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Published: 2007-07-31 - 00:20:00
Home Theater : Ask The Expert

How Can Analog Component Video Cables Support HDTV - Isn't That Digital?

By Chris Boylan

Question:

Hello, Big Picture Big Sound,

Your comments in two different "Ask the Expert" articles have confused me.

In a February 10, 2007 article, you suggest that when an HDTV only has one HMDI input it is best to connect an upconverting DVD player to the HDMI input and use the component video inputs for the cable signal since "high definition video signals can be carried over... component video connections." But in an article dated March 5, 2007 you state that "component video is an analog signal."

I always thought that HD signals were digital. I am therefore confused about how component video connections can carry HD signals. I am missing something?

Thanks.

Stephen A., North Hollywood, CA



Answer:

Hi, Stephen,

Thanks for your note. High Definition Television is not exclusively digital. In fact there is no requirement that high definition TV be digital at all. The broadcast of ATSC transmissions itself is done digitally (at least in the U.S.), but a digital HD signal can be converted to analog for transmission from a source to a display, as is the case with high def over component video cables.

The first few generations of HDTVs and HDTV-ready sets sold in the U.S. were all analog (CRT tubes and projection sets). They had no DVI or HDMI inputs as these did not exist back then. Today's high definition components still maintain compatibility with these early HDTV sets via high definition component video outputs.

Component video, similar to a computer's VGA connection, is an analog medium but it supports the full 1920x1080 pixel resolution of HDTV. Manufacturers of high definition source components such as Blu-ray Disc and HD-DVD players generally limit the output of analog HD to 1080i (interlaced), but it's not for technical reasons. It's primarily to support the adoption of HDMI with its inherent HDCP copy protection. 1080p could be done over analog component video connections, there's just no compelling business reason to do so.

If you want to compare this to another "digital" technology, consider the Compact Disc. Audio data is stored on a CD as bits and bytes - definitely a digital medium. But virtually all CD players include digital (coax or fiberoptic) and analog outputs (stereo Left/Right, usually on RCA cables). This was done for compatibility with both past, current and future receivers and preamps. The audio signal had to be converted to analog eventually, in order to be sent to the speakers. It was just a matter of whether that conversion was done in the CD player or in the receiver or processor. Similarly high definition digital video signals can be converted to analog (component video or VGA) without significant loss of detail.

As I've mentioned in these two referenced articles (and in other replies in this column), using HDMI to carry high definition audio/video signals is generally favored over analog component video connections. HDMI connections keep the audio and video signal in the digital domain as long as possible, and this can prevent generational loss as the signal moves from source component to receiver or processor to display device.

But if your current HDTV set only has one digital input (HDMI or DVI) and you find yourself with two (or more) high definition source components, and you do not wish to purchase an HDMI switcher or HDMI-switching receiver, then an analog component video connection is a viable alternative and will still give you high definition resolution.

The reason I recommended analog component video cables for the cable box (and using the HDMI connection for a Blu-ray Disc, HD-DVD or upconverting DVD player) is that cable broadcasts are currently limited to 1080i resolution, which component video is perfectly capable of handling. And the most advanced audio available on broadcast HDTV signals is Dolby Digital 5.1, which can be carried over standard "S/PDIF" coax or fiberoptic cables. Meanwhile Blu-ray Disc and HD-DVD players support native 1080p software titles with more advanced high quality audio options, and upconverting DVD players only upconvert standard def DVDs to HD resolution via HDMI or DVI outputs, so HDMI is a better choice for those particular technologies.

Hope that clarifies things a bit.

Regards,

-Chris

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