I'm betting there was some kind of audio compression off the webSDR
stream too.
That transmission was at 30W by the way, into a 40m/20m dipole, so not
ideal conditions.
I'll do some tests on 20m soon.
- Mark VK5QI
On 10/07/12 9:02 PM, David Rowe wrote:
Hi Mark,
Nice work, that's really interesting. Some observations:
+ In the modem wave file I can hear drop outs in the audio, perhaps due
to packet loss in the WebSDR packets (complete silence), and some others
that seem to be the tx signal dropping out.
+ The signal amplitude is fairly low which makes the scatter diagram
tiny, but this is probably not a practical problem. I boosted it by
10dB to give better plots.
+ The modem recovers fairly quickly with these drop outs.
+ The frequency offset is pretty low, just a few Hz. Good tuning :-)
+ When in sync, the timing estimate changes quickly indicating a
significant sample clock offset between the tx and rx. I should add
some code to convert this to an explicit ppm or Hz error.
+ The SNR is pretty low, between -5 and 8dB.
+ As well as multipath effects, the spectrum seems to have some sort of
null either side of the BPSK pilots. I am wondering if this is analog
audio filtering or perhaps a side effect of clipping of the tx signal
(i.e. insufficient headroom, peak clipping of the tx signal). Need to
make the modem signal robust to over driving the tx or have aids to set
up.
+ The decoded signal speech quality is fair when there are no errors,
but there are some loud annoying noises when sync is lost. I'll need to
work out a way to do something sensible when sync is lost or the BER is
high.
Anyway, suffice to say these diagnostics combined with real world
signals are pretty useful! Whyalla to Melbourne must be 1500km. Cool.
Thanks Mark & Josh,
- David
On Tue, 2012-07-10 at 09:40 +0930, Mark Jessop wrote:
Hi all,
I've been playing with codec2 over HF over the last few days, and I've
noted the following changes to get everything running on OSX:
In OSX 10.7, malloc is in stdlib, and malloc.h doesn't exist, so in
kiss_fft.h, I had to add the following around the malloc.h include line:
#if !defined(__APPLE__)
#include <malloc.h>
#endif
To get fl_fdmdv going, I installed the required dependencies (fltk,
portaudio) using macports, and used the following line to compile:
g++ fl_fdmdv.cxx -I../src/ -o fl_fdmdv -O3 -g /opt/local/lib/libfltk.a
-lpthread -framework Cocoa ../src/.libs/libcodec2.a -lm -L/opt/local/lib
-lportaudio -pthread -I/opt/local/include
Portaudio is quite nice on OSX, I can select the input device using
descriptive names, like:
./fl_fdmdv -s "Internal Microphone"
Josh VK3XJM and I had a go sending some fmdmv modulated codec2 data over
a 30m HF channel, from Whyalla South Australia. Josh used a webSDR in
Melbourne (http://websdr.comms.net.au:8901/) to receive the signal.
The recorded wave file from the test is here:
http://www.zindello.com.au/wordpress/wp-content/uploads/2012/07/Codec2-Mark3.wav
and the demodulated codec2 data (1400bit/s) is here (remove the .wav
extension, wordpress being annoying):
http://www.zindello.com.au/wordpress/wp-content/uploads/2012/07/rx_fd.c2.wav
Cheers,
Mark VK5QI
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