How are stereo audio signals sent over TRS connectors?

[cooperhow]

On a TRS connector, there are 3 connections: Left, Right, and Ground. I wonder how these connections function when sending audio signals.

Here are my questions:
1. Do the left/right connectors send both the positive and negative portions of the waveform, or is some form of DC offset applied first?
2. If they send both positive and negative voltages, how does the ground connector work?
3. Does it carry the sum of the left and right channels, but with opposite polarity? Does it just carry a constant voltage?
4. How is cross talk between the left and right channel avoided if they are both connected at the ground?

Any help would be much appreciated!

 

[rjenkinsgb]

It's just two single-ended (ground referenced) signals and the 0V or ground connection.

Mostly they are symmetrical around 0V / ground, but it's down to the application. eg. Microphone inputs on things intended to work with electret mics typically have a 470 Ohm pullup resistor to 5V (or some equivalent circuit) to power an electret mic capsule - so that has some DC offset, probably somewhere from 1 - 3V depending on the device and mic.

Most low level "Aux" type connections should have no voltage on the signal lines, but could be floating if they have coupling capacitors either end.

The ground will carry the sum of the load currents, but for signal interconnects with eg. 10K or higher input load, the voltages resulting from that should be tiny. Any other ground current superimposed on the cable will cause noise or hum etc, as with a "ground loop" effect.

The only time there could be significant ground voltage without an external cause is when driving a low impedance load, so the current involved causes voltage drop across the ground conductor - eg. such as headphones with a shared ground from both earpieces. Even then, it's probably not audible even if measurable.

In most cases for signal interconnects, electrically it's just treated as if there were a ground, left and right directly between the the two pieces of equipment.

Other interconnects like the DIN plugs and sockets used on older HiFi etc. have the exact same setup, electrically - though sometimes with four signals and ground, eg. the combined tape out & tape return on some gear.

Phono interconnects may have separate grounds / screens in the cable, but again generally the ground are connected together at both ends in the equipment, so no different in function as to ground currents.

 

[crutschow]

1. Do the left/right connectors send both the positive and negative portions of the waveform, or is some form of DC offset applied first?

They send the complete signal waveform, no offset applied.

2. If they send both positive and negative voltages, how does the ground connector work?

The ground connector carries the sum of the right and left signal currents.

3. Does it carry the sum of the left and right channels, but with opposite polarity? Does it just carry a constant voltage?

See answer to question 2.
It doesn't carry a voltage, it carries the sum of the currents.
A voltage has to be between two points.
It this case it's between each channel and ground, which is considered to be the zero voltage reference.

4. How is cross talk between the left and right channel avoided if they are both connected at the ground?

The ground connection is a low enough impedance, that the ground currents generate no significant voltage, and thus no significant crosstalk.

 

[Nigel Goodwin]

You're are MASSIVELY over complicating this - one pin is ground, one is left, one is right - that's really all there is to know.

The only 'extra' bit that might be useful?, is it the socket it plugs into it DC blocked or not (series capacitors on both left and right) - but you shouldn't need to know that anyway, and any input you build should have DC blocking capacitors, just in case the source doesn't (and likewise, and output you build should have them as well).

 

[rjenkinsgb]

any input you build should have DC blocking capacitors, just in case the source doesn't (and likewise, and output you build should have them as well).

Also, if you want to be really fancy, add high value bleed resistors (eg. 1M) to ground from the connector side of the caps.

That's one of the tricks with some "magic" audiophile cables, they include bleeds to prevent standing voltage, that can make any normal cable massively more microphonic.

 

[Tony Stewart]

With cable resistance being 1 ohm per 100 ft for AWG20 and input impedance being whatever it is the shared currents on sleeve (gnd) 0V may rise with crosstalk by that ratio. -60 dB is 0.1% for example 0.6 ohms or wire from 600 ohms load is 1:1000 or ~ -60dB, so it's pretty small.

 

[Lax Luthier]

It is not common to use 1/4 TRS connectors to carry the left and right signals of a stereo in the same cable. At least in 40 years of work with commercial and industrial systems, I've never seen that configuration. Can you give an example of where it IS used?

 

[rjenkinsgb]

Can you give an example of where it IS used?

The commonest thing I can think of is with traditional HiFi headphones; they always used to be 1/4" stereo & most decent ones now fitted with a 3.5mm plug still come with an adapter to allow use with 1/4" gear - I have at least six pieces of equipment with 1/4" headphone sockets.

The "Aux in" on this amp in another thread is also a 1/4" TRS stereo:

https://www.electro-tech-online.com/attachments/27789458-5fc6-493e-8daa-8278368390cd-jpeg.138660/

(Though did anyone ever specify 1/4" ? I'd assumed it was referring to the common 3.5mm TRS "aux" cables etc. as used with PCs and smaller audio gear...)

 

[Nigel Goodwin]

It is not common to use 1/4 TRS connectors to carry the left and right signals of a stereo in the same cable. At least in 40 years of work with commercial and industrial systems, I've never seen that configuration. Can you give an example of where it IS used?

As already mentioned, headphones are the prime example - but they are fairly common on PA gear as well, used as either balanced inputs, or as stereo inputs or outputs (for tape decks etc).

But as also mentioned, there's been no suggestion of 1/4 inch plugs in this thread, and 3.5mm stereo plugs are pretty well universally used.

So it's very common - what isn't common is the industries you're looking for them in.