Re: laptop microphone does not work

"BillW50" <BillW50@xxxxxxx> wrote in message
In news:47fb110a$1_1@xxxxxxxxxxxxxxxxxxxxxx,
M.I.5¾ typed on Tue, 8 Apr 2008 07:47:38 +0100:
"BillW50" <BillW50@xxxxxxx> wrote in message
In news:47f9cf72$1_1@xxxxxxxxxxxxxxxxxxxxxx,
M.I.5¾ typed on Mon, 7 Apr 2008 08:55:30 +0100:
"Paul" <nospam@xxxxxxxxxx> wrote in message
MikeCC4@xxxxxxxxx wrote:
I have a gateway MX3563 laptop with windows XP home. I tried to
use a mcirophone for Yahoo messenger kind software. The
microphone works perfectly on my other laptops. However, it does
not work on this one; i.e. no sound or no voice can be heard by
the person on the other side. The microphone must not be broken
because it works with other laptops.

I checked the device manager and did not find any problem. In
fact, I just used system restore CDs to completely reinstalled
the laptop to original manufacturer's condition. But I still
cannot use any microphones (I have two microphones both work on
all other desktops and laptops with Windows XP Home or Vista)
with this Gateway laptop. For the same reason, I cannot use any
microphone to record anything on this laptop. It is already out
of warranty. Can anyone help? thanks!

P.S. This problem happened quite long time ago. (it happened way
before I damaged my keyboard (as in my other post.))

Plug a Walkman or a tape recorder into the microphone
jack. Does the microphone jack produce a signal when that
is present ? Anything with a line level output could be
used to test the jack.

A line level signal will vastly overdrive a microphone input.

Hi M.I.5¾! This is usually true of most, but not for some Gateway
laptops at least (like the two MX6124 I have). They use the same
jack for both the mic and the Line In. It even has separate levels
for both as well. There is nothing to switch, it just autodetects
and switches all by itself. I just checked the manual and it says
nothing about the Line In feature. But I use it for both all of the

How on earth can it detect the difference between a line level jack
and an identical microphone jack?

The signal level of a microphone is typically 40 to 60 dB lower (a factor
of 100 to 1000) than a line input. The gain of a single OP amp back in the
80's if I remember correctly had a max gain of 50,000 times. So an AGC
(automatic gain control) circuit using one OP amp is well within this

Although I suspect it is more designed like my tiny FM transmitter works.
As if it detects voltage less than say 100mv for a minute, it shuts off.
Although in this case on my laptop, throws it in mic mode. If it detects
100mv or higher, throws it into Input mode.

Line in jacks are very uncommon on laptops and I would suggest that
the reason the manual doesn't mention it is because it isn't, in fact

Hmm... until my purchase of these Gateway MX6124 laptops back in 2006, I
always had aux in jacks on my laptops. So I thought they were very common.

Some Cardbus and Express Card plug in soundcards feature Mic in and
Line in on the same input jack, but the actual input has to be
selected in the mixer utility because there is no way for the card to
tell the difference (as I said the plugs are identical).

Hmm... you have me curious. Well it doesn't take long to hookup my MP3
player to my mic jack on this laptop so here goes. Well I remembered
somewhat incorrectly. While there is a line level control in the mixer, it
has no effect. But the mic level does. And the iRiver T10 FM/MP3 player
has a very high output level when compared to most other MP3/CD players.
But I have noticed that 50% volume is about as high as you can go with
most input jacks on most equipment without distortion from a T10.

The usual way of controling the input, is that either the MIC in or LINE in
has to be muted. The mixer then switches the sound card input to the other

This laptop, like most laptops and desktop sound card mixers I've seen in
the last 10 years also has a mic boost toggle in the mixer. And generally
a dynamic mic just barely has enough power for acceptable levels when the
boost is toggled on and the gain set as high as it can go.

Although when the mic boost is toggled off, the dynamic mic level can
barely be heard. Although feed a line in level into the mic jack with the
boost toggled off, it is just perfect. And with the T10 connected to the
mic jack at 50% volume, it is again just perfect.

So I remembered wrong and the mixer mic boost toggle switches between
input or mic levels. Although as an engineer, it isn't a big deal to
design a circuit to automatically switch between the two modes
automatically. But I guess nobody has bothered yet in the laptop market.

The MIC input on most sound cards these days is set for condenser
microphones. And even with these you usually have to set the 20dB boost on
for most purposes. Some, but not all, even provide the power for the
impedance converter, but the voltage is not standardised.

Although it would be fairly easy to build such an AGC circuit, it wouldn't
be a practical proposition. If the line input had no signal for any
appreciable time, it would ramp the gain up to MIC levels. As soon as a
full amplitude signal appeared, the input would be well overdriven for the
brief period that it takes the circuit to adjust its gain. If that
overdriven signal was fed to a class D amplifier (not uncommon these days in
PC sound systems) driving speakers, the speakers and possibly the amplifier
output could easily sustain damage (and even class A, AB, and B systems
might have a thing or two to say on the matter)