comparing RCA cables quality

[betwixt]

Even with 10 Ohms resistance you would find the picture quality varied wildly, I still suspect you have a measuring problem. Also note that it would be very unusual if the reading varied by itself, even a damaged cable would show a steady reading unless it was moved around.

I just measured a new 100m drum of cheap 75 Ohm cable and the center wire measured 22.3 Ohms end to end and the shield measured 3.2 Ohms. A good quality cable would have measured less than half that. Those figures would equate to 0.22 Ohms and 0.03 Ohms per metre and should be typical of domestic cables. Don't expect to measure 75 Ohms, that is the 'characteristic impedance' of the cable, not it's DC resistance. Measuring impedance is a lot more complicated!

Incidentally, resistance has nothing to do with the speed the signal travels and in this type of cable it travels at about 0.6 times the speed of light. Read up on 'velocity factor'.

Brian.

[budwzr]

Incidentally, resistance has nothing to do with the speed the signal travels and in this type of cable it travels at about 0.6 times the speed of light.Brian.

OK, I stand corrected, it would slow down to a paltry 111,600 miles per second.

[budwzr]

I just measured a new 100m drum of cheap 75 Ohm cable and the center wire measured 22.3 Ohms end to end and the shield measured 3.2 Ohms. A good quality cable would have measured less than half that.

Not to be an asshat, but where's the proof that a better cable produces a better picture?

There's a point of diminishing returns in everything, and that point is reached as you get closer to zero. How do we know that 10 ohms, or 5 ohms, or 1 ohm, has any tangible benefit?

The brain perceives less "this" equals more "that", and manufacturers use that against use. For instance, headphones that have a frequency response below what the human ear can hear, just to have a lower number to throw around.

[betwixt]

There is no proof, in the same way there is no way to subjectively grade a picture. All we can assume is that a very bad cable will have some noticeable effect on the image. As has been stated all along, the differences between domestic grade cables is so small that even an expert eye would be hard pushed to notice.

Joking aside, you should consider that even at modest video frequencies like 10MHz, several cycles of signal will go into one end of a short cable before anything has time to come out of the other end. This is why the impedance is important, if the driver, the cable and the terminator have different impedances, some of the signal bounces off them and goes backwards up the cable. While this may not sound important, it can cause some frequencies to meet an opposing wave and cancel out. The principle is used to advantage to make "quarter wave stubs" from open ended cable to reject specific frequencies.

Brian.

[jagabo]

even at modest video frequencies like 10MHz, several cycles of signal will go into one end of a short cable before anything has time to come out of the other end.

That must be a really long cable. 10 MHz corresponds to about a 15m wavelength (assuming half the speed of light).

[budwzr]

If a 10' cable has 10 ohms resistance, then does a 5' cable have 5 ohms? In that case, a 2" cable would be best.

If so, a 3' "Monster" $60 cable can easily be surpassed by a 3" cheapy cable.

If not, then 10 ohms is beyond the point of improvement.

There's the proof!

[budwzr]

After astutely ruling out a bad cable, another consideration, for the best connection, might be to double or triple up the cable using splitters/combiners. That would effectively double or triple the pathway.

Any loss due to the splitters/combiners would be made up due to sheer volume of pathway.

[betwixt]

Be careful to understand what the deficiencies in cable actually cause.
In an ideal World, the circuit driving the cable has an output impedance of 75 Ohms, the cable has a characteristic impedance of 75 Ohms with zero resistance and the load at the far end has an impedance of 75 Ohms. In that ideal World, all the video would be cleanly delivered to the load.
Now back in the real World, the resistance has two effects, it acts as a simple voltage dropper, if the resistance was 75 Ohms you would only get half the voltage at the load. The other half would be turned into a tiny amount of heat in the cable. Thankfully, the resistance is usually much lower and so is the voltage drop.
The other effect is to alter the impedance, this along with other impedance hazards, crates a mismatch at both ends of the cable. In an extreme case, the picture 'rings' , a non-technical name for all the edges being followed by diminishing outlines. I have seen it so severe that the picture is almost unrecognizable. In less severe cases, there may be a degree of ghosting or the wave cancelling effect I mentioned earlier might cause some frequencies to be 'lost' in the cable. Some frequencies are relatively incondequential, some, for example the color sub-carrier frequency might result in no colour but a good monochrome picture.

Brian.

[budwzr]

Hmmm...then the only prudent option left is to rig an amplifier at the output, and an attenuator at the end.

[betwixt]

No amount of amplification will make a distorted signal good again. What I do is use quailty cable and fit the connectors myself, RCA plugs and jack plugs are seldom 75 Ohms, I use BNC wherever possible. On long runs at the load end I use a 75 Ohm resistor as terminator and follow it with a compensation amplifier. That's one with high input imedance so it doesn't drop the 75 Ohm resistor and also has a 75 Ohm output to feed the following equipment. It also has an adjustment to increase the amplification slightly at higher frequencies to compensate for the drop in these frequencies in the cable. A test signal is sent out from the source and the adjustment is made to optimize it at the destination.

Brian.

[lordsmurf]

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?

Let's not bring in water hoses. I still regret my $20 cheapos. Kinky near-worthless crap.
The $50-75 hoses are worth it, built tough, no kinks.

Not really a good analogy for audio cables.

[budwzr]

No amount of amplification will make a distorted signal good again.

The signal is distorted?

OK, you win. But I got the LAST WORD!!!!!

[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?

Let's not bring in water hoses. I still regret my $20 cheapos. Kinky near-worthless crap.
The $50-75 hoses are worth it, built tough, no kinks.

Not really a good analogy for audio cables.

The best garden hose is the red hot water rubber type with brass fittings. They don't kink, period. I got a 100 footer custom made for me at one of those "Hose-Man" freeway stores. Cost me $80 12 years ago, and still going strong. I can leave the water on full pressure while it's hanging in the rack and not a drip of water, ever.