data from an optical media to the presentation of colors and the rendering of picture information on the screen.
What's more, many argue in favor of DVI and HDMI cables on the sole basis that if you combine these digital interconnects with a digital source such as a Blu-ray player and a digital display, e.g. DLP projector or plasma HDTV to get an all 'pure' digital setup, then you are guaranteed to enjoy the best 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, you would not incur 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' and vice-versa.
In other words, it is not possible to state which is better—HDMI or Component Video, digital or analog; this quality issue is highly variable and greatly dependent upon the characteristics of the electronics inside the respective source and sink devices.
This explains why say a 40-foot long HDMI cable may operate perfectly well with one source/display combination but then fails to operate with another source/display combination even at 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 setup.
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 a different system response to the same signal when applied across different inputs.
Connector and Cable Built Quality: What are the implications on signal conveyance performance?
We would not go into details here as this is a topic we discuss in significant length in two other articles appearing under this same section:
Rather, what we will be discussing are the implications of cable quality vis-à-vis the difference in performance between analog and digital interconnects. These differences would not normally emerge as long as one is dealing with high quality cables at short to moderate cable runs. Differences between DVI and HDMI cables versus RGB and Component Video however would show up with low quality cables and increased cable lengths.
The battle here would turn out in favor of analog interconnects. RGB and Component Video are extremely robust signal types; you can easily run 100 feet—even 200 feet—of good quality Component Video cable without without encountering any signal quality issues even when no signal repeaters are used.
Surely, at the longer lengths, cable and connector quality should be 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; reflections are the primary cause of ghosting.
The situation with DVI and HDMI cables is somewhat different; these would not support the long runs supported by analog interconnects. As expressed under the respective DVI and HDMI cable guides, these 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%. 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.
Yet there is more against digital interconnects when it comes to long cable runs. Whenever a digital signal passes through a cable, the leading and trailing edges of the voltage transitions representing 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 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 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. You have reached the 'digital cliff'; the term 'cliff' is used 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.
In practice, it is very hard to predict when a DVI or HDMI signal will fail. As stated earlier on, you may get away with a 40 feet run for an HDMI cable with no problems for a particular source/display combination; yet the same HDMI cable may still fail at shorter lengths with a different source/display setup.
So how you go about extending your HDTV over long HDMI cable runs that exceed the supported limits by this digital interconnect?
Unfortunately, there is no cheap HDMI extender solution; as further detailed in our article here, you will have to spend up to hundred dollars and more to extend HDMI over wireless or CAT5e, CAT6, or CAT7 cable for distances up to 330ft, and significantly more if you have to use fiber optic technology to cover larger distances.
There is no simple answer since 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 perfectly well using an HDMI cable between a source and display setup, may very 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.
It is always important to test your cables with your source/display combination, especially prior to proceeding with an in-wall installation. Do not proceed with an in-wall installation prior to doing your tests as it may be costly and time consuming to rectify later.
Remember also that long runs of good quality AV cables can turn out to be relatively expensive even though we are no longer seeing the exorbitant prices of a few years ago when a 25-ft HDMI cable from a top-tier cable manufacturer sold for $200! Yet this does not change the fact that AV cables would still amount to good portion of any home theater budget. So always check the return policy with the merchant before making an AV cable purchase.