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HD-SDI down RG59


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#1 Phil Rhodes

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Posted 12 May 2014 - 01:21 PM

In theory, HD-SDI should be transported down low-capacitance cable assemblies. Typically in my experience this means something like the teal-coloured cable that Van Damme call "HD Vision", which isn't actually any more expensive in 100-metre reels than the old cream stuff that everyone uses for, well, everything.

 

But has anyone ever actually had a problem with even 3Gbps SDI down normal, analogue-era coax? I've wired up entire drums of the stuff and never seen a problem, although that's only a third of the commonly-accepted 300m range of which it's supposed to be capable.

 

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#2 Rob Vogt

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Posted 12 May 2014 - 10:40 PM

Just took a look at the VanDamme website. I've never used this cable before, but I have assembled cables using Canare's CFB and CFW line and I can tell a difference in terms of physical strength and flexibility and performance. The CFB seems comparable to  VD's Standard line the noticeable difference being a single layer of braided copper strands separated by a single solid copper conductor. The CFW, like the "HD Vision" line has two layers of braided copper (tinned copper in the case of VD) which makes the cable more rugged and protects better against interference. I noticed a difference in performance when using it for recoding and sending trigger like from a codex/Gemini. If its just for monitoring, there wont be much of a difference or at least, I haven't noticed. But then again I typically only use them for 30-50m lengths at most.

 

When assembling the cable you can really tell the difference in the quality.

Hope that helps


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#3 John Miguel King

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Posted 15 May 2014 - 04:46 AM

It's got to be 75 ohms.

Now, a cheap option surviving a film set? Maybe yes, maybe no. I personally rather have the supposedly best of the best. This way, if it fails, I can say I did all that can be done.

Covering one's a$$ is the most important skill in the industry.


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#4 Phil Rhodes

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Posted 15 May 2014 - 02:15 PM

It's not a cost thing, the white and the lavender stuff is almost exactly the same price. I was just after any anecdotal evidence that it makes the slightest difference.

 

 

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#5 Dom Jaeger

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Posted 15 May 2014 - 08:09 PM

Doesn't seem to be a problem for monitoring, never tried RG59 coax for recording though. We followed Arri's recommendation of thicker stuff (Canare77S from memory) for REC OUT cabling on Alexas. I suspect for short runs RG59 would still be fine, but you don't really want to gamble with recording. It's probably a "better safe than sorry" thing.

 

I'm not electrically literate enough to know what exactly goes on inside these devices or at what point the image fairies decide to drop tools, but perhaps some applications or devices are more sensitive to a reduction in signal strength? 


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#6 Phil Rhodes

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Posted 18 May 2014 - 03:46 AM

It's not so much signal strength as the deterioration of the waveform due to capacitance. The teal stuff Van Damme sell does indeed have - microscopically - lower capacitance than some other types, although as far as I can see the capacitance of a bit of white stuff at 10m is about the same as a bit of teal that's 15m. So at short runs it oughtn't to make much odds.

 

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#7 Freya Black

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Posted 19 May 2014 - 06:09 AM

I thought the whole point of SDI was that TV stations wouldn't have to rip out all their existing BNC cabling so it was kind of backwards compatible.

 

I was also under the impression that the older stuff was also 75 ohms too.

 

I seem to remember that this was one of the reasons they couldn't get component video to take off completely back in the day as it would mean completely rewiring buildings so it tended to be just used to wire adjacent equipment together.

 

Freya


Edited by Freya Black, 19 May 2014 - 06:10 AM.

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#8 Phil Rhodes

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Posted 19 May 2014 - 06:59 AM

It was - but that's the 270 megabit version used for standard def, which makes considerably lesser demands on cable performance.

 

As a practical matter, the HD versions - certainly up to 3Gbps - generally work fine on cabling intended for standard-def analogue signals, with some concerns about reliability at long distance.

 

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