Explain the balanced signal and unbalanced signal in a popular way, and understand why there is noise in the long line.

When doing sound engineering and debugging, everyone has encountered an unsolvable problem:
As soon as the line is stretched, there will be buzzing current and rustling noise; Short lines are completely normal.
Many people think that the wire is of poor quality and poor shielding. In fact, in 90% of cases, there is only one fundamental reason: using unbalanced signals to run long distances.
The "balance and non-balance" in professional sound is not a metaphysical parameter, but the underlying logic that determines whether the system is clean or not. Today, I thoroughly explained it in the vernacular that the master can understand. After reading it, I will never connect the wrong line or step on the noise pit again.
First, remember the core conclusion first (directly avoid the pit)
1. Unbalanced signal: 2-core wire (signal+ground wire), which can only be used for a short distance, and there is a high probability of noise when it exceeds 3m.
2. Balanced signal: 3-core wire (positive signal+negative signal+ground wire), which is specially used for long-distance running and has almost no interference for tens of meters.
3. All long-line noises, ground loop buzz and lighting interference noise are basically caused by unbalanced wire transmission.
Second, popular disassembly: the working principle of two signals
1. Unbalanced signal: Single-line sound transmission, easy to eat interference.
The structure is the simplest: there are only two wires, one for transmitting audio signals and the other for grounding and shielding loops.
Common wires: 3.5mm small three-core, Lotus RCA and large two-core TS(6.35 mono).
It can be understood as: one-lane sports car.
The audio signal goes forward alone, passing through power lines, light lines, LED screens and routers, all of which radiate electromagnetic clutter.
The longer the line is drawn, the longer the antenna is, and the more interference noise it picks up.
The most fatal point: non-equilibrium has no noise reduction ability. When the interference is mixed into the signal, the receiving equipment can't tell which is the sound and which is the noise at all, so it can only be amplified together, and finally it is the continuous buzzing noise and rustling noise.
This is why: mobile phones and computers are connected to stereos, and when the lines are long, there will be a current sound.
2, balanced signal: double line cancellation, with noise reduction buff.
The structure has an extra layer of protection: three wires, positive original sound, negative anti-crosstalk and ground wire shielding.
Common wires: cannon XLR, big three-core TRS(6.35 stereo)
Popular principle, in a word: send out a pair of exactly the same sound with opposite positive and negative, the noise will be automatically cancelled, and the original sound will be kept twice.
The transmitter will output two signals:
Positive electrode: normal voice/music signal
Negative pole: the same signal, but the phase is reversed (positive and negative inversion)
Two signals are transmitted synchronously in the wire, and the electromagnetic interference and power supply clutter encountered along the way will be evenly coupled to the two signal lines, resulting in exactly the same noise.
At the receiving end (mixer, power amplifier and processor), the equipment will invert the negative signal again and superimpose it with the positive signal:
The same noise = positive and negative cancellation, directly disappear.
Original sound = superimposed enhancement, clean retention.
This professional principle is called common mode rejection (CMR), and it is also the core black technology of professional audio anti-interference. No additional noise reduction equipment is needed, and the hardware structure has its own noise reduction effect.
Third, why "the long line will make noise"? The core gap is here.
Many masters wonder: the same line, it's okay in a short distance, but it's useless in a long distance?
Unbalanced signal: the length is proportional to the noise.
The wire itself is an antenna for receiving interference. The longer the wire, the more electromagnetic clutter it receives. Unbalanced systems have no cancellation mechanism, and all interferences are mixed into audio signals. The longer the distance is, the greater the noise floor is, and it is basically inevitable to exceed 3 meters. Noise exceeding 10 meters will seriously affect the sound quality.
Balanced signal: the length hardly affects the noise.
No matter how long the wire is, the external interference falls on two positive and negative signal lines simultaneously and equally. Difference channel at the receiving end will uniformly cancel all common-mode noises, and even if the line is 30-50 meters away, it can still keep the sound clean and noiseless, which is why the balance line is used in all the stage, KTV and conference room projects.
Four, the common wire quickly distinguish (field direct control)
Unbalanced (short-distance home/audio source only)
1. RCA Lotus Line: set-top box, player, home audio connecting line.
2. 3.5mm small core: output line of mobile phone, computer and screen projector.
3. 6.35 Two-core TS: common musical instrument cable and some cheap microphone cables.
Taboo: Never use these wires to run cabinets and long lines of the whole field, which will cause 100% noise and crosstalk.
Balance (professional engineering/long-distance dedicated)
1. Canon XLR: wireless microphone host, conference gooseneck, professional microphone main line.
2. 6.35 Three-core TRS: Interconnection of mixer output, processor and power amplifier.
Advantages: Anti-lighting interference, anti-power clutter, anti-ground loop noise, suitable for all professional sound reinforcement scenes.
Five, 90% of the master stepped on three fatal misunderstandings.
Myth 1: The better the wire shielding, the less noise there will be in the long line.
Wrong! Shielding can only block part of external interference, but can't stop electromagnetic coupling and ground loop interference. Unbalanced congenital structural defects, no matter how thick the shielding layer is, can be saved. This is a circuit principle problem, not a wire work problem.
Myth 2: Big two cores and big three cores look the same, so you can mix them casually.
Absolutely not. The big two-core TS is unbalanced, and the big three-core TRS is balanced. A lot of noise at the scene is that the master tries to save trouble, using two cores to run the line all the time, and the length is mixed, and finally the noise is flying all over the sky.
Myth 3: If the equipment has noise reduction and feedback suppressor, there is no need to distinguish between balanced and unbalanced.
Put the cart before the horse. Peripheral equipment is used to optimize sound quality and prevent howling, not to cover wire errors. The bottom floor noise introduced by wire can't be completely eliminated by most noise reduction equipment, but the tone quality will get worse.
Sixth, the engineering practice standard: how to connect it absolutely clean?
1. Short distance within 3 meters: both unbalanced and balanced lines can be used, and there is basically no difference.
2. Interconnection of equipment exceeding 3m: All balanced signals must be used (Canon/Big Three Core).
3. Sound source of computer, mobile phone and screen projector: connect the DI box/audio isolator first, convert it into a balanced signal, and then enter the mixer to completely solve the current sound of the ground loop.
4. Unified specification for the whole project: all microphone lines and system signal lines are balanced, so as to prevent unbalanced long-distance transmission.
Seven, the ultimate summary in one sentence
Unbalanced single-line load, the longer it is, the more noise it eats, which is only suitable for household short connection;
Balance is a double-line interference cancellation with its own noise reduction, which is the core secret of long-term non-noise transmission in professional engineering.
For all the inexplicable long-line noise and current buzz at the scene, it is not necessary to blindly change lines and equipment. First, check whether the unbalanced line runs a long-distance signal and change it back to balanced transmission, and the problem is basically cured.