Hey, Joey. Some of the replies so far have given you information on breaking out the stereo line in jack to two mono jacks. That takes care of the mechanical interfacing – I’m sure you can figure that out if you give it a bit of thought, or post back to the list if you need more detail. I think somebody else answered the question about accessing each input separately. I just have a bit to add about the electrical interfacing.
You need to be aware that the output of a microphone is extremely low level, and line inputs are expected to be relatively strong, electrically. If you just wire two microphones to the left and right line inputs you will find the sound very low and possibly noisy or distorted. The right channel connection of the jack in that schematic is also connected to the mic input of the UCB1400 codec, which has basic analog pre-amplification circuitry built into the chip to boost the mic signal before the A/D conversion. If you only needed one mic input you could select the mic input in software, and use the chip’s preamp. However, it will probably be necessary for you to buy or build your own external 2-channel preamp and just use the stereo line inputs as has already been suggested. I don’t think the chip will let you use the mic input and one of the line inputs at the same time.
There are probably lots of circuits you can find on the net (like this: http://sound.westhost.com/project66.htm) or in DIY books for basic mic preamps if you want a challenge or need to keep it small or cheap. Just find a friend who is a musically-inclined EE student or something and you should have all the help you need. If you don’t want to go through that trouble you could go to a music store and buy the cheapest pair of mic preamps like this http://www.zzounds.com/item--ARTTMPSTU $30/each mail order (or cheapest 2-channel mic preamp like this: http://www.m-audio.com/products/en_us/AudioBuddy-main.html about $75 retail). These preamps are usually built for pro or semi-pro mics with XLR connectors (not ¼” or 1/8” jacks), but you can just use adapters to get things to hook up. You already know that you will need some adapters to get into the 1/8” stereo line in jack on the audiostix2. You will find that your results are a hundred times better if you do the proper preamplification.
I hope this isn’t too intimidating. Good luck!
PS – I am an EE and have been a professional sound technician, but I have never used these specific products and I am not recommending you to purchase these over any other brand or model. These are simply the first things I found with a quick google search.
On Behalf Of Joey Degges
Sent: Thursday, March 01, 2007 01:25
To: General mailing list for gumstix users.
Subject: Re: [Gumstix-users] 2 audio in ports?
Would it be as simple as buying a stero splitter, then plugging in 2 mono mics? You mentioned connecting the gnd to the gnd.. but I'm not sure what that actually entails.
If it is as simple as just buying the stero splitter, I was wondering how I would go about accessing each mic as a separate device. For example: recording from mic1 into foo.wav, and from mic2 into bar.wav at the same time.