Archived from groups: alt.comp.periphs.mainboard.asus (
More info?)
In article <BTI5d.122877$D%.392@attbi_s51>, "Bob" <rlongdon2001@yahoo.com>
wrote:
> Tom,
>
> I had the same problem. It seems that ASUS has put a stero plug on the MB
> for the Mic. You have to go to Radio Shack and buy a mono to serial adapter
> for the mic input for it to work. Good luck.
>
> Bob
>
> "Thomas A. Horsley" <tom.horsley@att.net> wrote in message
> news:uk6utzvhy.fsf@att.net...
> > Yep, I never got the normal mic input to work on my machine either
> > (fortunately I have no real use for it - I was just testing everything).
> >
> > When I was asking about it someone suggested that the socket on the
> > motherboard is a stero socket and you need some kind of stero->mono
> > adaptor to get a mic to work, but I never felt it was worth the
> > effort to explore, so I have no idea if it is true.
> > --
> >>>==>> The *Best* political site <URL:http://www.vote-smart.org/> >>==+
> > email: Tom.Horsley@worldnet.att.net icbm: Delray Beach, FL |
> > <URL:http://home.att.net/~Tom.Horsley> Free Software and Politics <<==+
This is what we are used to for a microphone input:
+------+ TIP / \ Motherboard
| |------+-------- audio --------| |---"MIC"-------------->AC97
| |mic+ | \ / codec
| mike | | RING +===+
| | +------ bias +5V -------| |---"MICPWR"--2Kohm----- +5V
| | |===| resistor
| |mic- SLEEVE | |
| |----ground -------------------| |---"GND"
+------+ | |
+=======+
| |
This is what you would get if you hooked up the FP_AUDIO header
on the motherboard to a front panel microphone jack. The middle
contact provides power for electret microphones. Notice how the
electret microphone in this example, is actually shorting the
MIC and MICPWR together, and the electret modulates the voltage
on the bias signal, to make a signal suitable for AC amplification.
There are probably some audio devices that don't joint the two
signals inside the microphone, such as maybe a dynamic mic with
its own amplifier of some sort.
I looked at the AD1985 AC97 Codec datasheet, as I have a
P4C800-E and downloaded the datasheet previously.
http://www.analog.com/UploadedFiles/Data_Sheets/1198969AD1985_a.pdf
There are a couple of ways to get microphone input. The mic mux
can be used to select center/LFE jack as an input. If the
speakers are in 2-spkr or 4-spkr mode, then the pink jack is
available for microphone input. If in 6-spkr mode, then the pink
jack is an output.
The other option, is the two signals called MIC1 and MIC2. It is
possible one of those is connected to the FP_AUDIO header, leaving
the other microphone signal unused.
I decided to do some measurements on the pink jack. I got the
Audio Wizard running, then plugged in a male to male audio cable.
This cable is what you use to connect a TV card to a Soundblaster
Line-In.
I got out two multimeters. First, with a multimeter set to
volts, I measured from Tip to Sleeve and from Ring to Sleeve.
Both of those measurements gave me 1.95V DC, more or less.
Next, I used two multimeters, and put them in parallel. One
was set to measure milliamps, the other to measure volts. Now,
I know this particular meter uses a 100 ohm precision resistor
to measure current, and I verified that by using one multimeter
on ohms, to verify the resistance of the current shunt in the
other meter.
With the 100 ohms load of the meter measuring current connected
from Tip to Sleeve and then again from Ring to Sleeve, I measured
78.6mV and 0.786mA in the first case and 78.3mV and 0.783mA in
the second case. This means the pink jack looks like this:
TIP / \ Motherboard
| |---"MICLEFT"------+----2.4Kohm-----1.95 REFBIAS
\ / | resistor
RING +===+ +---------------> To "Center?" pin
| |---"MICRIGHT"---+------2.4Kohm-----1.95V REFBIAS
|===| | resistor
SLEEVE | | +---------------> To "LFE?" pin
| |---"GND"
Pink | |
Jack +=======+
| |
I'm not really certain about which is Center and which is
LFE, as they are programmable and can be swapped. My assumption
is MIC1 is normally the left channel and MIC2 is normally the
right channel.
I didn't really need two multimeters for this test. I could have
used a 100 ohm resistor to do the "load" test. This is what my
assumption about the circuit looks like:
+V 1.95V
| |
| |
R_unknown R_unknown
| |
^ +---------------
| ^
measure voltage 100 ohm |
with no load, gives precision measure voltage
the value of V = 1.95V resistor in with 100ohm
meter set to load, and see
| milliamps 0.0786 volts
v | |
GND GND GND
First off, the AD1985 has a reference power source for the
microphone inputs. Other motherboards use the same +5V used
to power the AC97 codec. For some reason, this circuit is
using a lower voltage, which may not be enough to make
every electret microphone work. It would have no effect on
a dynamic microphone, an amplified microphone, or a walkman
used to drive the microphone input - they should still work.
To work out R_unknown:
0.0786 = 1.95 * ( 100 ) , solve for R_unknown.
---------------
R_unknown + 100
R_unknown = 2381 ohms, nearest likely resistor value 2.4K
There is an option in the AD1985, to boost Vref to 3.7V, which
might help a bit. Whether this is important or not, will depend
on the spec for the electret microphone you are using.
I took a look in the Digikey catalog, at electret microphones.
There were three popular ratings: 4.5V/0.8mA, 2.0V/0.5mA, 1.5V/0.5mA
In this case, only the 4.5V unit won't work on the P4C800-E.
And, it won't work even if the reference power source on the
AD1985 is cranked. It _should_ work, if you wire up your
FP_AUDIO header to a microphone jack on the front of the
computer (I haven't checked, because this measurement is
more of a PITA, and I'm assuming that the MICBIAS on the
FP_AUDIO header is wired to a +5V type source, and not
the same stupid built-in reference).
Since the average electret shorts TIP to RING, when plugged
into the pink stereo mic-in jack on the back of the computer,
both channels should get the same signal. There shouldn't
be any need for adapters from Radio Shack.
To test the jack, plug in a Walkman and see if the Audio
Wizard shows signals. I did the "hum" test, by touching my
finger to the TIP and RING terminals, and saw signal displayed
in the Wizard. I don't have a recording program handy to
do any more testing than that.
My conclusion right now - no reason to assume it is busted
HTH,
Paul
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