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I'm confused about how small audio signals are immune to interference. Headphone wires seems twisted but are they also differential signals?

As far as I know differential/balanced signals are carried by two wires and they are mirrored as shown here: https://upload.wikimedia.org/wikipedia/commons/thumb/e/e7/DiffSignaling.png/1200px-DiffSignaling.png

So the common mode noise is rejected.

But in headphone case the little headphone which goes to our ear is not a differential amplifier hence it is just an impedance. It DOES NOT subtract two incoming signals.

I accept my ignorance on the issue but couldn't find a clear explanation about this.

Lets say you have a smartphone or laptop and you plug your headphone to the device, what kind of signal is that audio signal passes through the vibrating earphones? And how come the wires reject the noise? Can this be shown with a simplistic circuit model?

EDIT:

I would be extremely glad if someone can reply these two point confuses me:

Premise: In below diagrams grounds are meant as analog signal grounds there is no earth involved. Imagine the circuits are in space to neglect confusion.

enter image description here

Sorry for the above quick hand drawn diagrams.

Question 1-)

I. represents the original question. As you see the phone's headphone driver circuit has a GND which is connected to the one terminal of the headphone coil. So some say "no way to tell which, if any, of the lines is grounded". But there is ground. All phone circuit's analog ground is tied to the one terminal of the headphone. (?)

Question 2-)

This is regarding diagram II above.

in A and B why B cannot reject the common mode noise? Isn't B similar to the headphone scenario in I ?

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    \$\begingroup\$ This question can almost be considered philosophical. Technically, the signals are not differential, because there is a ground. But in reality, how can you even tell the difference? There are just two wires at each speaker with a voltage between them. The speaker (inside the headphone) has no way to know if the signal is differential or ground-referenced, and it really doesn't care. So all answers to this question are valid. Yes, they are differential. No, they are not differential. Differential and non-differential are the same thing. \$\endgroup\$
    – user57037
    Commented Jan 13, 2018 at 19:22
  • \$\begingroup\$ @mkeith Is the audio circuit which outputs two wires for signals also have a ground? Then I can understand it outputs differential signals relative to its own ground. But what I wonder is when they reach the headphone it is not like when they reach a diff amplifier terminals. How are the nature of signals and how is the noise eliminated in this case? Im very curious yet still cannot express my question well enough. \$\endgroup\$
    – GNZ
    Commented Jan 13, 2018 at 19:28
  • \$\begingroup\$ I will think about it. It is hard to explain. If I can write a good answer I will. The basic idea is that in a twisted pair transmission line, any voltage couples equally onto both wires, even if one wire is named "Ground." Since the voltage is on both wires, it is purely a common mode voltage. The speaker does not respond to common mode voltages because it is just a coil connected between two wires. \$\endgroup\$
    – user57037
    Commented Jan 13, 2018 at 19:32
  • \$\begingroup\$ @mkeith No ground in those battery-operated items. Headphones respond to current. So just trace the audio current loop (it passes through the battery). That loop area can have disturbing noise induced by a magnetically changing source. But most often, the loop area is tiny, perhaps because all the wiring in the loop is very close, or it is a wire pair twisted together. \$\endgroup\$
    – glen_geek
    Commented Jan 13, 2018 at 19:36
  • \$\begingroup\$ @mkeith But then one terminal of the headphone/coil becomes ground. So one incoming wire and the one terminal of the coil is ground. Can we still talk about common mode? Common to what? \$\endgroup\$
    – GNZ
    Commented Jan 13, 2018 at 19:36

3 Answers 3

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A speaker is a two wire transceiver, as such it is differential by nature. The speaker reacts to the difference between the lines, it does not matter if one line is ground or an anti-phase signal, the speaker can not tell the difference.

schematic

simulate this circuit – Schematic created using CircuitLab

Looking at the image above you can see that the signal current that is driving the speaker forms a loop shown by the green arrows. These arrows flip back and forward in unison as the applied voltage changes polarity and excites the speaker.

Common mode noise on the other hand forms currents that go in the same direction in both lines, as such they form no voltage difference across the speaker terminals relative to each other.

As such any AC hum that may be picked up on the cable does not actuate the speaker.

RE YOUR EDIT

So some say "no way to tell which, if any, of the lines is grounded". But there is ground.

Yes but the difference is the ground on your headphone, or phone handset, comes from the same place as the audio. If you had an extra wire coming from one terminal of your headphones tied to a local ground it would be different, your would hear all kinds on noise.

in A and B why B cannot reject the common mode noise? Isn't B similar to the headphone scenario in I ?

In this case B has local ground tied to one side of the amplifier. That means the common mode current coming in each line sees a different impedance. As such a different voltage is generated on each pin and noise is introduced and amplified. Case A each signal sees the same impedance, generates identical voltages, and those cancel out.

