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Discussion Starter · #1 ·
I've searched the DVI cable topic and it's amazing the results different people have. I'd like to see if anyone has compared different DVI cables (brands) and had differences in PQ?


Lengths are another story for a latter thread.


I know, before some of you say... "It's digital information, there shouldn't be any difference". I bet there is.


With digital audio, there's big differences between optical digital and coaxial digital, and even sound quality differences between different brands of coaxial cables due to some that handle jitter and other transmission factors better etc.


What about digital video cables? I have a cheap $40. Pacific Custom Cable DVI cable that makes an incredible image between my Bravo D1 and my Marantz S2 at 1280 x720. I'm pretty hesitant to pay $500 for a fiber-optic DVI cable that might not look any better.
 

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My experience with DVI cables is that it either works (great, crisp picture) or does not (sparkles or worse). However, my experience is limited to the $20 to $100 range, but I learned that more $ not necessary means better PQ...)

On the other hand the longer cable you need the more $ you need to spend - here the length/$ ratio is definitely not a linear function...


_______

Axel
 

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Discussion Starter · #3 ·
Has anybody compared say a 15' $400 fiber cable to a 15' $30 crappy copper cable? Yes, both may work, but does the fiber have higher resolution or better color definition etc?
 

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Ernie,


You are absolutely correct that not all DVI cables are the same. However, your comparison to optical digital cables really doesn't apply. With digital audio, there are two potential sources of distortion caused by cables: bit errors (rare) and jitter (not so rare). With DVI, jitter is simply not a factor. Bit errors manifest themselves as "sparklies", and they're not too hard to see. (DVI cables also have a bandwidth limit---and when you reach that limit, you can lose sync. But that's produces catastrophic resuls that are not too hard to see :))


So Axel is fundamentally right: a DVI cable either works, or it doesn't. What's difficult is that proper operation depends on many factors: 1) the build quality (obviously), 2) the medium (copper/silver/optical), 3) the length, 4) the quality of the DVI transmitter, and 5) the resolution, and 6) the refresh rate.


Often a crappy copper cable will cause horrible sparklies or a loss of sync in one application---but if you crank down the resolution or the refresh rate, or change the video card being used, suddenly it will work perfectly. Not just better, but perfect.


My personal experience confirms this. For awhile, I was running a cheap 10-foot copper DVI cable. At [email protected], the copper cable had sparklies, and at [email protected] they went away and I had a perfect picture. But my final install demanded a longer cable, so I bought an 18-meter fiber optic cable. This cable works perfectly at 48Hz, 72Hz, 85Hz, and up to the bandwidth single-link DVI.
 

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Discussion Starter · #5 ·
Michael, Thanks for the info!


So, It makes sense to try the $40 cheapo cable and see if it will work without errors, before spending $500 only to see no difference in image quality.


man, I'm glad I only spent $40 instead of the $500 for the fiber-optic.


And yes, I understand that this cable may not work in the years to come if DLP's go up to higher resolutions like 1080i/p. That's great, maybe by then, cables will go down to 1/4 of the price too.
 

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I think that's a great way to go about it. By the way, it's easiest to see sparklies on dark scenes.
 

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I got a $80 cable to work in my application where a $200 cable did not. I must say you cannot draw conclusions from price alone. DVI is a "hit or miss" proposition, especially with HDCP.


Todd/Indy
 

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That's a very good point Todd---not all fiber optic cables carry the signals necessary to support HDCP.
 

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That's interesting, Todd, how long was the cable? If I understand

things correctly, the cable shouldn't affect HDCP data. HDCP is

just encrypted data that runs over the DVI cable. DVI employs

a synchronous data protcol (uses a clock signal) to move the

digital data. So basically that's why DVI is hit or miss. If the clock

signal makes it from end to end intact, so should the data. This just

pertains to the cable. Like Michael said, the DVI transmitter and

receiver must work too. Somebody please correct me if I'm wrong.


larry
 

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Ok, now I'm confused. :) Michael, can you clarify? I haven't seen

the HDCP spec, but didn't realize it employed additional signal paths.


thanks


larry
 

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I am making a DIY DVD-D cable out of personal need in my new HT. I'll post the results and where to get the parts. I did get male connectors but they are for injection molded cables. So this cable may be a bit kludgy but for running in walls or smaller than 2in conduits, a build-on-site cable is sometimes the only option.


