Mic input to pc

  • Thread starter Thread starter steve marchant
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steve marchant

Want to do some processing of the mic signal before it goes into the mic
input, but know nothing about the input/output conditions presented. Had no
luck so far in "googling" for technical info about the circuitry related to
the mic input area of a pc. Anyone out there with a clue?
 
steve marchant said:
Want to do some processing of the mic signal before it goes into the mic
input, but know nothing about the input/output conditions presented. Had no
luck so far in "googling" for technical info about the circuitry related to
the mic input area of a pc. Anyone out there with a clue?
Click to expand...
It's a line level input conforming to normal audio standards so if what
you want to do would work with a normal amplifier input, it'll work
with a PC soundcard.
 
steve marchant said:
Want to do some processing of the mic signal before it goes into the mic
input, but know nothing about the input/output conditions presented. Had
no luck so far in "googling" for technical info about the circuitry
related to the mic input area of a pc. Anyone out there with a clue?
Click to expand...
Hi Steve, your best bet is to use www.google.com ,and
type in `computer mic input`.

You`ll get plenty to have a read of, there.

bw..
 
steve said:
Want to do some processing of the mic signal before it goes into the mic
input, but know nothing about the input/output conditions presented. Had no
luck so far in "googling" for technical info about the circuitry related to
the mic input area of a pc. Anyone out there with a clue?
Click to expand...

You can research how AC'97 sound chips work and extrapolate from there.

Analog Devices are more clever about providing real specs at the
chip pin. If you look on PDF page 4, input impedance is 20K ohms
in parallel with 5pF. That would be 20K between input and ground.
If you were feeding it from a 600 ohm device, there would be
little loading from the 20K number. If you had a high impedance
output of some sort, then YMMV.

http://www.analog.com/UploadedFiles/Data_Sheets/AD1985.pdf

A popular AC'97 at Realtek, is the ALC650. The reason for
downloading the packages here, is a reference schematic
is provided. Realtek uses FTP servers for their files.

http://www.realtek.com.tw/search/default.aspx?keyword=alc650

ftp://152.104.238.19/pc/audio/ALC650_DataSheet_1.3.pdf
ftp://209.216.61.149/pc/audio/alc650_data.zip

The second package is a ZIP, with some separate ZIP files
inside. If you unzip everything, you should see
alc650_demo_circuit_ver_11.pdf . The thing to note there,
is analog I/O is all capacitively coupled. The input pin
has some kind of bias network, so the A.C. coupling is used
to prevent upset to the chip bias. Or at least that is my best
guess as to why they do it. Otherwise, a D.C. connection
would be desirable from the user's perspective.

Note that the jack sharing scheme shown in the ref schematic
would be present on a motherboard that had only three 1/8"
jacks on the rear panel. If a motherboard has five or
six jacks, then there is no need for a sharing circuit.

So based on that, you have a 1uF series cap, feeding a 20K
ohm or so resistor to ground. Roughly a high pass, with
8Hz corner frequency, using 1/2*PI*R*C.

http://members.aol.com/sbench/freqresp.html

Now, another issue, is MIC_BIAS. The 1/8" jack has three
contacts, tip, ring, and sleeve. Tip should be 1uF and
20K to ground. Ring can have a MIC_BIAS signal on it. The
purpose of MIC_BIAS, is to provide DC power to electret
microphones. The voltage is 3.3 to 5V perhaps, with a
series limiting resistor of 2K ohms.

On a monophonic microphone input, DC bias is only on
the Ring contact. If a microphone input is stereo, then DC
bias *could* be provided on both Tip and Ring. Each would
have their own separate 2K or whatever, limiting resistor.
This would be handy for things like the Andrea Superbeam
stereo microphone. (The following page is mainly for a mono
application.)

http://www.epanorama.net/circuits/microphone_powering.html

This is the Andrea Superbeam. I think it needs the equivalent
of mic bias on both Tip and Ring. The Superbeam is stereo,
and clever software on the computer can use the stereo signals,
to remove some background noise.

http://www.andreaelectronics.com/Buy/SoundMax/pdf_files/SuperbeamArray_spec.pdf

The reason I mentioned all of that, is if you have any
concerns about the Microphone jack, use the Line-In instead :-)

No matter what jack you use, you should test for the
presence of DC on Tip or Ring. (At least, if your proposed
circuit would be affected by a stray couple milliamps of
current.)

To test, first find a male to male, 1/8" cable, like the
kind that comes with a TV tuner card. Plug the 1/8" extender
cable into the Microphone jack. Using your multimeter, see if
Tip or Ring has a DC microphone bias voltage present. If there
is no DC present on the jack, then chances are you are safe to
assume just the series 1uF, and 20K to ground. You could then
use the Microphone input for your experiment.

Note - when doing the above DC check, go into the sound card
software, and tell it a microphone is connected to the
microphone jack. Some chips have the ability to turn the
bias on and off, and the DC bias may only appear when you
tell the software that a microphone is present. The DC
bias can come from the +5VA rail, but on some chips, the
bias actually comes from a regulated source on the sound
chip itself.

HTH,
Paul
 
Hi Paul
This is extremely helpful stuff, and I'll be taking a close look. It's a bit
of an over-kill for what I immediately have in mind, though, which is to
take what comes out of the pc speaker output(s), mix it with voice from the
mic, and push the composite back into Windows Sound Recorder via the mic
input. You also sorted out the mystery I found when looking for what pole of
the jack carried the mic bias. I was getting 2.7 volts on both the tip and
ring. Now I see this is for a stereo mic.
Thanks again. Great stuff.
steve
 
Hi Paul
This is extremely helpful stuff, and I'll be taking a close look. It's a bit
of an over-kill for what I immediately have in mind, though, which is to
take what comes out of the pc speaker output(s), mix it with voice from the
mic, and push the composite back into Windows Sound Recorder via the mic
input. You also sorted out the mystery I found when looking for what pole of
the jack carried the mic bias. I was getting 2.7 volts on both the tip and
ring. Now I see this is for a stereo mic.
Thanks again. Great stuff.
steve
Click to expand...

Can you connect the line output to the line input and the mike to the
mike input, and then enable both recording devices in your sound
card's mixer?

- Franc Zabkar
 
Franc Zabkar said:
Can you connect the line output to the line input and the mike to the
mike input, and then enable both recording devices in your sound
card's mixer?

- Franc Zabkar
--
Click to expand...
Didn't think of it Franc. Been spending a while setting it up. It works, but
the line input sound level is very low compared with the mic input it's
being mixed with.. I'm wondering if the signal from the speaker output needs
boosting. I'll try this next.
 
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