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