There is no black magic behind any of these newer interfaces, DVI, USB, 1394. While poorly thought out IMO for long legnth situations, this is nothing but an electrical engineering problem. It's resistance, capacitance and inductance. These probelms can be solved other than by fiber optics for less than 100 feet.
 

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Quote:
Originally posted by Michael Grant
That's a very good point Todd---not all fiber optic cables carry the signals necessary to support HDCP.
The problem with Fiber Optics and simpler copper repeaters and HDCP is the low speed single ended serial data line within DVI is bi-directional. In order to make a repeater for this line you must decode the protocol to know when to reverse direction of the fiber or repeater electronics.


Is that protocol public information???? Another complication is that data line is single ended and that alone invites noise and ground loop problems. The RGB data and pixel clock are balanced lines due to the high data rates. You could balance that line but once again we get into data direction control problems unless we use a simple transformer which has a myriad of problems too.


This also means a switcher presents a problem. The RGB data and pixel clock can probably be switched by simple ECL gates as they do not saturate. A high bandwidth analog switch should also work if it accepts differential lines. But the control data is forced to a relay switch unless that data cane be decoded.


Geffen makes a 4x1 DVI switch. They claim it's 100% solid state and HDCP compliant. I guess they must be decoding the control data.
 

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Gimmie,


anxious to know how your DYI DVI works out. I am using a cable cover for my SP7200 and there isn't room for the DVI to P&D adapter to fit in it. A DYI DVI to P&D cable would solve my problem. I can't find the 10M cable that I need, and currently have a 10M DVI cable.
 

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PooperScooper,

Glimmie explained it well--HDCP requires the low-speed serial channel to be intact, and some of the fiber optic cables don't support that channel. Some do, however. You just have to be careful.


Traveler,

I wouldn't necessarily recommend my supplier. But http://www.gefen.com sells good cables from what I hear...
 

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Quote:
Originally posted by PooperScooper
That's interesting, Todd, how long was the cable? If I understand

things correctly, the cable shouldn't affect HDCP data. HDCP is

just encrypted data that runs over the DVI cable. DVI employs

a synchronous data protcol (uses a clock signal) to move the

digital data. So basically that's why DVI is hit or miss. If the clock

signal makes it from end to end intact, so should the data. This just

pertains to the cable. Like Michael said, the DVI transmitter and

receiver must work too. Somebody please correct me if I'm wrong.


larry
All the cables I tried were copper. I tried combinations of 20' dual link cable and M1 adapter from Digital Connections, 9m single link cable from Better Cables, original cables supplied with STB's, Samsung SIR-TS160, Sony HD200, and my InFocus 7200. The ONLY combination that worked was the second cable sent by Digital Connections with the HD200. All other combinations produced no HDCP and/or noise in the image. This includes the vendor-supplied cables. Truly hit or miss... Anyone going down this road may be in for some painful experimentation. Digital Connections and BetterCables were both great to work with.


Hope this helps, but I doubt it...


Todd/Indy
 

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Discussion Starter · #18 ·
One of the guys on the Bravo D1 DVD player thread bought the Geffen (which say it's HDCP complient)and it did not pass HDCP. They had to send it back to Geffen so they could modified it .


So, be careful, talk is cheap.
 

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Sanetlla, if you can, you might want to wait for hdmi (next gen dvi) cables to come out as they are only 19 pin and therefore will supposedly have a 25 meter range without the need to use optics. It should also be able to handle 1080p with HDCP though at what refresh rates I have no idea. Supposedly the first cables are coming out this fall and you will be able dvi adapters for them.
 

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If I remember correctly from a chart in the DVI spec, 1920x1080 at 60Hz

will basically saturate a single link DVI depending on the display. Some

of it has something to do with "blanking" which I don't fully understand.

LCDs with 5% blanking require less bandwidth and other displays required

more with something call "GTF blanking". Higher refresh rates or resolutions require both DVI links (6 channels) to be used. I wonder if

any of this changed with HDMI.


larry
 
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