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Old November 30th, 2005, 12:33 AM   #1
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BnC cable to RCA input?

Is there any video quality advantage to using a b n C cable with an RCA adapter, into my XL1s' RCA inputs, to monitor video in the field (from a production monitor), vs simply using an Svideo cable?
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Old November 30th, 2005, 05:00 AM   #2
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The S-Video cable will give you better quality over the BNC or RCA connection.
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Old November 30th, 2005, 09:35 AM   #3
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Ok, that's what I thought. Yet people seem to hate S-video for some reason, and "pro" cameras don't even have that option.
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Old November 30th, 2005, 10:15 AM   #4
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Originally Posted by Josh Bass
Ok, that's what I thought. Yet people seem to hate S-video for some reason, and "pro" cameras don't even have that option.
BNC is really a connector type rather than a cable or signal type per se and is commonly used in pro equipment for composite video and similar high-bandwidth applications like timecode, word clock, etc. It's used in preference to RCA because it stands up better to repeated connection and disconnection without becoming worn and making poor connections and is less likely to introduce an impedance mismatch in critical circuits. Because it's a twistlocking bayonet type of connection it's less likely to accidently pull free at inconvenient moments. Essentially it's just a more rugged and reliable way of accomplishing the same type of connections that consumer gear uses RCA for.
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Old November 30th, 2005, 10:28 AM   #5
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"Essentially it's just a more rugged and reliable way of accomplishing the same type of connections that consumer gear uses RCA for."

Don't you mean S-video? Whenever I hook up RCA cables from my cam to monitor, it looks lower quality than the S-video.
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Old November 30th, 2005, 11:11 AM   #6
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I don't use BNC connections at all, but it would seem to me based on its structure that it is still just a 2-channel signal (similar to RCA-composite) but with a better connector tip - so you are still getting a composite level signal. S-video is going to produce cleaner results if that is the case. In either case, since you are going to use your BNC connector through the RCA adaptor, the best you will get will be the RCA-composite level (because the of the adaptor). Even if BNC is a better signal, running through the rca adaptor is not going to improve it - at most it will be composite. So s-video will still be a cleaner signal.
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Old November 30th, 2005, 12:35 PM   #7
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That's what I thought. Thanks.

So the idea is that the bnC cables used on the pro field cameras carry the s-video-quality signal with a different type of connector?
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Old November 30th, 2005, 03:54 PM   #8
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Originally Posted by Josh Bass
So the idea is that the bnC cables used on the pro field cameras carry the s-video-quality signal with a different type of connector?

Again, I may be wrong here, but I think the BNC is on par with the RCA type connection because I think it is just a two channel connection. It is used on field monitors because its major benefit is a secure connection (lock-twist) and is suitable for field monitoring, but may not be a very high quality color accurate feed. I would think s-video may be a little better quality.

As I mentioned, if....and that is a big IF the bnc connector is a better signal, you won't see it if you are running through the rca adaptor...at most it will be the composite level signal that rca produces. This would be the case whether you were running the s-video or some straight digital signal through the rca adaptor. With the adaptor, you are still going to end up with the maximum that the channel rca connection can carry.

On the other hand (and this is only uninformed speculation...I wonder if it would even be the very slightest of better if you are using the rca adaptor, but only at the beginning of the signal flow. I wonder this because if it at the end, you are putting out the 2 channel video signal - however if running the adaptor at the beginning and then theoretically adapting it to (say) s-video - while you won't get the native s-video signal, I wonder if the cable would then be trying to split the RGB signal somewhat for at least a slight attempt at cleaning it up a little. This is probably something that would need some signal analysis gear to determine. ....Not my area of expertise.

On my XL2, the bnc and composite is switchable between the two, but they are on the same circuit - while the s-video is on a separate circuit, because I think the bnc and composite are the same signal level.
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Old November 30th, 2005, 04:14 PM   #9
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i used to wire up mobile production semi-truck trailers for a company called centro... we built full-on pro facilities that were used for things like golf tournaments for nbc sports.

bnc connectors are used with quality shielded cable, better than anything you'd get with an rca cable... at one point i was cutting and terminating matched-length cables in sets of 12, one set of which was 100 yards long... you would never try something like that with an rca plug situation.

s-video(really y/c) has better picture quality than the composite video signal that's typically sent over a single rca plug wire, but it's still a color-under format that does not hold up in production like the video signal quality of component video... component video jacks are found on the back of better tv's and such.

the y/c video signal coming off of your xl1s will have better quality than the composite signal coming off of your camera, regardless of whether or not you are using bnc or rca cables on the composite jack of the camera... it's just that the y/c cable is typically not shielded, so it could be subjected to interference, and you may not want to wear out the y/c jack on the camera.
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Old November 30th, 2005, 04:22 PM   #10
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Originally Posted by Dan Euritt

bnc connectors are used with quality shielded cable, better than anything you'd get with an rca cable... at one point i was cutting and terminating matched-length cables in sets of 12, one set of which was 100 yards long... you would never try something like that with an rca plug situation.
okay...that is excellent info to know - many thanks for the insight. Yes, it makes sense with the shielding and cable integrity that bnc would certainly retain a higher signal integrity.

