Mixing cable types

[sandall]

Does anyone have any hands-on experience of mixing RG59 (75R) & RG58 (50R) cables? I've just been warned that I may get a call on Monday morning for a video link for a big public/political meeting, rigging that afternoon, supplying camera, BP projector on-stage (about 30m run), daisy-chained via VDA to another projector in an overflow room (another 50m). I've got nearly enough (but not quite) made-up lengths of RG59 & have just found 10m of RG58, which I can terminate tomorrow, By the time the call does (or doesn't) come it will be too late to source a reel of RG59, let alone have time to cut & terminate it, hence my query.

[david.elsbury]

Composite ? HDSDI? it sounds like you intend to downconvert the pc to composite... I wouldn't recommend it. Poor resolution. It will probably work for composite but unlikely for SDI.

 

my 2c :)

[timsabre]

I'm not sure any PC's are involved...

Over that distance I think it will work fine, either composite or SDI. I've had to run HDSDI over all sorts of dodgy cables and it seems to be very resilient unless you are trying to go a long way (100's of m)

[sandall]

Sorry, I should have said - it's composite. DV camera on a stick feeding comp out on a phono to the comp sockets of 2 XGA projectors via a cheap'n-cheerful VDA. Being brought up in telly-land I'm a bit cautious about mixing impedances ("reflections dear boy"), so I was wondering if anyone had had any problems with mixing 75R & 50R cables (I must admit I never bother which impedance BNCs I use). It's a bit high-profile, & there won't be time to think up a Plan B, so I don't want to end up with egg on my face. Last time I did this in the venue we used wi-fi, but it was a bit nail-biting.

[david.elsbury]

Ah my apologies. I too have done it before and it seemed to work fine, but for what sounds like such a high profile gig, I'd not chance it! :-)

[abbotsmike]

CPC do reasonable quality, cheap, ready made coax cable sets. Might be able to click & collect monday morning?

[Whiskers]

Sorry, I should have said - it's composite. DV camera on a stick feeding comp out on a phono to the comp sockets of 2 XGA projectors via a cheap'n-cheerful VDA. Being brought up in telly-land I'm a bit cautious about mixing impedances ("reflections dear boy"), so I was wondering if anyone had had any problems with mixing 75R & 50R cables (I must admit I never bother which impedance BNCs I use). It's a bit high-profile, & there won't be time to think up a Plan B, so I don't want to end up with egg on my face. Last time I did this in the venue we used wi-fi, but it was a bit nail-biting.

 

you will lose signal at every transition to 50ohm and possibly pick up reflections as well,so if you must do this wicked thing reduce the number of transitions to a minimum ( presumably 1) or perhaps run VDA to one projector in the wrong cable, you can pretest this part of the system, feed the vda with a known good signal with a white black transition and look for repeats usually to the right. The "letter box" in standard test cards is good for this.

 

For the cost of a reel of cable I would make up more vid cables and code them so the right cable goes to the right job,

 

Note that barrel connectors also come in 50 and 75R these can give you troubles to

[paulears]

Is the only real problem the load impedance at the distant end, in trams of padding the input, so it's at that point the impact of the impedance mismatch will be fine or not. This is where the signal remains stable decision is made. The loss in level might just be a slight drop or total loss of syncs, no real way of telling. My experience of kit with phonos is that the output levels are never exactly 75/1v p-p anyway! As for reflections, that's the other unknown. Long lengths of the wrong cable impedance do seem to impact on standing waves, and with non standard impedance too, those horrible double edges may well be visible. You'll just have to give it a try and see what happens. Although the recent test results on mixing 50 and 75ohm connectors suggest there are few problems, I just find the 50ohm pin does 'loosen' up a 75Ohm socket for when you then stick the proper connector back in. I've got quite a few RF cables and it's a bugger keeping them separate.

[sandall]

For the cost of a reel of cable I would make up more vid cables and code them so the right cable goes to the right job,

 

Yup, as the saying goes "I wouldn't start from here", & given an extra 24 hours notice it would have been an overnight from CPC, on the grounds that a reel of RG59 is never wasted.

 

As it is, I can spend my weekend trying to track down a retailer with some over-priced cable, only to find that the client has got cold feet & gone to one of the big AV companies, or take a punt that my daisy-chained lengths will be fine. Ho hum.

[bruce]

Is there nobody locally you can scrounge from? or hire? If you’d been local to me, I’d lend you some psf 1/3 - pretty sure there’s loads of it out in the garage somewhere - but since I’m several hundred miles away, that’s not a practical option.....

 

Can you test the daisy-chained lengths now? That would inform you whether it’s likely to be good enough (I’d guess it will be) or whether you need to do something better.

[J Pearce]

There must be a sub-hire possible in London surely?

[sandall]

Is there nobody locally you can scrounge from?

 

After lots of calls (all my usual suspects seem to be on holiday) I think I've tracked down an unwanted reel of the thin stuff. It's a bit lossy compared with standard RG59, but better than nowt.

 

Many thanks to all for the various suggestions. The job will probably now go to one of the big boys at 10x the fee

[sandall]

The job will probably now go to one of the big boys at 10x the fee

Far from it as it turns out. It's still all up in the air; the latest word being that rather than having to pay someone to run the audio & video they will go with an unattended sound system, & attempt to stream the politician with the beard using 4G.

 

As to my original query; having a bit of spare time yesterday, I daisy-chained 100-odd metres worth of mini-RG59 (CPC & RS),fairly thin RG58, something unlabelled that might be RG59 & that horrible thin black cable with yellow phono plugs that you get with consumer stuff, all mixed up to give as many mismatches as possible, plugged together with audio phono couplers (which led to a few dodgy connections) & fed with Testcard-F & Colour Bars from .ppt slides, via a cheap'n'cheerful VGA-CV converter (which later got very hot & died!).

 

Result, compared with a short length of RG59, when projected onto shiny white card? Possibly a split-screen would show up a marked difference, but all I could detect was a slight loss of contrast. Inserting my £9 VDA at various points gave a slight loss of definition (HF loss), as might be expected, but allowed me to increase the level, & thus restore the contrast. No visible reflections on eitherTestcard or Bars. Replacing the projector with a cheap LCD TV/monitor gave similar results.

 

I then replaced the VGA-CV box with a reasonable, but several years old (so only 1MP), consumer SD DV camera. The shortcomings of the camera totally masked any defects in the cabling.

 

Conclusion - with an SD (or XGA) source, unless you are using identical screens fed by vastly different cable lengths, mixing up cable-types is unlikely to cause any problems. Cable-losses might become an issue with an HD source, but probably only if feeding monitors rather than projectors.

[david.elsbury]

I once did a 4 camera live mix at high school for a talent show. Camera 4 (onstage roaming on a diy steady cam) was about 90mtrs away, and since I had run out of coax (even 50r!) Mic cable it was.

 

The run was so long that we simply lost the colour information from the signal. So we fudged it and referred to it as an "effect" ;)

[sandall]

I once did a 4 camera live mix at high school for a talent show. Camera 4 (onstage roaming on a diy steady cam) was about 90mtrs away, and since I had run out of coax (even 50r!) Mic cable it was.

 

The run was so long that we simply lost the colour information from the signal. So we fudged it and referred to it as an "effect" ;)

Might be worth adding a pair of XLR-phonos next time I have a "piece of string" link to do. I don't suppose you know how far you could get before you lost all colour.