Chromatic aberration / video over Cat5

[rawshark]

Hi Everyone,
I'm getting serious chromatic aberation in a video over Cat5 run to a Sanyo XF-47 projector. Tweaking the level and peak dials on the unit change it somewhat but don't eliminate it.
It's an Extron MTP unit (sorry don't have the exact model number to hand right now) and the Cat5 run is approximately 200'. Tried different sources and no change, and a straight 5-wire run to the same projector is fine as well.

Anyone experienced this before or have any ideas?

Thanks for any help - I'm working on a show in China so my resources are severely limited!

 

[ruinexplorer]

Can you take a picture of it to post or more consicely explain what you are seeing in the aberation? Does this unit have skew correction? Do you know if you are using Cat 5/5e/6/7, or some other variant?

 

[rawshark]

Hi thanks for getting back to me. I have switched out the Extron for regular 5-wire, and the problem is fixed...so I can't post a pic I'm afraid.

Basically wherever there was a hard edge I was getting green fringe on one side and pink fringe on the other.

The unit had LEVEL and PEAK controls.
I don't know the exact cable but I'm guessing Cat5.

Although the problem is resolved I'd still be curious to know what might have been causing it for the future.

Thanks!

 

[ruinexplorer]

It sounds as if this is a problem with skew. Basically, over longer runs of Ethernet cable, there is a difference in lengths of the twisted pairs (which makes a difference in the type of cable) and can cause this issue. Some of the converter boxes will have skew correction to compensate for this. You will generally get less skew difference with Cat 5 than other types of cables, even Cat 5e.

 

[museav]

Or with Extron you can use this, Extron Digital & Skew-Free.

Here's a picture showing the typical effects of excessive skew, Model URA-SKU Video and Audio over UTP Receiver with Skew Correction. That looks a lot like your description of the problem. The Extron MTP boxes have both versions that incorporate a skew compensation control labeled "Delay" on the front panel and versions that have no adjustable skew compensation, it sounds like yours does not have the adjustable delay so there is no way to compensate other than physically making the pairs match lengths or changing out the cable.

It does throw people that in terms of skew CAT5 is better than CAT5e which is in turn better than CAT6. Some of the very things that make the cables better for data make them worse in terms of the skew introduced.

 

[rawshark]

Yes that picture shows exactly what I was experiencing.
Thanks so much for all the help guys - that is really valuable information!

 

[FMEng]

Or with Extron you can use this, Extron Digital & Skew-Free.

Here's a picture showing the typical effects of excessive skew, Model URA-SKU Video and Audio over UTP Receiver with Skew Correction. That looks a lot like your description of the problem. The Extron MTP boxes have both versions that incorporate a skew compensation control labeled "Delay" on the front panel and versions that have no adjustable skew compensation, it sounds like yours does not have the adjustable delay so there is no way to compensate other than physically making the pairs match lengths or changing out the cable.

It does throw people that in terms of skew CAT5 is better than CAT5e which is in turn better than CAT6. Some of the very things that make the cables better for data make them worse in terms of the skew introduced.

Receivers with skew correction are bigger bucks. Skew happens because, in Cat 5/5e/6 cables, each pair has a different twist rate to lower data crosstalk and that makes them different lengths over a long distance.

Belden and some other companies make low skew Cat 5 cable for this exact purpose. (I think it is Belden 7987 and Full Compass carries it.) It works perfectly for long video runs, and they caution against using it for data because it is designed specifically for video performance.

Using the proper cable is much simpler and cheaper than electronic skew correction. Electronic skew correction makes sense where you cannot change existing cable.

 

[museav]

Using the proper cable is much simpler and cheaper than electronic skew correction. Electronic skew correction makes sense where you cannot change existing cable.

Or when corporate or campus standards require the use of a specific UTP cabling, especially when that means CAT6. Or when the cabling may be installed by others and be Value Engineered by them or a different cable substituted, another example of Value Engineering saving money in one place only to then add even more cost in another place.

 

[thesigma]

"Value Engineering" has become my #1 enemy. AS you say, save money one place......only to spend it later somewhere else 10 times over........ugghhh....

 

[museav]

"Value Engineering" has become my #1 enemy. AS you say, save money one place......only to spend it later somewhere else 10 times over........ugghhh....

The original concept of Value Engineering was to allow Contractors to offer ideas that they felt could provide the Owner the same results for a lower cost or provide superior results for the same cost. It was about the value and was not supposed to involve compromising performance or the results. Unfortunately, it has evolved into too often addressing only the impact on the direct costs without identifying or quantifying the potential effects on the quality, results or indirect costs.