“Most computer microphones use the third segment to carry bias power for the microphone.” https://en.wikipedia.org/wiki/Microphone_connector
I had thought it was to deliver sound to both sides (rather than left- or right-only) but here we are.
What is bias power?
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I have some headphones with a mic, but there is no bias voltage contact, only Left, Right, Mic(return?)and Ground. How does that work?
When you speak into a microphone, there is a membrane that picks up the vibration from the air, and either by moving a coil, compressing a piezoelectric crystal, or moving a MEMS system, it changes the voltage between ground and output.
Now, every time that membrane moves, it converts sound into electricity (same happens if you speak into a speaker, BTW), but… all real world materials have a resistance, capacitance, reactance and a resulting impedance, which need to be overcome for the signal to resemble the sound the membrane is picking up.
The difference is:
- Without bias power, the sound itself needs to power the system, meaning any sound below some threshold will get “used up” by the mic and not transmitted
- With bias power, the whole mic system is primed and ready to react to the slightest variation in sound, which gets quickly reflected in the voltage of the output wire
Basically, without power, the mic is “deaf” to part of the sound, which might be fine if you give it sound loud enough, but will always “sound” worse than a powered mic.
Without bias power, the sound itself needs to power the system, meaning any sound below some threshold will get “used up” by the mic and not transmitted
This is false. I suspect this myth came about because this is how magnetic audio tapes work (tape bias).
Dynamic microphones do not benefit from bias. They can tolerate a small amount but too much will burn them out (depending on their resistance & the voltage applied) or increase distortion (depending on the mechanical construction & how much the diaphragm is moved by the DC). Some dynamic mic units are built with capacitors in them to intentionally block bias voltages, preventing them from burning out.
I have never seen a datasheet or research paper showing improved dynamic mic performance due to DC offset. If it helped then a manufacturer would be recommending it in the datasheets (so they could claim better distortion & sensitivity specs).
Mics with in-built amplifier circuits require bias voltage to function. Many small “electret” modules contain jfet amps, you have to check the datasheet because they look identical to non-amplified versions on the outside. This is very common in small computer & headset mics. Some might work without bias, but they will sound poor because the amplifier circuit is not designed to work this way.
Condenser mics need some form of bias voltage to function at all. Electrets provide this themselves through some magic materials science that’s similar to a battery that lasts for years/decades/centuries. The other types of condenser mic require you to apply an external bias voltage (aka “phantom power”).
Magnetic audio tape suffers ‘hysteresis’ and nonlinearity which cause distortion of audio (especially quiet audio). Applying a bias voltage works around this problem. DC biases work, but high frequency AC ones are typically better.
I suspect the source of this myth is a confusion between the magnetics of tapes and the magnetics of dynamic mics. I think I recall a year 8/9 science class where I was taught that audio could be amplified slightly by putting a battery in series with a microphone and speaker. I failed to find any sources to support that at the time, but the teacher was adamant that this used to be a legitimate method. Perhaps if the coils were not glued properly in the speaker & mic? It was supposed to be a solution before the days of tube amplifiers but I think the true information turned into nonsense somewhere along the chain.
but… all real world materials have a resistance, capacitance, reactance and a resulting impedance, which need to be overcome for the signal to resemble the sound the membrane is picking up.
Resistance, capacitance and inductance are linear. They will affect all signals the same way, they will not only affect small signals.
To affect the small signals differently to the large signals you need nonlinear elements, like diodes and transistors. EDIT: there are also nonlinear capacitors and resistors, but they’re from more exotic materials than what you find in standard headphone wires & mic designs.
Thankyou for asking this question, I have no clue and you’re making me think that a recent frontpanel audio TRRS jack board I designed might be wrong :D
There are two possible options I can see:
- There is no bias voltage and your mic works fine without it (ie it’s a dynamic mic or an electret mic without a jfet amplifier)
- The bias voltage is provided through the mic pin (via a resistor and/or inductor). The mic then overlays AC onto this DC signal.
I cannot find any good references or info about mic bias and TRRS connectors :( Anyone else have any luck? Wikipedia says it’s a standard referred to as “CTIA” or “AHJ” but those appear to be company names, not standard names.
My current headset uses a TRRS, but also provides an extension cable that splits into two 3.5mm TRS just like yours. I might probe it out and find out what it’s doing (but that doesn’t mean it’s the right/universal solution).
Most likely the mic is simply powered by the voltage which also powers the headphones.
But there’s also mics which don’t need voltage to work at all (unlikely for headsets tho).
Some types of mic capsules require a voltage to work
As well as everyone else’s answer here about bias power: it could also just be because a 3-pin TRS are cheaper/easier to buy and get assembly tooling for than 2-pin TRS. Economies of scale.
(For a good example of this: 3-axis accelerometers are cheaper than 1-axis and 2-axis ones. Everyone wants 3-axis for mobile phones, drones, human inputs and the like. You’re better off buying a 3-axis chip and ignoring the extra channels)
LOL that’s not a stereo wire, that’s a headset combo jack.
I see two three pin 3.5mm stereo plugs (one of them color coded for the headphones and one for the mic), and zero 4-pin combo plugs?
No that is a stereo mic lead, hence the two rings.
I don’t think we can tell from just a picture of the plugs. These are definitely two 3.5mm stereo jacks, colored headphone and mic. Maybe have a look at the manual? Or type the model number into Google? Or use a multimeter and measure the resistance.
This was in a shop so yeah
What do you mean by stereo wire? It’s got 3 contacts on the 3.5mm jack, that’s enough to transfer analog stereo (GND, L, R).
Microphone is mono They’re wondering why there’s a third contact
It’s for bias to the mic. Condenser mics need it to apply bias to one of the leads of the mic so it can amplify the sound before sending it to the input of the card. Some mics don’t require that (self-biased) so in that case, the R pin (middle ring) goes to GND.
Ah, seems you’re right. I didn’t know that. But that seems to be an old way of doing it. I’ve only ever seen 2 contacts on a seperate microphone jack or the 4-contact combined ones in modern laptops.
The link doesn’t open, says connection refused 🤷.
Regardless, if it doesn’t require the bias pin, the mic is self-biased or biased through another source (use the same wire for the signal to get bias, this is easy, you just use a cap to decouple the signal from the bias).
So you have two low-quality mics that can distort your discord call twice as effective.
Innovation!
It has that wire to output audio on both speakers? That’s the green one btw. The pink is for mic input. USB could be for a few different things depending on the features that particularl set has.
Edit: Nvm, thought the question was for a headset, not a microphone. Unless the mic was part of a headset or something. Idk, other comments may have the answer.
The mic is part of the headset, I was wondering why there are 3 contacts on the mic wire unstead if 2 (signal,ground)
