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I own an HDTV with YPbPr component inputs, and an AccessDTV card with a loopback from a Radeon 8500.


Currently, I have the ADTV set to output RGB and an AA transcoder between the ADTV output and the HDTV, which is a standard configuration that allows for switching between fullscreen HDTV and the desktop via software on one HDTV input.


I was wondering if there is any signal degradation when using the transcoder. With my current setup, when watching full screen HDTV the YPbPr data in the transport stream is converted to RGB by the ADTV card, then back to YPbPr by the transcoder, then back to RGB again by the HDTV to drive the CRTs. At each stage various artifacts such as ringing, and issues with less than ideal frequency response and color space transformations could degrade the signal. So I am considering alternate ways to wire the system to remove the transcoder from the fullscreen HDTV output, which removes two conversions, leaving only the conversion from YPbPr to RGB by the HDTV set.


Option 1:

Since my HDTV has 2 component inputs, I could remove the loopback cable and connect the ADTV output in YPbPr mode directly to one input on the HDTV set with a VGA to component RCA cable, and connect the Radeon output to the other input on the set using the transcoder. With this setup I would switch between them using the HDTV remote, but that's a bit of a pain since the only way I know of to switch inputs is to cycle rather slowly between about 6 options. It also raises a couple questions that hopefully someone here can answer: Will the ADTV card output the HDTV signal all the time (while operating) when no loopback cable is connected? What would the Full Screen button in the ADTV interface do?


Option 2:

Would it be possible to connect the transcoder between the Radeon output and the loopback input of the ADTV card, so I can use YPbPr output and the keep the loopback switching between desktop and FS HDTV? It seems like it should be possible with the right cables, since the Full Screen mode button still works when YPbPr (component) output mode is selected. With my current setup the FS mode is messed up since the transcoder is getting YPbPr input, but the desktop works as before. Theoretically moving the transcoder from after the ADTV ouput to inside the loopback connection should not affect the desktop mode. Has anyone tried this? I would use a normal monitor cable to connect the Radeon output to the transcoder, but how would I connect the component output of the transcoder to the loopback cable's VGA input (or directly to the loopback input on the ADTV card)? Would simply connecting the ground of the RCA outputs to the ground of the input, and the Y, Pb, and Pr signals to what normally would be the G, B, and R inputs suffice? Is that the right mapping of connections, and would any other signals from the VGA output to be fed into the loopback input for it to work or sense the presence of a loopback connection? Are there existing cables that do this?


Lastly, is this more trouble than it's worth? Would I be likely to notice the difference in full screen HDTV signals with the transcoder removed?


- Dave
 

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Option 1 sounds like the best idea, ignoring the problem of cycling inputs. Have you looked into getting discrete input codes for your set from something like RemoteCentral?


As for the transcoder harming the picture...I believe you will not notice a difference. Reviews of good transcoders, like the KeyDigital, have shown no noticeable loss of PQ when using them.
 

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You'll have to excuse me, I'm a PC guy first, HT guy second... But I have always failed to understand the video industry's fasination with YPrPb. Why can't we all just use RGB?


As pointed out in thes thread, it is the native format of the display device. And it is potentially the highest bandwidth interface, having the same bandwidth for luminance and chromanance unlike all the Ywhatever interfaces which only allow full bandwidth for the luminance (Y) channel.


I'm still waiting for someone to come out with a 50-60" rear projection MONITOR, not TV... RGB only, no speakers, PIP, line doubler, etc. that I don't want or need... Just a big monitor, please! (My room is too bright for a FP)
 

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helzerr,


Having worked in the 'video industry' many years ago, the 'fascination' with YPrPb (or more appropriately YUV) is that:


a) This is the format all broadcasts (NTSC) are in

b) Nearly all CRT-based sets (including many today) process the video in this colorspace. Why you ask? Because controls like brightness=Y+n, Tint = U+n and Saturation=U*n and V*n, and Contrast=Y*n are easily processed in the YUV domain. They can be *very* difficult to process in accurately in RGB, especially in analog.

c) There is *no* reason YUV can't have the same bandwidth as RGB. However, as the 'video industry' learned many years ago, there is no reason to waste extra bandwidth on something your eyes can't see. This is why many *PC* video codecs as well use YUV4:2:2, etc. formats to place more bits in the luminance domain. This is not to say that you won't see lack of chroma info, but if you're using, say, 8 bits to represent luma, you can use 5 for chroma and you won't be able to see the difference from using 8 bits for chroma.


dave77,


Although I don't own an accessDTV, I do own a HiPix and also have designed numerous video processors and RBG->YUV transcoders. *If* the AccessDTV uses simple bilateral switches for the loopback switch (I think it does), then you could theoretically construct a modified loopback cable that ran through your transcoder and fed theYPrPb signals into the AccessDTV's loopback inputs. The analog switches don't care what they're being fed; the video is not 'processed'


-Jon
 

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Very good points. Indeed, YUV4:2:2 is fine for video sources, but IMHO sucks for text and graphics on computer displays.


For HTPC purposes most video overlays are done in the YUV colorspace, so hue and saturation adjustments are still simple. I just hate to see perfectly good high bandwidth RGB VGA outputs get converted to YPrPb that is almost always bandwidth limited to 4:2:2 at the display.


I don't own a transcoder, do most of the transcoders support full bandwidth Y and PrPb output?


BTW I did not intend to attack the 'video industry', maybe I should have said large format display manufacturers instead.


I just think it's short-sighted to stick with or even promote the YPrPb YUV4:2:2 interface (notice I am talking strictly interface, I have no problem with YUV4:2:2 overlay) in an HTPC forum when full bandwidth RGB is commodity and should be a feature HTPC'ers demand in their displays.
 
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