J. P. Gilliver (John) wrote:
In message ,
writes:
On Tue, 10 Sep 2019 11:10:53 -0400, 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?

It monitors the impedance of the output or input.

Impedance checking wouldn't work with nothing plugged into the other end
of the cable. (Well, very clever impedance checking might, of the sort
used for finding where the break is in a cable, but not the sort I'd
expect in computer audio circuitry.)

It's probably a switch in the socket that detects something being
plugged in - probably detected by the audio chip and switched
electronically, as described by Paul, but it _could_ be just the
mechanical type of switch as described by Rudy - in the same way
headphone/earphone sockets have muted the speaker on plugging in the
headphone/earphone (or anything else) since at least 1960s transistor
radios, probably longer ago. No OS intervention needed!

AC'97 used two contacts (stereo) in the jack, to direct the
LineOut signals where appropriate. If a headphone was plugged
into the Front Panel headphone jack, the plug would cause the
contacts to open so that the signal could not flow via "FP_Return"
and get to the computer speakers via LineOut jack. This is a
DPDT mechanical switch function.

This allows one DAC to support two jacks. If the front headphones
were not using the signals, then the signals were looped to the
lime green LineOut on the back of the computer.

Whereas HDAudio has enough output channels for everything.
The front headphone jack has its own DAC. There's no need to
share with LineOut. The side contact on the jack, is a
logic indicator. Usage of the front headphones can still
cause the speakers to be muted, but this is a digital
function. In fact, some HDAudio chips, in the driver, support
enabling two headphone amps at the same time, for a total of
four channels of output. This allows two people to listen
to different music feeds (this requires the right kind of
application, like something in the WinAmp class).

The differences between the two schemes are summarized here
in picture form.

https://www.sevenforums.com/sound-au...l-layouts.html

HDAudio has signals for 8 jacks. Headphone and Mic on the front.
7.1 audio out on the back (4 jacks), MicIn, LineIn on the back.
The generic pinout is 48 pins. If a "cheesy" laptop HDAudio chip
is used, a large number of pins are NC. A top end HDAudio chip
uses all the pins and supports the 8 jacks. With AC'97,
fewer jacks were provided, so some multiplexing was needed
here and there.

AC'97 used mechanical switching.

HDAudio uses a single side contact per jack, and that
is a digital logic indication (plug is in/plug is not in).
The driver uses the digital logic indication, to control
which channels are enabled. Classical speaker muting
can be implemented in the driver, using the front
panel headphone jack and its logic indicating signal.

Paul