Record simultaneously on two microphone inputs

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Record simultaneously on two microphone inputs

Permanent link to this post Posted by eseb » Mon Jan 30, 2017 11:20 am

Hello,

The problem is in the title.
How to record with the front and rear input of the PC two mono microphones ?

It seems that it was possible with Win XP.
I can not find with Win 7.
Is there an old version of Audacity with multiple channels?

I'm lost.

Thank you.

Eseb
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Re: Record simultaneously on two microphone inputs

Permanent link to this post Posted by steve » Mon Jan 30, 2017 11:44 am

For conventional (not USB) microphones, you need to use an audio interface ("sound card") that has two microphone inputs, and then record in Audacity with "2 channel (stereo)" selected in the device toolbar.
The two mics can be separated when the recording is complete using "Split Stereo to Mono" (see: http://manual.audacityteam.org/man/spli ... racks.html)

For USB mics it is more difficult. Recording from multiple devices at the same time is not supported directly on Windows, so you would need to "aggregate" the mics into a single "virtual device" using a third party application such as "Virtual Audio Cable" or "VB-Audio VoiceMeeter". On Mac, you would need to set up the two USB mics as an aggregate device in the Audio MIDI setup (see: https://support.apple.com/en-us/HT202000). On Linux one way would be to use Jack Audio System as described here: http://www.jackaudio.org/faq/multiple_devices.html
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Re: Record simultaneously on two microphone inputs

Permanent link to this post Posted by eseb » Mon Jan 30, 2017 3:24 pm

Thanks steve.

i read this FAQ about 2 micro http://manual.audacityteam.org/man/faq_recording_how_to_s.html#twomics
We need a sound card with two input.


We have only one input micro in front of the tower and one behind the tower.
The jack are connected to the mother board.

In Win7, the bargraph is active for one micro and one "on line".

Audaticty read only one input : the micro input or the "on line" input.
And if i put "Canal" to "2 stero", i have the same sound for the 2 tracks.

Some People say it worked several years ago.


Can do this ?

Thanks.

eseb
eseb
 
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Re: Record simultaneously on two microphone inputs

Permanent link to this post Posted by steve » Mon Jan 30, 2017 4:51 pm

eseb wrote:We have only one input micro in front of the tower and one behind the tower.

I doubt that will work on its own. It is most likely that the "line" and "mic" inputs are really one and the same, but with an additional, mono, microphone gain stage for the "mic" input.

Another problem is that the built-in microphone inputs on most computers have bad sound quality. If you intend to make high quality recordings, then you will almost certainly need to upgrade the sound card. The most economical way of doing this is to buy a USB sound card with two microphone inputs.

It may help us to make appropriate recommendations if you describe what you are wanting to do and what your budget is for equipment, and what equipment you already have.
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Re: Record simultaneously on two microphone inputs

Permanent link to this post Posted by eseb » Tue Jan 31, 2017 8:55 am

Thank you for your informations.

This is in no way for music !!!
It's for a school ad we want to use the sound card as an acquisition card. The advantage is the price.

The aim is to measure the propagation speed of waves in different media for geology. One causes a shock that one piezo sensor registers and another one retrieves further in a material: the propagation time of the wave in the material is measured.

The system was working.
I can not get it to work. Change from Win Xp to Win 7.

Thank you for your help.

Eseb
eseb
 
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Re: Record simultaneously on two microphone inputs

Permanent link to this post Posted by steve » Tue Jan 31, 2017 11:23 am

eseb wrote:The aim is to measure the propagation speed of waves in different media for geology. One causes a shock that one piezo sensor registers and another one retrieves further in a material: the propagation time of the wave in the material is measured.

OK, sounds interesting.

So I presume that you are able to solder up your own leads to connect your "sensors" to the sound card?

Piezo devices produce extremely low current, but relatively high voltage. The impedance ("resistance") of the input that the transducer is connected to is therefore very important. If you connect a piezo to a low impedance (Low Z) input, the signal level will be extremely small. Ideally the input impedance should be 1 MOhm or more.

I don't know what level of electronic experience you have, but a good way to interface piezo transducers with the "Line" input of a sound card is to use a simple JFet Op Amp buffer. As a starting point, choose components to give an input impedance of 10 MOhms. A suitable (and cheap) Op Amp would be "TL051" or similar (TL052 for two channels in one chip). The basic circuit is very simple:

Image
R1 would be a high value to provide high input impedance.
R2 sets the gain - higher values give higher gain.
For a split rail supply, vref would simply be grounded. For single rail supply, use a voltage divider between vcc and vee.

To connect the outputs to a standard computer "line in", the output of one op amp would connect to the tip of a stereo mini-jack, and the output of the other op amp would connect to the ring. The sleeve of the mini-jack would connect to common ground. For safety (so that you don't blow up the sound card, you should place DC blocking capacitors between the op amp output and the jack plug, and start with a low value of R2.

If that all sounds too complex, then you could try just connecting the piezo transducers to the stereo mini-jack plug via 500 Ohm resistors. Again, one should connect to the tip, and the other to the ring, with the sleeve as common ground. If the signal level is very low, it may not be visible in Audacity until you amplify / normalize it ("Amplify" or "Normalize" effects).
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