VGA/RGBHV distribution amplifier needed for 8 displays? - AVS Forum
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P719C1's Avatar P719C1 07:18 PM 06-05-2011
I'm hoping to have at least eight VGA monitors display the same thing. They won't be too far apart from each other, so I'd imagine no more than about 25 feet absolute max worth cables. I'm assuming I'd be best off buying a RGBHV distribution amplifier or three, rather than relying on just D-sub VGA splitters? I know the voltage would stay the same across each splitter but I'd be concerned that the current being split so many times would adversely affect the picture quality. I'd estimate requiring no more than 250 MHz of bandwidth.

petern's Avatar petern 08:56 PM 06-05-2011
I'd be surprised if you could get that many splitters to work. Do you just need to display 1 VGA source? Hard to beat $50.
P719C1's Avatar P719C1 11:57 AM 06-06-2011
Yes, I plan on just displaying one VGA source on all displays at any given time.
Gizmologist's Avatar Gizmologist 12:39 PM 06-06-2011
We frequently have 4 projectors and 10 monitors or more fed from a single source through a DA. We usually use RGBHV DAs as opposed to VGA. Some of our cables are 200+ ft in length and run from the backstage control area to the flying trusses for projectors, to the stage for the presenter's confidence monitor and ancillary rooms as well as multiple monitors on the floor.
P719C1's Avatar P719C1 11:44 AM 06-07-2011
So I should definitely invest in a distribution amp at every split-off/junction (to daisy-chain or cascade)--or one with many outputs. If I need 200-250 MHz bandwidth max, could I get away with buying a model rated for 250 MHz, or am I truly better off getting a model rated for 300 or 400 MHz? I don't want to spend a small fortune on overkill if I don't have to, but I don't want to buy something that's going to give me noticeable ghosting or blurriness either.
Gizmologist's Avatar Gizmologist 01:20 AM 06-08-2011
I would use one larger sized unit as opposed to daisy chaining. The better units will have a solid buffering and line amp on each output as opposed to simply a resistive bridge between the output jacks.

The big box stores are famous for selling distribution amps with only a 75 ohm series resistor tying each output to a common output buss. This is a big problem. The reason is that each termination device (display) has its own built-in load resistor so without active buffering you would get a substantially reduced voltage to each unit. The other issue is reflections and ringing in the lines of differing lengths. That is usually not an issue with separate buffered line drivers.

Look into Extron and Kramer for top quality gear. They both have dealers all over the country.

If you opt to run RGBHV, I would strongly suggest Canare V53C-5 conductor jacketed coax. Super flexible and excellent specs. It is the pro industry standard. It does use only Canare BNCs due to a slightly smaller diameter than other brands, but it is worth the money, IMO. You can find this cable and the BNCs and the crimp tool at most industrial electronics supply houses.

If you opt for VGA, Kramer, QVS and ATEN have great quality and reasonably priced, larger scale DAs, and you can get excellent grade VGA cables in stock lengths up to 100 ft and very reasonably priced from QVS (NOT QVC)
P719C1's Avatar P719C1 05:43 PM 06-08-2011
Thanks so much. I decided to go with the model that petern suggested as it seemed to provide the most bandwidth for the money, and while that professional-grade equipment from Exetron, Kramer, and others looks and sounds great, I don't want to shell out too much money on this.

One last question though: would a model like that one (this is it) require a load or termination on all eight outputs when in use to prevent an impedance mismatch as I believe you had referenced?
Gizmologist's Avatar Gizmologist 09:29 PM 06-08-2011
In the old days monitors had a switch for termination. Today, not so much. Each input RGBHV has its own 75 ohm termination internal to the circuit.
I cannot say with 100% certainty that the DA shown has individually buffered outputs that operate independently of each other. I have seen units close in design and marketing claims that do and some that do not have line buffered outputs. Looks like it is a good probability that this one does, given the specs.
P719C1's Avatar P719C1 06:26 AM 06-09-2011
All right. Thanks again!
Glimmie's Avatar Glimmie 10:42 AM 06-09-2011
Quote:
Originally Posted by Gizmologist View Post

In the old days monitors had a switch for termination. Today, not so much. Each input RGBHV has its own 75 ohm termination internal to the circuit.
I cannot say with 100% certainty that the DA shown has individually buffered outputs that operate independently of each other. I have seen units close in design and marketing claims that do and some that do not have line buffered outputs. Looks like it is a good probability that this one does, given the specs.

It's quite common for NTSC analog DAs to have 75ohm series resistors tied to a high current low impedance driver - like an ohm or less. This was a standard design during the good old NTSC days and most broadcast manufactures did it this way. And now some VGA DAs just have a high bandwidth OPAMP for each output ties to a common input buss. Both work quite well although the OPAMP per monitor does offer better isolation and crosstalk rejection from sister loads. And remember that cost effective 500mhz OPAMPS didn't really come around until the 1990s. (note I said cost effective!) SDI and HDSDI DAs are most always a driver per load due to the high frequencies involved.

Note that VGA standard H&V sync is TTL and should not be terminated in 75ohms.
P719C1's Avatar P719C1 07:19 PM 06-17-2011
Quote:
Originally Posted by Glimmie View Post

And now some VGA DAs just have a high bandwidth OPAMP for each output ties to a common input buss. Both work quite well although the OPAMP per monitor does offer better isolation and crosstalk rejection from sister loads.

No idea which design mine uses, but I've started using it and it works great even at 2048x1536 @ 72 Hz and with just one or two monitors connected.

Quote:
Originally Posted by Glimmie View Post

SDI and HDSDI DAs are most always a driver per load due to the high frequencies involved.

You piqued my curiosity mentioning SDI and HD-SDI distribution amps, and I see they're actually not as expensive as I would have thought. Guess it makes sense since it's just one signal, rather than multiple components as with VGA.
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