Ali, buzz wrote half the Bible here. I was busy today....
I'll try to make these answers brief. I said try.
On March 14, 2019 at 20:02, buzz said...
At home one might have a few components to hookup and the distances are relatively short, perhaps 10 or 15 feet at most and everything plugs into the same outlet. As the system becomes larger and more complex with longer cables, nasty issues creep in that are very difficult to resolve. Issues such as picking up hum or the local radio station. Further, too long cables can cause loss of high frequencies.
I had a client in an apartment with living room on one side of the wall and bedroom on the other. He had surround systems in each room that shared inputs. He moved to a place where the living room and bedroom where fifty feet of wire apart (once done). He had me wire up his two systems.
Halfway through connecting things up, I left him with working components, and the TV had hum bars (this was back in NTSC TV days) running up it. He sort of freaked.
I told him that we'd hunt down the hum problems once the wiring was finished. When the wiring was finished, there was no longer any hum.
Lesson: A ground loop is a ground loop that causes hum. A ground loop that doesn't cause hum is not called anything because there's no problem with it. Keep adding interconnections and hum might just go away.
In a commercial environment there could be dozens and dozens of sources and cable runs of more than 100 feet.
The way to avoid problems in these cases is to use twisted wire, balanced connections, and have low source impedance.
The commercial guys will not hesitate to run their cables "over the river and through the woods". At home we start to worry when the length approaches 20 feet or so for audio cables. This is because the commercial guys use "balanced" cables.
And low source impedance.
If you look at the balanced cable ends, there are multiple pins
I think calling three "multiple" is an exaggeration. RCAs have two -- the pin and the shield -- and we don't say "several" with that.
, not just one pin that is typical of home setups.
One pin plus shield, remember. Also, that shield is the ground connection from one component to another.
There is a fierce amount of hostile (to audio signals) energy floating around out there. We use "shielded" cable in an attempt to protect the signal from being polluted by noise. The shield is a flexible metal sheath enclosing the signal wire(s). The theory is that the shield will intercept the noise energy and preventing it from reaching the enclosed conductor(s) carrying the signal.
Note that the shield can be connected at both ends or at one end; if it's connected at one end, it should be connected at the source, so noise can sink into the low impedance situation at the source. If it sinks into the input (think AUX IN), the noise will be amplified along with the signal.
Unfortunately, the shield is not 100% effective and a certain amount of noise reaches the central conductor(s) anyway. Obviously, the longer the cable run and in very noisy environments, enough noise energy reaches the center conductor(s) to cause trouble.
True, and here he's speaking of unbalanced wiring. Plain old Consumer Krap (POCK).
Balanced cables use an interesting trick to cancel most of this noise. The reason for the multiple pins is that two versions of the signal are sent down the wires inside a shield. You can think of them as (+) and (-) versions.
Now this is the opposite of exaggeration. They are EXACTLY opposite polarity versions of one another. And three pins still isn't multiple.
As noise enters the cable along the way it effects
affects
the (+) and (-) wires equally. At the receiving end the two versions are subtracted. From algebra, remember that (+1)-(-1) = 2. The (2) is our signal and the noise faces (+1)-(+1)= 0 in that subtraction and the noise is cancelled.
I'd say it this way: the following stage amplifies the signal on both halves of the wire, and adds them up. The two halves that are reverse in polarity add up to the signal. The two noise portions that are identical appear at the following input, but they are both the same signal, resulting in zero signal. The technical name for the same signal (neither reversed) on both wires of a balanced cable is the Common Mode Signal. See, the noise is common (and identical) on both wires. But since the amp amplifies the difference between the two signals, and there's no difference, that noise is not seen. It sill might be possible for the amp to somehow be sensitive to noise on both wires. The spec that describes this issue is Common Mode Rejection Ratio (CMRR) which lists how HOT a common mode signal (identical on both wires and identical polarity) before it will influence the audio signal.
Now to the 600 Ohms. This is a little more difficult to explain. Note that this explanation is a little crude and I'm sweeping a few things under the table that should be included, but it will help you visualize what is happening and save me from writing a book, rather than a paragraph.
As can often be said of me, too late!
If you measure the resistance of a consumer audio input jack you might measure 10K, 20K, or even 100K Ohms. In pro audio gear you'll measure something on the order of 600 Ohms. In our shielded cable only a small amount of energy leaks through the shield. Unfortunately, if this small amount of energy becomes a significant fraction of our signal energy this will cause trouble. In consumer equipment a 100K input will not dissipate much of this energy and it mixes easily with our signal. In the 600 Ohm environment the small noise energy is easily dissipated.
Why don't we use balanced at home? Simple -- balanced is much more expensive than unbalanced. First, we have three times as many conductors inside the shield and three times as many pins in the connectors.
buzz... wha?
Balanced cables have two signal leads and one shield. That's three. Unbalanced have on hot lead and one shield. That's two. There's no three times anything (except the price of the wire and the price of the connector).
Next, we need an inverter as part of the source to generate the (+) and (-) versions. Then, we need a subtracter at the receiving end to support our cancellation trick. Finally, we need to supply more signal power from the source so that the the signal does not disappear along with the noise at the 600 Ohm receiving end. Overall, the balanced approach is several times the cost of unbalanced -- multiplied by however many inputs and outputs the equipment supports.
Basically, yes.
There is more to this story than I've indicated above.
True.
Real pros are loath to incorporate unbalanced signals into their system. The pros will use special boxes
Called Direct Boxes in PA use.
to isolate and convert the unbalanced equipment to balanced before introducing this signal into their mixer. Managing noise becomes so complicated for large systems that there are engineers who make a good living by going to these installations and eliminating the noise.
Ali,
I remember one question you asked: Yes, we connect an unbalanced source to a balanced input by connecting the unbalanced hot to the balanced +, and the unbalanced ground to the balanced - and to the ground. Note that this means that only half the voltage (-6dB) is presented by only the +. Beyond that, pro level is hotter anyway, as buzz mentioned.
One thing to remember is to not try to simply mix balanced and unbalanced stuff. And if you EVER have to play a stereo source in mono, DO NOT use a Y cable. Rane has a white paper on this. Download it.
