BillAhearn wrote:
How does the operating system know that an audio cable extension is plugged
in?

I have a 20 foot audio extension cable which is just male on one end and
female on the other.

If I plug in a headphone into that extension cable the sound comes out the
headphones and not out the computer speakers.

When I just plug in the extension cable, the sound stops coming out of the
speakers.

How does the OS know that an (unused) audio cable is plugged in?

On a RealTek HDAudio chip, the "standard" method is jack
side-contact. The "stack" on the back of the computer, can
feel when the plug is inserted and that causes a switch closure.
The standard for 48 pin HDAudio chips, has a scheme whereby
two pins on the package, can sense the status of eight jacks
total. (The switch closures are encoded and sensed by a
crude ADC.)

The RealTek chip does not know whether the load is a 32 ohm
headphone or a 10K ohm audio input from an amplified speaker.
It will put up a dialog asking for confirmation.

A SoundMax HDaudio chip supports two methods. It has the side
contact sensing like all other brands. However, it also has
its own unique method. It applies a 25KHz AC signal to the jacks
to sense current flow (at 25KHz) and work out the impedance of
the load. That product family knows the difference between
a 32 ohm headphone and a 10K ohm amplifier, all by itself.
It could tell if a "remote load" was plugged into the end
of your extension cable.

*******

It seems some RealTek chip in this thread, does now
have a sensor. But the method would have to be different
enough, to not be a patent violation (on the SoundMax method).
Due to the relatively small range of measurements it offers, my suspicion
is they're measuring the current flow used by the output
pad driving LineOut or similar.

https://www.reddit.com/r/audioengine...eeps_changing/

Since they're talking about "output sensing" and not
"I/O sensing", we don't know whether that RealTek also
can sense input types.

*******

When impedance sensing is involved, plugging a headphone
into the remote cable, should be the only time the speakers
are muted.

If a solution only has "side contact detection", then as
soon as the extension cable is plugged in, the speakers
will be muted the whole time. Without actual impedance
measurement, the chip can't tell whether a "remote load"
is present or not.

And even when these techniques are present, the datasheet
may not properly delve into the details.

Some computers do not have the right kind of jacks
with the side-contact on them, in which case "YMMV" :-/

Paul