[Discuss-gnuradio] Source of annoying problem found

[Ian Wraith]

Hello
     For the last week I have been chasing an annoying bug in my
experimental DQPSK demodulator which was causing the I & Q channel
waveforms from my quadrature demodulator to be distorted. To try and
pin down this bug I borrowed a Agilent signal generator and set it
to produce a 18 KHz waveform. I fed this into the sound cards line in
and and set my program to write the data captured (from GrAudioSource
<VrComplex>) to a file. When I looked at the initial data which had
been through the root raised cosine filter then sample rate converted
the sine waves had wildly varying amplitudes and the square waves
looked very similar. Even when I removed the root raised filters
and the sample rate converters the waveforms were still distorted.

 Just to confirm this wasn't due to a problem with GrAudioSource I
used Audacity to record from the signal generator and to my suprise
the waveforms looked the same. However when I lowered the output of the
generator to 10 KHz the waveforms were much larger and not in the
slightest distorted. I can only conclude therefore that the soundcard
built onto my motherboard (an Asus A7V8X-X) has a hardware lowpass
filter on its line inputs which is affecting my 18 KHz signals. Are such
filters normal on a soundcard ??

 I'm afraid looks like my idea to demodulate PI/4 DQPSK with a soundcard
isn't possible with this motherboard. So its roll on the USRP (I have a
credit card burning a hole in my pocket !) then I can get back to this mode.
Until then I might look at writing something to decode one of the simpler
data modes in use over here.

Regards

Ian