Download QEX article Some thoughts on "Real Time" SSTV Processing
Thoughts on "Real-Time" SSTV Processing, Lionel and Roland (F2DC)
Cordesses -- How to improve our present SSTV programs.
QEX, May/June 2003
News from 2012: see the update and the links at the bottom of this page.
A few years ago, I decided to publish the whole method behind my digital slow-scan television demodulator (SSTV). It was the end of a work that started in the days of the IBM PC XT, and it proved to be very effective in low signal to noise ratio conditions. The main ideas were:
- A robust horizontal synchronization, allowing slanted pictures to be displayed vertically. It relies on linear signal processing (IIR filter, FIR filter) and a robust line estimation method: The Hough transform (later followed by a least-mean squares algorithm known as total least squares).
- A robust frequency estimator for the video signal, based on a fast Fourier transform (FFTW 2 in those days).
- And last, but not least, a signal to noise estimator so as to choose the most likely window for the FFT.
The article also includes
the exact timings of the Martin M1 mode, kindly given by
G3OQD in those days.
The proof of concept software is available on F2DC SSTV web page. You can download it, but please keep in mind that it has been coded for the Windows 95 operating system and for the Borland C++ Builder compiler. If I had more time, I would rewrite it for the GNU GCC tools, and I would switch to the new FFTW 3 library. The other DSP filtering routines could also benefit from the SSE (SIMD) instructions.
I recently discovered (in January 2012) that one SSTV software mentions our article and uses similar techniques, namely the Hough transform, FFT and variable size windows based on signal to noise ratio. The software is slowrx by Windy OH2-250. The source code is available, along with resulting pictures.
have also found another web site sharing similar thoughts
on SSTV and commenting upon our article: it is the blog brainwagon.org of
K6HX. The author also suggests moving to a more modern
version of FFTW. He then goes back to black and white SSTV
with modern approaches, along with plenty of interesting
ideas. See thoughts-on-sstv
to start with this great series of posts!
slowrx by Windy, a SSTV software based on FFTW3 and the Hough transform.
thoughts-on-sstv: a post on modern approaches to SSTV by K6HX.
more about black and white SSTV by K6HX.