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README_fdmdv.txt
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README_fdmdv.txt
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README_fdmdv.txt
David Rowe
Created March 2012
Introduction
------------
A 1400 bit/s (nominal) Frequency Division Multiplexed Digital Voice
(FDMDV) modem based on [1]. Used for digital audio over HF SSB.
The FDMDV modem was first implemented in GNU Octave, then ported to C.
Algorithm development is generally easier in Octave, but for real time
work we need the C version. Automated units tests ensure the
operation of the Octave and C versions are identical.
Quickstart
----------
Built as part of codec2-dev, see README for build instructions.
1. Generate some test bits and modulate them:
$ ./fdmdv_get_test_bits test.c2 1400
$ ./fdmdv_mod test.c2 test.raw
$ play -r 8000 -s -2 test.raw
2. Two seconds of test frame data modulated and sent out of sound device:
$ ./fdmdv_get_test_bits - 2800 | ./fdmdv_mod - - | play -t raw -r 8000 -s -2 -
3. Send 14000 modulated bits (10 seconds) to the demod and count errors:
$ ./fdmdv_get_test_bits - 14000 | ./fdmdv_mod - - | ./fdmdv_demod - - 14 demod_dump.txt | ./fdmdv_put_test_bits -
Use Octave to look at plots of 1 second (1400 bits) of modem operation:
$ cd ../octave
$ octave
octave:1> fdmdv_demod_c("../src/demod_dump.txt",14000)
4. Run Octave simulation of entire modem and AWGN channel:
$ cd ../octave
$ octave
octave:1> fdmdv_ut
5. NOTE: If you would like to play modem samples over the air please
convert the 8 kHz samples to 48 kHz. Many PC sound cards have
wildly inaccurate sample clock rates when set to 8 kHz, but seem to
perform OK when set for 48 kHz. If playing and recording files you
can use the sox utility:
$ sox -r 8000 -s -2 modem_sample_8kHz.raw -r 48000 modem_sample_48kHz.wav
For real time applications, the fdmdv.[ch] library includes functions to
convert between 48 and 8 kHz sample rates.
6. Send 20 seconds at 2000 bit/s (20 carriers) to demod and count errors:
$ ./fdmdv_get_test_bits - 20000 20 | ./fdmdv_mod - - 20 | ./fdmdv_demod - - 20 | ./fdmdv_put_test_bits - 20
7. Test with timing slips due to sample clock offset of 1000ppm:
$ ./fdmdv_get_test_bits - 30000 | ./fdmdv_mod - - | sox -t raw -r 8000 -s -2 - -t raw -r 7990 - | ./fdmdv_demod - - 14 demod_dump.txt | ./fdmdv_put_test_bits -
octave:98> fdmdv_demod_c("../build_linux/src/demod_dump.txt",28000)
27552 bits 0 errors BER: 0.0000
References
----------
[1] http://n1su.com/fdmdv/FDMDV_Docs_Rel_1_4b.pdf
[2] http://n1su.com/fdmdv/
[3] http://www.rowetel.com/blog/?p=2433 "Testing a FDMDV Modem"
[4] http://www.rowetel.com/blog/?p=2458 "FDMDV Modem Page" on David's web site
C Code
------
src/fdmdv_mod.c - C version of modulator that takes a file of bits and
converts it to a raw file of modulated samples.
src/fdmdv_demod.c - C version of demodulator that takes a raw file of
modulated samples and outputs a file of bits.
Optionally dumps demod states to a text file which
can be plotted using the Octave script
fdmdv_demod_c.m
src/codec2_fdmdv.h - Header file that exposes FDMDV C API functions. Include
this file in your application program.
src/fdmdv.c - C functions that implement the FDMDV modem.
src/fdmdv-internal.h - internal states and constants for FDMDV modem,
shouldn't be exposed to application program.
unittest/tfdmdv.c - Used to conjunction with unittest/tfdmdv.m to
automatically test C FDMDV functions against
Octave versions.
Octave Scripts
--------------
(Note these require some Octave packages to be installed, see
octave/README.txt).
fdmdv.m - Functions and variables that implement the Octave version of
the FDMDV modem.
fdmdv_ut.m - Unit test for fdmdv Octave code, useful while
developing algorithm. Includes tx/rx plus basic channel
simulation.
Typical run:
octave:6> fdmdv_ut
Eb/No (meas): 7.30 (8.29) dB
bits........: 2464
errors......: 20
BER.........: 0.0081
PAPR........: 13.54 dB
SNR.........: 4.0 dB
It also outputs lots of nice plots that show the
operation of the modem.
For a 1400 bit/s DQPSK modem we expect about 1% BER for
Eb/No = 7.3dB, which corresponds to SNR = 4dB (3kHz
noise BW). The extra dB of measured power is due to the
DBPSK pilot. Currently the noise generation code
doesn't take the pilot power into account, so in this
example the real SNR is actually 5dB.
fdmdv_mod.m - Octave version of modulator that outputs a raw file.
The modulator is driven by a test frame of bits. This
can then be played over a real channel or through a
channel simulator like PathSim. The sample rate can be
changed using "sox" to simulate differences in tx/rx
sample clocks.
To generate 10 seconds of modulated signal:
octave:8> fdmdv_mod("test.raw",1400*10);
fdmdv_demod.m - Demodulator program that takes a raw file as input,
and works out the bit error rate using the known test
frame. Can be used to test the demod performs with
off-air signals, or signals that have been passed
through a channel simulator.
To demodulate 2 seconds of the test.raw file generated
above:
octave:9> fdmdv_demod("test.raw",1400*2);
2464 bits 0 errors BER: 0.0000
It also produces several plots showing the internal
states of the demod. Useful for debugging and
observing what happens with various channels.
fdmdv_demod_c.m - Takes an output text file from the C demod
fdmdv_demod.c and produces plots and measures BER.
Useful for evaluating fdmdv_demod.c performance.
The plots produced are identical to the Octave
version fdmdv_demod.m, allowing direct comparison of
the C and Octave versions.
tfdmdv.m - Automatic tests that compare the Octave and C versions of
the FDMDV modem functions. First run unittest/tfdmdv, this
will generate a text file with test vectors from the C
version. Then run the Octave script tfdmdv and it will
generate Octave versions of the test vectors and compare
each vector with the C equivalent. Its plots the vectors
and and errors (green). Its also produces an automatic
check list based on test results. If the Octave or C modem
code is changed, this script should be used to ensure the
C and Octave versions remain identical.
Modelling sample clock errors using sox
---------------------------------------
This introduces a simulated 1000ppm error:
sox -r 8000 -s -2 mod_dqpsk.raw -s -2 mod_dqpsk_8008hz.raw rate -h 8008
TODO
----
[ ] implement ppm measurements in fdmdv_get_demod_stats()
[ ] try interfering sine wave
+ maybe swept
+ does modem fall over?
[ ] try non-flat channel, e.g. 3dB difference between hi and low tones
+ make sure all estimators keep working
[ ] test rx level sensitivity, i.e. 0 to 20dB attenuation
[ ] make fine freq indep of amplitude
+ use angle rather than imag coord
[ ] document use of fdmdv_ut and fdmdv_demod + PathSim
[ ] more positive form of sync reqd for DV frames?
+ like using coarse_fine==1 to decode valid DV frame bit?
+ when should we start decoding?
[ ] more robust track/acquite state machine?
+ e.g. hang on thru the fades?
[ ] PAPR idea
+ automatically tweak phases to reduce PAPR, e.g. slow variations in freq...
[ ] why is pilot noise_est twice as big as other carriers