ossaudiodev full duplex

I'm messing around with trying to record sound reflections off of
various objects at different frequencies. I'm using python 2.4 on
Fedora Core 3 and I have an SBLive card.  Basically, I want to send
some samples through a speaker and then record from an input source
(the line input on the sound card which I have the output of a mixer
going into and a mic plugged into the mixer). So I want to using the
soundcard in full duplex mode.  I'm trying something like this:

number_of_channels= 1
sample_rate= 44100
sample_width= 2
frames_out= get_frames("test.wav")

fh= ossaudiodev.open("/dev/dsp", "rw")
fh.setparameters(ossaudiodev.AFMT_S16_LE, number_of_channels,
sample_rate)
fh.writeall(frames_out)
frames_in= fh.read(number_of_samples * sample_width)
fh.close()


However, even if I turn off the speakers I get the original output tone
back from the read (as well as the input from the mic).  I'm assuming
it's the same internal buffer somewhere on the soundcard so when I do
the read I'm really reading what the write just wrote (because I get
that orginal tone in the background even though I turned off the
speaker so it's just not the reflections coming back through the
mic...it's happening internally somewhere). If I unplug everything from
the sound card I'll still get it.

I tried closing the filehandle after the write and then reopening it
before the read and I also tried opening two filehandles, one to
/dev/dsp and another to /dev/dsp1 and still have th same problem.  In
the above example I tried the ossaudiodev's sync() and reset()
functions inbetween the write() and read().  Also still have the same
problem even when I do this:

fh= ossaudiodev.open("/dev/dsp", "w")
fh.setparameters(ossaudiodev.AFMT_S16_LE, number_of_channels,
sample_rate)
fh.writeall(frames_out)
fh.close()

fh= ossaudiodev.open("/dev/dsp", "r")
fh.setparameters(ossaudiodev.AFMT_S16_LE, number_of_channels,
sample_rate)
frames_in= fh.read(number_of_samples * sample_width)
fh.close()


Any ideas?  Am I doing something fundamentally wrong?  Any help would
be greatly appreciated.
Thanks.
-richie