Question about signal loss through BNC-to-RCA adapters

Discussion in 'Home Theater Projects' started by Brandon_H, Mar 22, 2004.

  1. Brandon_H

    Brandon_H Stunt Coordinator

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    The setup: I have several component-video sources (HDTV cable box, DVD player, PS2, Xbox, and Gamecube, and occasionally my computer.) To accommodae all these, I bought a used Extron HDTV component-video switcher (six in, one out) off eBay a couple of years ago.

    The sources are all in close proximity -- no cable length greater than six feet (for most, more like three feet) into the switch. The switch's "out" runs about 20 feet to a ceiling-mounted Sony VPL-HS10 projector.

    Except in cases where I have to use proprietary cables (PS2 and Gamecube), everything is using reasonably spec'd component-video cable (Monster, or mid-grade Acoustic Research.) The picture certainly is very nice. So what's my issue?

    The Extron has BNC connectors, so I have to use BNC-to-RCA(female) adapters on it. I've often wondered about if, or how much, picture quality I'm sacrificing through using these adapters. I've given some thought lately to ordering some custom-made cables with RCA connectors at one end and BNC connectors on the other (from Blue Jeans, probably.) But if it's not going to make any discernible difference, there's no point.

    So, any experience or opinions? Would the picture improvements and peace of mind be worth a couple of hundred bones for the custom cables? Thanks in advance.
     
  2. Bob McElfresh

    Bob McElfresh Producer

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    Hi Brandon.

    The issue is not really signal-loss, it's impedence-mismatch.

    Having an adaptor is like having a speed-bump in a road or sidewalk. The signals can 'stumble' and reflect/distort.

    Like a speed-bump: it's more of a problem for the higher frequency signals.

    My advice: stick with the adaptors for any source that is 480. Consider a custom cable for your higher-frequency sources that provide 1080. Any device that provides 720 - judgement call.

    Hope this helps.
     
  3. Chu Gai

    Chu Gai Lead Actor

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    Your loss is neglible through the connectors especially considering it's going through the Extron which would introduce a greater loss but still not something to write home about.
    As far as impedance mismatching goes, this is really not much of an issue with your adapters. Consider that the BNC connector adds maybe 1/2 inch if that to the total length of your connector. In order for an impedance mismatch to start becoming a factor, the length of mismatch (let's call that the connector length) must be a sizeable fraction of the wavelength of the signal you're passing through it. Let's calculate some numbers.

    The speed of an electromagnetic signal is approximately 300,000,000 meters per second in a vaccum. Assume a velocity of propagation of 0.8 (electromagnetic signals travel slower through wire) and let's assume that you're pushing 1080 through the cables which is a frequency of approximately 36 MHz. The length of the signal is estimated at:

    300,000,000 * 0.8/36,000,000 = 6.7 meters.

    IIRC, an impedance mismatch starts becoming significant in terms of reflections when the length of the mismatch is ~ 1/8 of the signal length. Let's be conservative and call it 1/10. Well that means that your mismatch would have to be 2/3 of a meter or around 2 feet before you have to start getting concerned.

    My recommendation is that you keep your connections clean and tight and don't let that long length hang unsupported or else you'll introduce connection problems eventually.
     
  4. Brandon_H

    Brandon_H Stunt Coordinator

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    Thanks for the help, guys. Much appreciated. Given that Blue Jeans' cables are so (relatively) cheap, I think I'll pick up one to test the connection between the cable box (1080i) and see if I notice a difference. Given how good the picture is already, I probably won't, but I won't feel terrible for spending $30 on a cable just to try.

    The long length was an installation issue, Chu, since the projector was installed in an already finished basement and tearing up drywall was not really an option. I ended up running s-video, component, DVI, and composite to the projector. I used a drywall hook (50 lb rated) in the ceiling right behind the projector to hold the cables up right at the connections and relieve any strain there.

    To hold the cables tight to the ceiling, and then back down the wall to my equipment rack, required some creativity. I used those tiny plastic cable hangers with the brad nails --about 20 of them. I ran a cable tie through them, then tied the tie around the cable bundle to hold it tight against the hanger. Of course, individually, one hasn't nearly the strength to hold up those heavy cables, but collectively, they have no trouble keeping the cables right against the ceiling. And as a bonus, the holes made in the ceiling are negligible. It's not a pretty solution, but it works.

    Thanks again for the help. I can't believe how much good, free advice I get through this forum. Time to make another donation...
     

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