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Product Review
 

Gefen HDTV Repeater for Use with Long DVI Cables

October, 2004

John E. Johnson, Jr.

 

Specifications:

● Allows DVI Cables up to 50 Feet to be
   Used

DVI Amplifier Bandwidth: 1.65 GHz

HDCP Compliant

● External Power Supply: 5 Volts Wall Wart

Dimensions: 1" H x 2.75" W x 4.5" D

● Weight: 2 Pounds

● MSRP: $249 USA

 

Gefen

www.gefen.com

Introduction

If you have a satellite box and/or DVD player with DVI ouput jacks, and your DVI input display is across the room, you may have run into a problem as I did, namely, the image has sparkles. Like this:

The sparkles are due to the DVI cable being too long for the small DVI output voltage in your sources. Each sparkle is a pixel that is white instead of whatever color it is supposed to be, and is a missing bit of information in the stream.

It is sort of like the "snow" that we saw in our old terrestrial TV broadcast images when our antenna was too far away from the TV station.

Enter Gefen

Whenever I run into problems with DVI or HDMI distribution, I first check out Gefen's website, as they seem to market a solution for everything in this area. Switchers, Extenders, Repeaters . . . you name it, they have it.

The problem comes from the voltage at the DVI output jacks on sources being too low to work over cables longer than about 12 - 15 feet. The voltage from video scalers seems to be OK, but from satellite boxes and DVD players, you get the sparkles.

For this particular situation, I obtained a Gefen HDTV Repeater, which sells for $249. It is also sold as a DVI Repeater, but the DVI Repeater is not HDCP compliant. So, be sure to get the HDTV Repeater rather than the DVI Repeater.

The Repeater

Whether it is called a repeater or extender, the process is basically the same. The incoming DVI signal voltage is amplified so it will work over a longer cable.

On the Gefen unit, you plug one end of the supplied DVI cable into your source, and the other end into the DVI In jack on the Gefen. The other side has a DVI Out jack that you connect your long DVI cable to, with the other end going to your display. Since the amplifier will amplify faulty signals, it is important to keep the cable length between the source and the Gefen unit as short as possible. If you use a long cable before the repeater, you will still get sparkles because they are in the signal being amplified.

The supplied cable is 6'. In fact, I would go to a 3' or even a 1' DVI cable between the source and the Gefen unit. You could also use a Gefen switcher before the repeater if you have two DVI sources.

In Use

I tried the Gefen HDTV Repeater between the DVI jacks of an LG satellite box and an Hitachi projector, as well as between a Denon DVD player and the projector. In both cases, if the DVI cable was 18 feet long, with no repeater, I got sparkles (a 12 foot DVI cable by itself had no sparkles). Addition of the repeater eliminated the sparkles, even if the total length of the DVI cables was 30 feet. So, the product works beautifully.

In setting up your own system, keep all the cables as short as possible. Just because you have the repeater, don't get a longer cable than you need. With 6 feet of DVI cable between the DVD player and the repeater, and 24 feet of DVI cable between the repeater and projector, I could still see an occasional sparkle, due, no doubt in part, to the fact that I had two 12 foot DVI cables connected together between the repeater and projector (signal loss will occur at the junction between the two 12' cables). For a permanent install, I would put a 1 foot DVI cable between the player and repeater, and a single 18 foot DVI cable between the repeater and projector. If I were to use a DVI switcher for the two sources, I would have a 1 foot DVI cable from the DVD player to the switcher, a 1 foot DVI cable between the switcher and repeater, and a 6 foot DVI cable between the satellite box and switcher (the satellite box is on a different equipment rack).

We are in the middle of a transition, now, with HDMI replacing DVI. The life of DVI in home theater was very short, much shorter than anyone expected, and many of us have DVI products. So, what do you do if you have a DVI DVD player or a DVI projector, and you are about to get a product with HDMI that connects to them?

Although HDMI and DVI are pin-for-pin compatible, and our tests indicate that sources with one or the other connector will work with displays that have the other connector, there are issues you should know about if you plan to integrate the two types of connectors in your system.

DVI is an 8 bit RGB signal, while HDMI can be 8 bit RGB, or 8 bit, 10 bit, or 12 bit YCbCr. If you have a DVI source and DVI display, there will be no problem. If you have a DVI source and an HDMI display, again, no problem. If however, you have an HDMI source and a DVI display, the below-black video information may be lost in the translation. There is a bug in the Silicon Image HDMI transmitter that pops up when converting YCbCr to RGB. The HD TiVo and Pioneer 59AVi do not have this problem.

Even though source information (DVDs, HD) is all 8 bit color, if DSP is applied in 8 bit, such as in a video processor, rounding errors will toss out some of the data. On the other hand, if the data are 10 bit, such as with YCbCr, then the rounding errors don't occur. In fact, 14 - 16 bit is optimum for processing. Also, DVD data are YCbCr, and are converted to RGB in the player for the DVI output. RGB cannot represent all the data in YCbCr, and this is why the below-black information gets truncated.

So, if you have a DVI display, and are buying a DVD player, you should probably get one that has a DVI output on it along with an HDMI output, as the DVI-DVI connection will give you the optimum picture. If you have a DVI source and an HDMI source, along with an HDMI display, then use an HDMI switcher and repeater (Gefen has these products), rather than the DVI switcher and repeater. Use a DVI-HDMI converter cable between the DVI source and the HDMI switcher. Lastly, although DVI has better resolution and lower noise than component video, DVI can introduce posterization. Since all modern DVD players have component video outputs, don't forget to compare your component video image with the DVI image. You may like component video better.

Conclusions

For those of you with DVI sources and a DVI display that are so far apart, you get the sparkles, the problem is solved with the Gefen HDTV Repeater. It is inexpensive and it works.



- John E. Johnson, Jr. -

 

Copyright 2004 Secrets of Home Theater & High Fidelity

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