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  • \$\begingroup\$ Interesting. But why is then many differential outputs are mirrored? And why I always read differential ended signals are more immune to noise than single ended signals? Is that because in single ended outputs it is hard to make the output impedances equal? This is what confuses me: dataq.com/blog/wp-content/uploads/2011/09/se-diff.png \$\endgroup\$
    – GNZ
    Commented Jan 13, 2018 at 20:43
  • \$\begingroup\$ I mean in your picture one of the terminal is all the way goes to the GND of the cellphone's audio amplifier(headphone driver). So one terminal's ground is common to one end of the headphone. There is a ground at the end which is wired to common to the headphone terminal and the circuit GND. Does it make this a single ended system? \$\endgroup\$
    – GNZ
    Commented Jan 13, 2018 at 20:48
  • \$\begingroup\$ @161776 mirrored gives you double the voltage swing for the same supply voltage = twice the volume. \$\endgroup\$
    – Trevor_G
    Commented Jan 13, 2018 at 20:49
  • \$\begingroup\$ @161776 as far as the speaker is concerned there is no such thing as single ended. \$\endgroup\$
    – Trevor_G
    Commented Jan 13, 2018 at 20:50
  • \$\begingroup\$ Do you mean that terminology makes sense if the headphone was an opamp? \$\endgroup\$
    – GNZ
    Commented Jan 13, 2018 at 20:51
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Headphones aren't particularly susceptible to induced noise, when compared with audio amplifier inputs.

An audio amplifier may have an input impedance of many kilohms. Headphones are typically around 32 ohms.

If a nearby signal induces a small imbalanced current into the signal cable, then that can produce a significant voltage across the input of a high impedance amplifier (V = IR). Whereas to a 32 ohm headphone speaker, the current is too trivial to be heard.

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  • \$\begingroup\$ If the audio amplifier is balanced and using twisted cable it will reject that noise. If you use it single ended then it will pick up noise as you say. But what I ask is this: Is the amplifier inside the cellphone like gadgets single ended signalling? So we are in a situation where the transmitter is single ended and receiver is just an impedance. Noise might be small but signal is also small what matters is SNR isnt it? And what is to point to use twisted wires in this case if there is no common mode subtrator? I would love to see a realistic diagram but couldn't find any. \$\endgroup\$
    – GNZ
    Commented Jan 13, 2018 at 20:34
  • \$\begingroup\$ So far as I can see, it makes little difference what sort of wires you use. Most of the headphones I have disassembled use screened, rather than twisted pair, cable. But I think that's just to make a nice round cable. Ultimately, the headphones are differential. They respond only to the voltage across the two wires, and don't even know what ground potential is. \$\endgroup\$
    – Simon B
    Commented Jan 13, 2018 at 20:44
  • \$\begingroup\$ What do you mean by ground? Do you mean one of the terminals which is wired to the GND of audio amplifier or the earth? \$\endgroup\$
    – GNZ
    Commented Jan 13, 2018 at 20:46
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    \$\begingroup\$ @161776 That's sort of the point. The headphones have no way to tell which, if any, of the lines is grounded. Depending on the source amplifier, the "sleeve" terminal on the headphones may be grounded. But if the amplifier is battery operated, it won't. \$\endgroup\$
    – Simon B
    Commented Jan 13, 2018 at 21:09
  • \$\begingroup\$ I made an edit. Please see my confusion. I think that's the point where Im stuck at. \$\endgroup\$
    – GNZ
    Commented Jan 13, 2018 at 21:38
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Any time a voltage is applied thru a pair of wires to a load, it is differential but not necessarily balanced.

However some signal pairs have differential drivers such as car audio power amplifiers and RS485. But don't confuse that with the fact that what is received is a differential voltage, even if the return reference is 0v or ground. However, differences in earth grounds can cause noise when using earth ground as a common reference. Many audio sources use a floating ground unlike PC towers earth-grounded for EMI reasons.

When an additional voltage or current is coupled to both wires equally by say mutual inductance or capacitance then a Common Mode voltage is applied according to the CM impedance* CM current.

This becomes a Differential voltage if the impedance of each line is significantly different. For a speaker the driver is <<1 Ohm just like the ground return, so the difference is not significant. But for a 600 ohm microphone with a ground return the difference is significant so these are used in balanced mode.

For a long digital signal, the line inductance is significant so a balanced differential low impedance RS-485 works better and over longer distances with a balanced receiver.

A coaxial cable is unbalanced but the inner wire is shielded so that any ingress voltage is common mode, so it behaves like a balanced signal. However poor grounding along the distributed cable can cause stray CM signals such as from a nearby train to get into CATV coax.

Whenever the stray magnetic or electric field interferes with the unbalanced impedance of a pair of wires to create a "significant" differential signal , then a BALUN choke must be added or high quality STP with balanced driver and receiver must be used.

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