Horus 37 – High Speed SSTV Images

Today I was part of the AREG team that flew Horus 37 – a High Altitude Balloon flight. The payload included hardware sending Slow Scan TV (SSTV) images at 115 kbit/s, based on the work Mark and I documented in this blog post from earlier this year.

It worked! Using just 50mW of transmit power and open source software we managed to receive SSTV images at bit rates of up to 115 kbit/s:

More images here.

Here is a screen shot of the Python dashboard for the FSK demodulator that Mark and Brady have developed. It gives us some visibility into the demod state and signal quality:

(View-Image on your browser to get a larger version)

The Eb/No plot shows the signal strength moving up and down over time, probably due to motion of our car. The Tone Frequency Estimate shows a solid lock on the two FSK frequencies. The centre of the Eye Diagram looks good in this snapshot.

Octave and C LDPC Library

There were some errors in received packets, which appear as stripes in the images:

On the next flight we plan to add a LDPC FEC code to protect against these errors and allow the system to operate at signal levels about 8dB lower (more than doubling our range).

Bill, VK5DSP, has developed a rate 0.8 LDPC code designed for the packet length of our SSTV software (2064 bits/packet including checksum). This runs with the CML library – C software designed to be called from Matlab via the MEX file interface. I previously showed how the CML library can be used in GNU Octave.

I like to develop modem algorithms in GNU Octave, then port to C for real time operation. So I have put some time into developing Octave/C software to simulate the LDPC encoded FSK modem in Octave, then easily port exactly the same LDPC code to C. For example the write_code_to_C_include_file() Octave function generates a C header file with the code matrices and test vectors. There are test functions that use an Octave encoder and C decoder and compare the results to an Octave decoder. It’s carefully tested and bit exact to 64-bit double precision! Still a work in progress, but has been checked into codec2-dev SVN:

ldpc_fsk_lib.m Library of Octave functions to support LDPC over FSK modems
test_ldpc_fsk_lib.m Test and demo functions for Octave and C library code
mpdecode_core.c CML MpDecode.c LDPC decoder functions re-factored
H2064_516_sparse.h Sample C include file that describes Bill’s rate 0.8 code
ldpc_enc.c Command line LDPC encoder
ldpc_dec.c Command line LDPC decoder
drs232_ldpc.c Command line SSTV deframer and LDPC decoder

This software might be useful for others who want to use LDPC codes in their Matlab/Octave work, then run them in real time in C. With the (2064,512) code, the decoder runs at about 500 kbit/s on one core of my old laptop. I would also like to explore the use of these powerful codes in my HF Digital Voice work.

SSTV Hardware and Software

Mark did a fine job putting the system together and building the payload hardware and it’s enclosure:

It uses a Raspberry Pi, with a FSK modulator we drive from the Pi’s serial port. The camera aperture is just visible at the front. Mark has published the software here. The tx side is handled by a single Python script. Here is the impressive command line used to start the rx side running:

#!/bin/bash
# 
#	Start RX using a rtlsdr. 
# 
python rx_gui.py & 
rtl_sdr -s 1000000 -f 441000000 -g 35 - | csdr convert_u8_f | csdr bandpass_fir_fft_cc 0.1 0.4 0.05 | csdr fractional_decimator_ff 1.08331 | csdr realpart_cf | csdr convert_f_s16 | ./fsk_demod 2XS 8 923096 115387 - - S 2> >(python fskdemodgui.py --wide) | ./drs232_ldpc - - | python rx_ssdv.py --partialupdate 16

We have piped together a bunch of command line utilities on the Linux command line. A hardware analogy is a bunch of electronic boards on a work bench connected via coaxial jumper leads. It works quite well and allows us to easily prototype SDR radio systems on Linux machines from a laptop to a RPi. However down the track we need to get it all “in one box” – a single, cross platform executable anyone can run.

Next Steps

We did some initial tests with the LDPC decoder today but hit integration issues that flat lined our CPU. Next steps will be to investigate these issues and try LDPC encoded SSTV on the next flight, which is currently scheduled for the end of October. We would love to have some help with this work, e.g. optimizing and testing the software. Please let us know if you would like to help!

Links
Mark’s blog post on the flight
AREG blog post detailing the entire flight, including set up and recovery
High Speed Balloon Data Link – Development and Testing of the SSTV over FSK system
All your Modems are belong to Us – The origin of the “ideal” FSK demod used for this work.
FreeDV 2400A – The C version of this modem developed by Brady and used for VHF Digital Voice
LDPC using Octave and CML – using the CML library LDPC decoder in GNU Octave