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To my layman's understanding (which is not good) it's because of electronic resonance. In other words, how cables can give off/pick up radio waves.
In early radio, this was a problem with microphones picking up signal from radio transmissions, which obviously is unwanted when you're trying to have people hear your voice, and not your voice mixed with competing radio signals.
From what I understand, when it comes to microphones, there is a "hot" a "ground" and a "cold", and the cold is the same signal as the hot but inverted, and apparently this helps prevent picking up radio signals. It is called "balanced," and I'm not sure why other than the inverted signals. So while it looks like a standard stereo cable with three connections, it's apparently actually a standard microphone cable.
There's a lot more radio/electric wizardry going on than that, but that's my understanding of it, as a person who thinks Electricians are real life Wizards. This is based on some cursory internet research, so please anyone with more understanding correct me if I am wrong.
"Balanced" is used for signal transmission with twisted pair cables. The idea is that any interference will get picked up at the same time by both cables, but be of opposite polarity, so the receiver can just average them out to make like the interference never existed.
They don't need to send the same signal inverted, just allow both cables to react in the same way to any interference (maintain the same impedance).
In microphones in particular, there are other two possible uses for a third cable:
- "phantom power", which uses the balanced twisted pair to deliver power to a microphone's built-in preamplifier, which can then send back the sound over the balanced twisted pair (kind of works like PoE)
- "bias power", which is used to prime a transistor connected to an electret microphone in order to overcome its impedance (which can be seen as a kind of preamplifier like with phantom power, but doesn't send the audio back over a balanced pair, and uses lower voltages)
Sorry Jarfil if I'm being nitpicky :|
They don’t need to send the same signal inverted, just allow both cables to react in the same way to any interference (maintain the same impedance).
These are both the same thing, just viewed from different angles. Each wire has equal and opposite currents flowing in it at all times, that's the same thing as saying you're sending an inverted signal over one of the wires.
"phantom power" [...] "bias power"
Stage audio almost universally uses "phantom power" to mean 48V balanced, which is a nice standard meaning for the term, but I'd never claim someone is wrong for claiming they are doing balanced signals + "bias power". It'd raise an eyebrow (have they made a mistake? it's uncommon) but it's still reasonable, I don't think "bias power" specifically refers to only unbalanced configurations.
Albeit my mind might be poisoned by working with badly translated technical documents all of the time :D
These are both the same thing, just viewed from different angles
Not exactly. The balanced cable pair is just a loop closed by transformers at both ends, with the pair twisting intended to induce opposite currents which will cancel each other (convert to heat at whatever part of the loop). The signal you send, is the same over both cables at the same time, relative to ground, so it isn't "a signal + the opposite", the interferences are what becomes "interference + the opposite".
"phantom power" [...] "bias power"
Following with the loop from above, "phantom power" is 11-48V power applied by the receiver to the whole loop relative to ground, which gets used to power whatever circuitry is in the mic, and the signal gets sent back as a variation in that.
"Bias power" on the other hand, is a 3 wire connection: ground, a 5-9V positive, and the signal coming back through the third wire.
It's always best to double-check whatever data sheets, but the general difference is that if you hook up a 5V "bias powered" mic to a 48V "phantom power" system, suddenly you get: ground, a wire at +48V, and another wire at +48V. The circuitry prepared for 5-9V, is probably not going to like that.
That's why "bias powered" mics tend to use 3.5 TRS, some come with adapters to 6.35 TRS or XLR that have decoupling capacitors inside, and warn against using simple "dumb" 3.5 to 6.35 adapters or hooking them to a "phantom power" system (... although some systems can automatically disable phantom power when they detect a bias powered mic hooked up, but others don't).
claim someone is wrong for claiming they are doing balanced signals + "bias power"
Not necessarily wrong... but it would raise an alarm 🚨 and I'd ask for clarification on what exactly are they doing, just in case they let some magic smoke out.
You're describing balanced/differential signalling. This is used in stage or professional audio (typically over XLR connectors, but not always).
The 3.5mm TRS connectors that the OP has pictured are extremely unlikely to be using balanced signalling. If they did then they would not be compatible with the headphones jacks on computer motherboards or case fronts which follow the AC'97 or Intel HD Audio standards.
Is this better than a normal 4 contact jack without the cold?
"Cold" suggests you're thinking of balanced signalling. You don't have any balanced options with standard headphones and computer PC jacks, everything is unbalanced. Both the 4-connector (TRRS) and 2x3-connector (TRS) variants of your headphone connectors are unbalanced audio.
There might be a difference in crosstalk between the speaker and mic wires (ie signals going to your speakers leaking through the wire insulation and into the mic wires), but it should be inaudible if the cables and headset are designed correctly.
"Better" in terms of less radio interference? I suppose, probably.
I am not an audio engineer. It fascinates me, but most of it is beyond me.
Like how Apple used to be able to send video out of an iPod over a 1/8th connector, using the "sleeve" part as a 5th connector for video.