Re: [LAU] lost sound (it had been working)

On Tue, Jan 20, 2009 at 04:29:01PM -0500, Reid Vail wrote:
> I get nothing at all from the head-phone jack either.
>
> The funny thing is, this all started when after I tried plugging in a
> mic. And that's it.

That might relate. Was the plugging in successful? What sort of
microphone? Was the cable attached to any other device? Have you tried
plugging it in and out again? Was the plug tip undamaged? Was it
difficult to plug in?

A story: this might not be your problem, but it was fun to fix, and it
might give you some ideas to explore.

Friend of mine bought a laptop, and used it for showing audio and visual
content at church. It is an Intel HDA sound device, with three 3.5mm
TRS sockets at the front. One is marked as an input, the other two as
output. He connected the output to the church sound system, which might
have had phantom power.

After this, the integrated speakers would not work, and the left-hand
headphone channel would not work.

Booting and old Linux on CD, Knoppix 3 something, could make the
speakers work, but not Windows, and not a modern Linux. Therefore this
was a software related problem.

The Intel HDA sound system has a Conexant 20549 codec attached, and the
codec has electrical sensing of the presence of the headphone plug. Not
physical sensing like a switch, but electrical.

Since we know the headphone jack is electrically damaged, it goes to
show why the speakers are not working ... unless an older driver is
installed that doesn't enable the "mute speakers when headphones are
inserted" feature. This mute is not an ALSA control.

Here is my write-up:

"Success. I have the speakers playing and I understand the problem
better. I made a change to the Linux kernel source code to ignore the
headphones. Here is what seems to have happened;

0. there are two headphone sockets on the front, one is also an SPDIF
optical output, and there is a set of speakers on the body of the
laptop,

1. electrical damage to the headphone circuit, caused by either
manufacturing fault or something peculiar about the church sound system,
or static discharge, or power surge between power supply of laptop and
earth conductor of church wiring (would be almost impossible to prove
either way),

2. the left headphone amplifier section no longer functions, the right
headphone amplifier section works fine, (both headphone sockets exhibit
this symptom),

3. the left headphone amplifier is used as the method to detect whether
headphones are present, since it is connected to the sharp end of the
headphone plug, and so a half-inserted plug won't normally trigger it,

4. because the left headphone amplifier section has failed, the
headphone detection operates as if the headphones are present,

5. the driver for the sound device notes the headphones are present and
turns off the amplifier for the speakers."

References:

http://www.alsa-project.org/main/index.php/Help_To_Debug_Intel_HDA
(developer section),

cxt5045_hp_master_sw_put() function in patch_conexant.c source file,
comment reads:

        /* toggle internal speakers mute depending of presence of
         * the headphone jack
         */

http://quozl.linux.org.au/2008-12-19-hp-present/

-- 
James Cameron    mailto:quozl@email-addr-hidden     http://quozl.netrek.org/
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