AFAIK there are no cheap and cheerful stereo mic input
soundcards. The really cheap headphone dongles have stereo phones
ouptut and mono mic - specifically aimed at headsets for Skype etc.
If you are prepared to homebrew something, there is the PCM2900
USB Stereo codec which needs very few extra components for a complete
good quality stereo soundcard. Howeverm to get stero microphone as
opposed to stereo line in you'll have to "get at it" in teh driver. By
default it sets the inputs as line level. (It is the same Codec as
used inte DG8SAQ Vector Network ANalyser)
Actually there are a whole family of themPCM2900, PCN2902,
..04, ..06 etc. I've only ever used the PCM2900. I think teh higher
numbered ones are improved, and the 02 is certainly cheaper from
Farnell now. The PCM290x devices are available from many of the
catalogue suppliers like Farnell and RS
You don't say why you need stereo, but I'm guessing / assuming
an SDR I/Q input for Softrock or similar. If so, AND YOU ONLY NEED
A NARROW BAND there is a quick and simple bodge that works
wonderfully with a mono Mic input. But it is only applicable to
bandwidths of no more than a couple of hundred Hz, say 1400 - 1600Hz.
Certainly not a full SSB bandwidrth .. For example, it worksif you
want to receive a single beacon like the 5.29MHz ones (which is where
I did this for several years on a dedicated Softrock ) or just about
good enought to cover teh WSPR audio subband from 1400 - 1600Hz. It
works like this :
Take a resistor / capacitor pair whose RC combination give a
break point at the centre frequency of interest. ie. 1/(2.pi.C.R) =
Freqeuncy in Hz. Connect the resistor from say the I channel to the
mono mic input. Conenct the capacitor from the Q channel to the same
point. You will get sideband cancellation for a narrow band around
the centre freqiuency. make the resistor adjustable to be able to set
this freqeuncy to what you need. Eg for 1500Hz (WSPR band centre)
use a 10nF capacitor and 10.6k (10k resistor and 10k variable). The
sideband rejection worsens either side as you depart from the C Freq.
The interesting thing about this simepl RC network is that teh
phase shift from I and Q inputs to the mono summing point is completely
independent of any load resistance to ground there. So the mic
conenctor input impedance doesn't mattter. What does matter,
however, is any residual output resistance in the driver. Outputs from
opamps are fine, but there should be no significant additional series
resistance there, certainly any present shoudl be less than 5% of the
Xc and R values making up the simple phase shift network.
More complex wideer band I/Q all pass networks can be made up
from chains of OP-amps, and ther eis plenty of documentaion aroudn for
that.
Andy G4JNT
