LOSTCIRCUITS

The appearance on the market of flat panel LCD computer monitors brought with it a new connector: DVI, short for Digital Video Interface. Traditional CRT (Cathode Ray Tube) monitors are analog displays, so it made sense in the past to convert the computer’s digital image to an analog signal and send it over an analog VGA connection to the analog monitor. An LCD monitor, however, is a natively digital display. It is possible to convert a PC’s video output to analog, send it to an LCD, convert it back to digital, and show the image, but the conversions become unnecessary and cause degradation of the image quality. The purpose of DVI is to preserve the quality of the video signal by avoiding digital to analog and analog to digital conversions.

In the consumer electronics / home theater arena, a similar situation has arisen. With CRT displays being eclipsed by fixed-pixel digital technologies (coinciding with the advent of high definition television and displays), interfaces have been migrating from analog to digital. While the familiar 15 pin analog VGA plug has been used outside the computer realm, the most common analog connection capable of carrying a high definition signal for consumer electronics is component video, which uses 3 RCA connections to carry the picture. DVI has also been used in consumer electronics for the same quality-preserving reason it is used on computers.

An additional consideration in consumer electronics is the complexity of cabling. Where a classic hi-fi system might have consisted of a turntable connected via stereo RCA to an amplifier with speaker wire running to two speakers, modern systems quickly become a quagmire of cabling. Adding video to the situation means adding cables to carry the video signal. Adding 5.1 and 7.1 surround sound requires cabling for up to eight speakers positioned throughout the room. Adding source components (CD player, VCR then DVD player, cable or satellite box, etc) of course requires all the cables to carry the various audio and video signals. Most home entertainment systems center around an AV receiver, with the source selection, processing, and amplification integrated into a single box. Higher end systems use a separate preamp/processor and amplifier which naturally require more cable to connect them. Any means to simplify the mass of cabling is welcomed.

Since many common source components like DVD, satellite, and cable send both audio and video to the receiver, it would be great if we could use a single cable to carry both the audio and video. That is the idea behind HDMI (High Definition Multimedia Interface). The video portion of HDMI is essentially the same as DVI. In fact, with adapters or cables with HDMI on one end and DVI on the other, the two ports become interoperable for the purpose of carrying video. It is the ability to carry digital audio that separates HDMI from DVI. HDMI is designed to carry not just the common SPDIF digital audio that contains the compressed Dolby Digital and DTS signal from our current DVDs. HDMI also is capable of carrying signals that require much higher bandwidth, like uncompressed multichannel audio that is possible on the next generation HD-DVD and BluRay formats.

The digitization of video in consumer electronics has accelerated the convergence of PC and home theater. When Microsoft designs a version of Windows specifically tasked with integrating a PC into a home entertainment system, the idea has gone mainstream. Windows MCE (Media Center Edition) was everywhere at the CEDIA Expo 2006, a major tradeshow for the consumer electrics industry. To continue the integration trend, we now have PC video cards with HDMI output for easy connection to a home theater system.

       

Yet another “feature” of both DVI and HDMI is HDCP, or High-bandwidth Digital Content Protection. HDCP is a form of Digital Rights Management (DRM) intended to prevent copying the digital video signal carried by DVI and HDMI. HDCP will most commonly be seen on HD-DVD and BluRay discs. In order to view content protected with HDCP it is necessary to have an HDCP compliant source and an HDCP compliant display. This could pose a problem, since not many current video cards support HDCP, and not many computer monitors support HDCP yet. In fact, a great number of the high definition televisions in homes today to not support HDCP.

The way around the HDCP problem is to use an analog video connection like component or VGA. The catch is that not only are there possibly extra analog/digital conversions going on, but there is another “feature” of the HD disc formats called the Image Constraint Token (ICT) that can automatically cut the resolution of a video signal fed over analog outputs. Both HD-DVD and BluRay have their content encoded at a resolution of 1920x1080. When activated the ICT will cut that to 960x540, which is only slightly better than the resolution of material on standard DVD at 720x480. The good news is that movie studios have agreed not to activate the ICT until at least 2012, and there is no requirement for it to be activated after that – it will be up to the studio whether or not to implement ICT for a certain title. A full discussion of HDCP and ICT would be an article of its own, but these are issues that users need to be aware of when planning their purchases.

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