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Old 17th November 2007, 03:30 AM   #1
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Default DVI switch, worry about frequency?

I want to make a relay based DVI switch for qxga. Should i worry about keeping the inductance down? Is just the inductance of the relay going to cause problems?
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Old 22nd November 2007, 04:31 AM   #2
1audio is offline 1audio  United States
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Depends- It might just work, but the data rate is over 500 MHz per lane and the cabling is 4 sets of very precise diff pair cables. Get a cheap HDMI switch and adapters. Do something more fun with your time.
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Old 23rd November 2007, 02:11 AM   #3
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Ok

It is going to be 10 channels by the way :P Time for 1000+ solder joints of fun
But it beats spending big $$$ to buy one.
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Old 23rd November 2007, 02:51 AM   #4
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Default relay's aren't going to cut it.

couple of good white papers here:
http://www.hdmi.org/learningcenter/publications.aspx

check out this one:
http://www.hdmi.org/pdf/whitepaper/S...IWhitePaperv73(2).pdf

qoute:
Signal Resolution * Refresh Rate * Color Depth = Necessary Bandwidth

so 480p = 0.81Gbps, or 27MHz

a simpler relay scheme is not going to work unless you have *very, extremely* small relays. The inductance will kill the signal, as the impedance discontinuity will certainly cause nasty reflections and losses that will completely close off the digital eye at the receiver.

As 1audio stated, the cables are impedance controlled twisted pairs for high speed differential signaling. If you can find relays that match the characteristic impedance and have minimal isnertion and return loss, you can likely make it work. Otherwise, just cascade a couple of off the shelf HDMI switches, and you should be golden. Yes, this could cast 400 to $1K.

good luck!

-CK
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Old 23rd November 2007, 03:31 AM   #5
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Default um, OK...

Sorry, sent that too fast without quite finishing... had to run take care of something. I equate DVI=HDMI these days for most purposes - hence the links to HDMI spec. (the video portion is basically the same)

in any case, what I wrote is essentially correct. XXGA is 2048x1536, so that will be quite a high bandwidth requirement. No simple task for any relay. (though for many SMT transistors)

good luck,
Chris
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Old 23rd November 2007, 06:29 AM   #6
1audio is offline 1audio  United States
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HDMI and DVI use the same signalling except HDMI adds the CEC wire and doesn't have a dual link option. The impedance issue is critical at these speeds. You can get switches for HDMI pretty cheap today, look at Monoprice. You can get distribution amps from Extron and Geffen. Build it yourself won't be too successful. Your price on the connectors will exceed the cost of the switch. The bandwidth is a big issue- 1080p/60Hz is 4.95 Gbps total or around 1.65 Gbps per lane. About the same as a 1600X1200 display at 72 HZ. The numbers must also include the retrace time which is a 40% overhead (I checked this with Silicon Image who invented it all). PC layout is very critical to success. Freehand wiring is even more critical. The relays to do it that way are the $25-$50 RF cans. You may as well layout a PCB and get the switch chips from Maxim.
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Old 23rd November 2007, 07:50 AM   #7
fizzard is offline fizzard  Canada
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It's probably not feasable to switch the signal mechanically. There are high frequency microwave relays but they're not cheap. At HDMI frequencies you would need to switch the signal digitally, and 7400 logic won't do it either.
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Old 23rd November 2007, 06:56 PM   #8
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Click the image to open in full size.

Thats the relays. Looks like i am going to have to spend the time wireing for low inductance to keep the signal as best i can.

Edit: Oh ya, they have solid silver contacts.
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Old 23rd November 2007, 07:39 PM   #9
1audio is offline 1audio  United States
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What is inside the relay? SPDT or DPDT (also known as form A or form C contacts). Silver isn't good for low level signals, it needs too much energy to breakdown the oxide.

The impedance is really critical- the signal is on a 100 Ohm differential pair with a 70 picosecond risetime. Any discontinuity will cause a reflection and bad data. There is no error correction on DVI so you will just get a bad picture.

You need to switch 4 data pairs, DDC, and assert hot plug detect to restart everything when you switch.

Are you making a board or discrete wires? The wires in DVI cables are 30 or 28 AWG on short ones and go up to 24 AWG on long cables. The pairs are individually shielded usually with a drain wire.

You must have plenty of time available because this will use a lot of it. 10 channels X 5 or or X9 relays plus the control etc. will have you very busy. Build one channel and test it before you get too deep.
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Old 24th November 2007, 12:50 AM   #10
fizzard is offline fizzard  Canada
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The size doesn't matter so much as will it maintain the correct impedance when the contacts are closed, and how much capacitance does it put on the line when the contacts are open. I can't tell just by looking at the picture if those will work, but what you have doesn't look anything like the small signal RF relays I've seen. The RF relays I've seen are 50 ohm, there might be 75 ohm kicking around somewhere too.

To save you a lot of time this project is a complete non-starter.
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