But what about the question of running the bnc through the rca adaptor? I would think that this is down-rezzing the signal to rca level signal. Would this put the signal below the s-video signal level?
-Jon
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Old November 30th, 2005, 05:02 PM   #11
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Ok, so I guess what I'm really curious about is whether this "quality shielded cable" better than an S-video cable. Is the shielded cable component? I have no idea what I'm talking about.
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Old November 30th, 2005, 05:21 PM   #12
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The y/c signal(svideo) is better than the composite signal( RCA and BNC)
However the advantage using svideo termination is gone if your cables are longer than( and the exact distance is debatable) 10 feet. Many pro cameras do in fact have Y/c outputs most of these use a totally different connector than the svideo 5pin .
Point is a huge advantage when shooting tethered (with output cable to mixer in studio or truck) is the BNC doesnt pull out if you step on the cable or if the cable puller goofs.On a live shoot this is a disaster and surely the director will tan your hide.
If you use a cam like XL1 etc. get a shoulder mount aka Ma100,200 bracket,use a board mount BNC mounted to it with 8" cable to RCA.Use the best cable and connectors yuo can(Neutrik,etc.)
The quality is the same with BNC or RCA or Svideo (over 10 feet)
Just noticed your reference to component. What Dan is talking about that composite and y/c(svideo) are color under formats.They send the color information in the picture (reds,greens,blues)together. Composite also sends the luminace(black&white) with the color info. Y/c seperates the luminance on a wire by itself.Its one of the 5 pins in svideo.
Component seperates the colors like your computer signal RGB.red ,green,blue. Sorry about the long post ,hope this help.
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Old November 30th, 2005, 05:54 PM   #13
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Originally Posted by Josh Bass
Ok, so I guess what I'm really curious about is whether this "quality shielded cable" better than an S-video cable. Is the shielded cable component? I have no idea what I'm talking about.
Sorry Josh...

I may be complicated things for you - sorry about that. And that's okay, because I almost don't know what I am talking about either. What you are using is not component. (The component is in reference to what looks like rca connections, but each one is its own connection for red, green, and blue. You will find this setup on the back of some DVD players, digital cable boxes, probably directv and tivo sets also. It is a good quality connection - with good separation for the colors, but it is not what you are using. The composite is also rca connection, and not separated - so the rgb is carried together and is a low quality video signal.

In basic answer to your question - use the s-video signal if you are running a cable 12 feet or less. More than 12 - 15 feet or so, the bnc shielded cable will maintain a higher signal integrity. My understanding is that even if you have to pump that through the rca adaptor that's okay because at longer lengths, the signal loss through of s-video will be worse as it is not shielded.

If your cable is 12 feet or less (optimally 6 feet or shorter) than the s-video connection is currently the best connection available to your given setup.
-Jon
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Old November 30th, 2005, 08:46 PM   #14
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Really? That's weird. I have used, for a very long time, a 50 ft svideo cable (probably overkill, but all I used to have was the little 6 ft one, and that just wasn't gonna cut it in every situation), and I can't see a difference between how it looks with that cable vs. the 6ft one. Still looks better than RCA, to me. With RCA, there's kind of a "fuzziness" around edges, and it makes the whole image look lower res. Maybe it's me. I dunno.
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Old November 30th, 2005, 09:09 PM   #15
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My 2 centavos -
If you are using BNC connectored cables, chances are the cable itself is of higher quality than anything you will find in a comperable length RCA cable. It's the cable that will be better. Quality BNC cable is actually pretty inexpensive compared to S-Video (more on S in a moment).

If you are running over 6' or so, you should probably try to use a good 75 ohm video cable like Canare or something from Gepco, Mogami, etc as they will be much better shielded, be heavier and stand up to more usage and have thicker conductors for less signal loss. BNC center pins are gold plated and don't oxidize. My recommendation is over 10', get a good grade BNC cable and a few good BNC to RCA male adapters with gold center pins. You'll thank me later.

On S-Video. S uses 2 coaxial cables that are smaller (normally) than a single BNC or RCA type cable, and bonded together or in the same sheath. The signal is less encoded than a single cable BNC or RCA system. (Too hard to explain encoding here, you'll have to look that one up if you need more info.)

The signal is luminance and chrominance info left seperated out and sent via 2 coaxial lines. This provides a much better signal. S-Videos' huge downfall is the stupid connectors they chose to use with it. Let's see a show of hands for those of us who have jammed the things in sideways in the dark and the bent the pins - I thought so. Bad bad bad design.

It is possible, and sometimes preferable to run 2 BNC lines and split out the S-Video cables with an S-Video to dual BNC adapter. As the BNC cables are better shielded and have less signal loss, you can run a pair over longer distances with less degradation than a single S-Video cable. If you are going to do the dual BNC run, make sure the BNC cables are really close to exactly the same length. There can be phase errors over long mismatched length runs.

Hope that answers some questions.

Sean McHenry
NBC Engineering for 16 years,
Post Engineer for 5,
Broadcast equipment sales 3 years.
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