comparing RCA cables quality

[eadmaster]

I have quite a few RCA cables collected from various products
and i'd like to choose which one should offer the best quality for video capture.

Apart for the general rule "the shorter the better",
i'd like to compare cables of the same length too.

I've tried measuring the (DC) resistance with a multimeter but i always get "unstable" readings.

[edDV]

This might help. Difficult to know what you have without splicing the cable.

Their premium line.
http://www.bluejeanscable.com/store/composite/index.htm

Same company, budget line.
http://www.tartancable.com/composite-video-cables/index.htm

[fooledagain]

There is very little difference, save for the amount the companies spend on marketing to have writers come up with pseudo science ad copy to persuade you to part with more money. If you doubt this do a blind comparison test.

[lordsmurf]

There are good and bad cables,
but brand name cables are not any more special than any other good no-name cable.
Radio Shack branded cables used to be excellent. (Discontinued.)

[Nelson37]

Somebody a while back did just such a comparison test, there were some significant differences among cables, IIRC the expensive ones definitely were not worth it.

[jagabo]

Aside from a few obviously defective or completely unshielded cables you won't see any difference.

https://forum.videohelp.com/threads/221249-Test-Caps-various-composite-and-s-video-cabl...ight=composite

[eadmaster]

Can lower cable resistance be considered a quality factor?

[rcubed]

Can lower cable resistance be considered a quality factor?

Impedance (ac resistance) is the real thing of interest. The cables should match the input and output impedance of the source and sink. For component cables I believe the value is 75 ohms. http://www.audioholics.com/education/cables/component-video-cables-the-definitive-guide . That and as someone else pointed out shielding. Somewhere I remember reading an article that pointed out that one of the "premium" cable manufactures had a really bad impedance match on one of their component video cables, just one of the three. Audio cables since they are lower frequency 20K Hz or so max. are not so sensitive to impedance matches. With reasonable quality cables you probably won't see a difference.

I don't think DC resistance will tell you much other than if the cable is open (broken or poor connection) or not. Usually the DC resistance is quite low.

rcubed

[betwixt]

The resistance from inner to inner and shield to shield should be almost unmeasurably low (< 1 Ohm). The resistance from inner to shield should be very high (> 10M Ohms). There are some special AUDIO cables which have a carbon impregnated insulaton around the inner wire which is slightly conductive and there to prevent static charges being developed as the cable is moved around. These are commonly used on roving microphones to prevent crackles as the user drags the cable. these are NOT suitable for video work.

As stated earlier, what really matters is the degree of shielding and the impedance. The best shielding is a weave of copper or steel wires which completely surrounds the inner wire and it's insulation. The worst shielding is a metalized plastic foil wrapped around the insulation, this tends to crack and the metalizing falls off, leaving the connection open-circuit or prone to interference pick up. Some cables use both, plastic to keep costs down and a thin copper weave to 'join up' any broken metalizing.

The impedance is derived from the ratio of the inner conductor diameter and the outer conductor diameter. For radio use the normal impedance is 50 Ohms for video it is 75 Ohms, although both will work in both applications, only the correct one will work optimally. The effect of using the wrong impedance will be 'ghosting' although n short runs the ghost image will be so close to the original it might not be visually obvious. It can result in color cancellation (ghost is one half color subcarrier cycle behind the original) or 'ringing' where sudden changes in contrast are followed by a series of outlines. Always use the correct impedance.

Sadly most cables are just the cheapest stuff the manufacturer can lay their hands on and even the plugs and connections are likely to be the wrong impedance. They are not mechanically stable so bending them alters their impedance. If you really want the highest quality, buy decent 75 Ohm cable from a reputable electronics supplier and make your own. I feed video down cables here which are nearly 100 metres (> 300 feet) long and have the equipment to measure the loss and impedance, the degradation is almost unmeasurable.

In summary, yes there are differences in cables but 99% of them are technically at the 'crap' end of the scale, regardless of their cost and the name on them. Whether you would ever notice the difference is another matter, despite 'high end' brand names, most domestic video equipment is incapable of showing the errors a bad cable would introduce.

Brian.

[eadmaster]

Ok, since the multimeter can only measure DC resistance it will be of no use in this case.

Do you know other scientific methods/instruments that could help measuring cable quality (apart of blind tests)?

[edDV]

Ok, since the multimeter can only measure DC resistance it will be of no use in this case.

Do you know other scientific methods/instruments that could help measuring cable quality (apart of blind tests)?

Not clear what you are trying to do. Identify shorted cable or test performance?

Those links I gave above are representative of pro level (Belden 1694A) and acceptable consumer grade (about $3). The pro level cables are good for about 300 ft without equalization. Cheaper cables will have shielding issues, return loss and attenuation over longer lengths. For simple VCR recording, a short consumer cable is good enough.

http://www.bluejeanscable.com/pages/technicaldocs/1694tech.htm

[eadmaster]

Not clear what you are trying to do. Identify shorted cable or test performance?

Test quality.


What about using component cables like these instead?
They should have better specs.

[redwudz]

If you want to spend money for no improvement or affect, go ahead and buy the expensive cables. As stated, very cheap cables may have poor shielding and poor quality connectors, but will likely work exactly the same as very expensive cables for short cable runs. They are just pieces of wire. A better quality cable may last longer if the construction is good, with flexible wires, good quality cable insulation and sturdy connectors. This may be useful if you move or unplug your cables very frequently.

Oxygen free, gold plated connectors or other tags are just hype to sell expensive cables, IMO. Electrons move the same speed in a copper wire, irregardless of the brand name on it. Belden is one of the major producers of cable, so their specifications are pretty much industry standard.

