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Key Digital ClearView3 CTCA3 Transcoder for Converting YPbPr to RGBHV

November, 2004

Brian Weatherhead

 

Specifications:

  • Outputs: One VGA Video (RGBHV), One Component Pass-Through

  • Input Formats: 50 Hz or 60 Hz refresh, 480p, 5/6p, 1080i/540p,720p

  • Negative TTL Sync for H and V in RGBHV Mode

  • Negative Sync-on-Green for RGsB mode

  • Polarity of H&V Sync for Pass-through RGBHV is the Same as Input H and V Drive Polarity

  • HDTV Colorimetry Matrix

  • Video Bandwidth -3 dB at 110 MHz

  • Size: 0.75" H x 4.75" W x 2" D

  • MSRP: $299 USA

Key Digital

www.keydigital.com

Introduction

Although just about any new digital projector you purchase now will have DVI, HDMI, or YPbPr component video inputs, the projectors of the past used RGBHV (H stands for Horizontal Sync, and V stands for Vertical Sync). These high end projectors, usually CRT, were capable of very high resolutions, some as high as 1080p.

As time has passed, companies moved to the Component Video format, YPbPr, which DVD players use predominantly.

So how do you glue Component Video sources to high end CRT projectors? Key Digital's ClearView3 CTCA3 Transcoder is the answer.

Component Video (YPbPr)(YCbCr)

Component Video, found on just about every DVD and HD source is a compressed video stream. It is calculated from the original RGB (native) source as follows:

Y  = + 0.299R    + 0.587G    + 0.114B
Pb/Cb = +(B - Y) / 1.772 + 0.5
   = - 0.168736R - 0.331264G + 0.5B      + 0.5
Pr/Cr = +(R - Y) / 1.402 + 0.5
   = + 0.5R      - 0.418688G - 0.081312B + 0.5

Here, R, G, and B are assumed to range from 0 to 1, with 0 representing the minimum intensity and 1 the maximum. YCbCr was defined in the ITU-601 standard for use with digital component video. The same color space for use in analog component video was called YPbPr, although the term YCbCr is often used for both systems. Where Y carries the luminance, the PbPr carries the color space difference encoding.

RGB Video (RGB)(RGBHV)(RGBs)(RGsB)

RGB video can be organized with different synch configurations to create the four recognized RGB formats. These formats keep true RGB information in separate signals without compression, and each component has full luminance information. RGBHV carries the synchs on separate signal wires. In essence, RGB video is the best possible analog video format, as there is no signal compression.

The "Glue"

Since YPbPr is derived from RGB using known formula, it is then possible to restore the signal to RGB format by doing what is called transcoding. This is done by restoring each RGB signal, and restoring the synch to the RGB or RGBHV format. While some information is lost from the original compression to Component Video, the result is video quality that is equivalent to the component signal but in a format that "legacy" RGB display devices can understand.

The Key Digital ClearVideo3 CTCA3 provides for color space correct transcoding of component signals to RGB. It's a small but powerful device. Not much larger than a deck of cards, this single device brought HDTV to my Runco DTV-933 projector. It's simple to use. I connected my Dish 811 receiver to the component input, and the projector to the RGB output. The CTCA3 is configured for RGBHV out of the box. I powered it on, selected 720p on my dish receiver (this produces the best image) and created a profile for it, using 720p, in my projector.

That was it. It worked flawlessly. There was no more configuration, no fiddling. Just 5 minutes of plug and go. I later connected the RGBHV output of my Runco scalar to the RGB pass-through. This way, when I wasn't watching HD, the unit would pass through my scalar's signal automatically. The unit also provides for a Component Video pass-through.

While I cannot do a direct comparison (my projector only accepts RGBHV), I can say that the unit performed very well. The colors looked spot on, there was no loss in black level or blacker-than-black. I didn't see any color clipping. The 1080i picture was far superior than that of my up-converted DVD signal from my Runco scalar.

Conclusions

The Key Digital ClearView3 (CTCA3) will become an essential part of my reference theater. Only a very select few of the very high end processors provide transcoding as part of their package. As more processors focus on HDMI, DVI, and digital displays, the CTCA3 will become invaluable as we CRT users upgrade our video processors to the latest technology.



- Brian Weatherhead -

Copyright 2004 Secrets of Home Theater & High Fidelity

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