Coax to phono connector wanted

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Re: Coax to phono connector wanted

Post by ef37a »

Organ Grinder wrote: Tue Oct 04, 2022 7:13 pm
ef37a wrote: Tue Oct 04, 2022 5:32 pm No, it's not that fussy, just that there are about 4 or 5 parts to a BNC connector and you have to get the wire right or it won't go together or it will short.

Well now I'm confused! I though you had live and earth (or ground)

Ha! Ha! viz...https://uk.rs-online.com/web/p/coaxial- ... rs/5464875

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Re: Coax to phono connector wanted

Post by Hugh Robjohns »

Organ Grinder wrote: Tue Oct 04, 2022 7:13 pm Well now I'm confused! I though you had live and earth (or ground)

It's signal and screen, but yes, two connections.

However, video (and digital audio and RF) are all high frequency signals that require the cable to have specific characteristics if the signal is to get to the other end intact.

One of those characteristics is called 'characteristic impedance' and its why the cable is identified as 75 Ohms or whatever. The connectors must have the same characteristic impedance too.

The characteristic impedance is determined by the dimensions of the cable and plug/socket – the spacing between the signal core and screen – and the insulating materials between and around them.

That's why BNC connectors have a specific insulation sleeve and screen terminating ferrule, and why the cut cable dimensions are critical. They are fiddly to assemble with the proper tools, and a nightmare without.

Crudely joining two dissimilar cables will create an impedance mismatch, degrading the cable's ability to convey the signal.

RCA-phono plugs/sockets aren't actually 75 Ohm connectors, but they're close and cheap enough for non-professional applications. That's why they're used on consumer video and digital audio equipment in place of the standard professional 75 Ohm BNCs.

And that's why the best solution for interconnecting video gear using BNCs with RCA-phonos us to use standard BNC cables with phono adapters as I linked above. That way the characteristic impedance is maintained and everything works.
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Re: Coax to phono connector wanted

Post by Guest »

Hugh Robjohns wrote: Tue Oct 04, 2022 9:44 pm However, video (and digital audio and RF) are all high frequency signals that require the cable to have specific characteristics if the signal is to get to the other end intact.

Are, the higher the frequency the shorter the wave band, they don't tend to travel very far or travel well though buildings and hills unlike long wave radio or base notes do they
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Re: Coax to phono connector wanted

Post by James Perrett »

Organ Grinder wrote: Tue Oct 04, 2022 5:09 pm
I used an old phono cable and a coax cable soldered together

As Hugh says, just any old phono cable won't work. It needs to have low loss at high frequencies and a characteristic impedance of 75 ohms. That's why the CPC cables I linked to use RG59 cable which is likely to be similar to the cable used by the BNC cables.

A random join in a video cable is likely to have a very different impedance which will result in much of the signal being reflected back to the source and very little actually getting through. There are ways of joining video cables to preserve the impedance but it requires very careful trimming of the cable and very neat soldering.

Oh - and just to confuse things further - BNC connectors and cables are available in both 50 ohm and 75 ohm versions. You need to make sure that you have the 75 ohm version for video and digital audio.
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Re: Coax to phono connector wanted

Post by Guest »

James Perrett wrote: Tue Oct 04, 2022 10:49 pm As Hugh says, just any old phono cable won't work. It needs to have low loss at high frequencies and a characteristic impedance of 75 ohms. That's why the CPC cables I linked to use RG59 cable which is likely to be similar to the cable used by the BNC cables.

Oh - and just to confuse things further - BNC connectors and cables are available in both 50 ohm and 75 ohm versions. You need to make sure that you have the 75 ohm version for video and digital audio.

But what about when the connector arrives and if I use one of my cheep phono cables to connect to the fitting. I mean I bought them from the pound shop and not Dawsons. But they are OK for sound, but will there be any different for video?
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Re: Coax to phono connector wanted

Post by James Perrett »

Organ Grinder wrote: Wed Oct 05, 2022 12:01 am But they are OK for sound, but will there be any different for video?

They're no good for video.
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Re: Coax to phono connector wanted

Post by Guest »

So I need to get a 75 Ohm one. Dam! I wish I ordered it at the same time when I ordered my plug as part of the fiver handling charge
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Re: Coax to phono connector wanted

Post by ef37a »

Organ Grinder wrote: Wed Oct 05, 2022 2:55 am So I need to get a 75 Ohm one. Dam! I wish I ordered it at the same time when I ordered my plug as part of the fiver handling charge

Hang on a minute there OG and other chaps. I have an external camera which has a very thin, ~3mm cable with it. Some 5mtrs long and BNC to BNC. Yes it is just your bog standard composite colour signal but it is pin sharp on a 40" FSTV. There is no trace of reflection bands. The camera was only 50 quid from Maplins years ago so I doubt the cable is anything special. In fact I know it isn't because I had to chop one of the plugs off to get it through a window frame. I then used a pair of RCAs and a coupler.

In fact the video and sound go into an RCA to CAT5e extender box, round the roof space, 30mtrs easy and into a recorder in the living room. Yes, that does degrade the video somewhat but again, no reflections.

I very much doubt you will have any bother friend. 4k maybe.

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Re: Coax to phono connector wanted

Post by Mike Stranks »

Couple of points...

You can get - and I have used a few times - solderless BNC connectors. They're more expensive, but if used properly and carefully can save a lot of hassle.

Hugh has already mentioned, and Dave has endorsed, that for short runs then 'bog-standard' phono-phono leads should be fine. When I first came across this (in composite video days of yore) I raised an eyebrow, but was assured that for our application it would be fine. It was. As Dave says, whether I'd want to attempt it with 4K and large screen is another matter... :)

BTW; for the longer runs to and from the control room, we always used 'proper' 75-ohm coax into a Distribution Amp at the 'downstream' end.
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Re: Coax to phono connector wanted

Post by MarkOne »

The thing I love about this forum, is it's probably the only place on the internet where a simple question about a BNC connector can get to be 3 pages long! :thumbup:
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Re: Coax to phono connector wanted

Post by Guest »

Mike Stranks wrote: Wed Oct 05, 2022 9:45 am BTW; for the longer runs to and from the control room, we always used 'proper' 75-ohm coax into a Distribution Amp at the 'downstream' end.

So the longer the cable the longer the signal and the more resistance therefore the better quality cable needed, fortunately for me this is just to test my cameras so a short distance of say a meter cable from the camera to the TV should be enough
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Re: Coax to phono connector wanted

Post by ef37a »

Organ Grinder wrote: Wed Oct 05, 2022 2:52 pm
Mike Stranks wrote: Wed Oct 05, 2022 9:45 am BTW; for the longer runs to and from the control room, we always used 'proper' 75-ohm coax into a Distribution Amp at the 'downstream' end.

So the longer the cable the longer the signal and the more resistance therefore the better quality cable needed, fortunately for me this is just to test my cameras so a short distance of say a meter cable from the camera to the TV should be enough

The cable resistance matters not a jot because it will be far less than the 75 Ohm source. I dare say there is a limit but CCTV is piped around buildings,100s of mtrs.

I have not got clear picture of what you intend to do. If you intend to 'splice' a BNC cable to an RCA cable a twisted join offends us but up to a few mtrs would probably work fine. Better to solder the join and 'lay' the inners in line, tape up then lay the outers on top, more tape.

We have all had to do 'Terrible Things' to get out of shtuck in the PA and TV game!

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