Now if you have a very long cable run, then cable quality can make a difference, mostly with shielding or signal attenuation. But for six feet or less, just about any but the very cheapest cables work the same as the most expensive.

There is equipment to judge the quality of cables, but it is well out of the price range of an average user, several thousands of dollars most times.

[budwzr]

Does a $100 garden hose spray more water than a $10 one? Sure, it has the new "SuperHydroTech" 3-stage design, but what you need is simply water delivery from point A to point B, right?

[betwixt]

I didn't even touch on "oxygen free copper". Any chemist how much oxygen there is in copper and they will give you the same answer.
I never watch TV in a vacuum anyway, my eyesight is bad enough already without having my eyeballs pop out.

Brian.

[edDV]

Not clear what you are trying to do. Identify shorted cable or test performance?

Test quality.

Difficult to test a short cable. As said above, consumer grade cables will be adequate for 1.5 meters. Failures would be shield breakdown or poor connectors.

[redwudz]

Units like the Tektronix 1502C are commonly used to test cables. They use a process called Time Domain Reflectrometry.
From Wikipedia: http://en.wikipedia.org/wiki/Time-domain_reflectometry

A Tek 1502C lists for about $12,000 and there are newer units that have mostly replaced it. It's more commonly used to test long buried cable runs. And it's more suited to test cable integrity than quality.

[edDV]

Cables are a third or forth order issue for capture.

#1 is source player + TBC if needed
#2 is the capture device
#3 is disk system if capturing uncompressed.

[jagabo]

Once you've exceeded the requirements to carry a 15 MHz composite signal it doesn't matter how much better the cable is. Pretty much all <10 foot composite cables can handle a 15 MHz signal without appreciable attenuation, ghosting, blurring, or noise from outside sources. Beyond that it doesn't matter if the cable can handle up to 60 MHz or 100 MHz or 10 GHz.

[budwzr]

Units like the Tektronix 1502C are commonly used to test cables. They use a process called Time Domain Reflectrometry.


[Attachment 6829 - Click to enlarge]

Whew! Is that thing turbocharged?

[edDV]

The time domain reflectometers (TDR) are mainly used by telcos and cable companies for long coax cable diagnostics. Cable companies need equalized 1GHz from head end to home. By contrast, SD capture needs 6 MHz and HD needs ~30MHz (60 MHz max).

When I was at Tektronix, we did optical TDRs for telcos that tested fiber out to 93 GHz over miles and could ID a bad splice or break to within a meter so the telcos could open the right man hole.

[eadmaster]

That looks a pretty expensive equipment!

Btw, i was curious to know why i get variable/unconstant readings with the multimeter.
I expected the (DC) resistance to be low and constant, as rcubed said.
I tested 2 cables, and the readings where moving between 10 and 110 ohms in both cases.

[redwudz]

Resistance measurements somewhat depend on the testing voltage used by the multimeter. Then with very low resistances, you have to take into account contact resistance through the leads.

But it shouldn't vary that much if you have a solid connection. It may be a problem with the multimeter. I would expect the resistance of a short cable to be 0.1 Ohm or probably a lot less. All but expensive multimeters don't really have much resolution below 0.1 Ohm, so really not much use except to show a high resistance cable or a broken/open cable.

[betwixt]

As Redwudz says, measuring very low resistance is a science in itself. If your meter does not have four test probes but two like most have, it will not accurately measure low values. The reason is simple, the meter probes, wires and the physical connection to your video cable will have more resistance than the video cable itself. 'Four wire' probing, where the test current and test voltage are carried on different wires is far more accurate but you are starting to talk in the realms of scientific equipment, not home video!

If the readings you see are actually changing between 10 and 110 Ohms there is something seriously wrong and it would be immediately visible in the picture. A cable resistance of 75 Ohms would halve the contrast level and probably cause synchronizing problems too. I suspect the problem you see is more to do with how your meter probes are touching the cable than the cable itself.

If you measure between the inner wire and shield you *might* get variable readings because an Ohm meter is also sensitive to static charges in the cable and the capacitance across the insulation plastic, as long as this is a very high reading > 1M Ohms you shouldn't worry, different meters will give different results.

Brian.

[jagabo]

I suspect the problem you see is more to do with how your meter probes are touching the cable than the cable itself.

I agree.

[budwzr]

I use Mobil 1 full syn in both my vehicles because it's an engineered product, not petroleum based, and allows me to go up to 10K miles between changes, although I usually change it at 5K.

But if I buy a $59 Monster component cable, vs. Amazon Basics $4.99, there's no upside except bling.

[budwzr]

I suspect the problem you see is more to do with how your meter probes are touching the cable than the cable itself.

I agree.

Since meters use their internal battery to test resistance, wouldn't a low battery cause wild readings too? And you can't use your meter to test the battery, because it needs the battery to turn on, so it's like a Gordian Knot.

[jagabo]

Since meters use their internal battery to test resistance, wouldn't a low battery cause wild readings too?

Yes, that's a possibility. But I think it's likely he's just holding the probes against the ends of the cable and not getting good contact.

[eadmaster]

I've done the tests again paying more attetion to have a good contact between the probles and the cable ends.
This time the readings were lower (<10ohms), but still unstable.
In the end i've found it is quite difficult to compare the cables this way.

[budwzr]

Since electrical signals travel at 186,000 miles per second, the same as light, 10 ohm's of resistance probably wouldn't slow it down enough to notice. I'd say you're good to go.