The TriplePIC Slow-Scan Television video scan converter is breathing new life into vintage SSTV! The Summer and Fall 2014 issues of ATV Quarterly carried the full details full details on the history, design, and operation of KD2BD's TriplePIC SSTV Video Scan Converter. The author's original manuscript may be accessed here in PDF format.
Firmware for each of the PIC microcontrollers employed in the TriplePIC SSTV Scan Converter is made available by the author under the GNU General Public License, and may be downloaded here:
Here is comparison of 120 line 8 second SSTV video images displayed on a Robot Model 70 P7 CRT SSTV monitor, a Robot Model 400 digital scan converter, and KD2BD's TriplePIC Scan Converter. Enough said. :-)
How does the TriplePIC's performance
compare with that of modern-day SSTV software, such as MMSSTV? Take a look:
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The TriplePIC shows about 2 dB of A/D sampling jitter in the high frequency portion of the test signal. Some ringing in the demodulator is also evident after the low-frequency black-to-white video transitions. By contrast, the MMSSTV plots shows nearly all high video frequency detail is lost. This fact is further evident in the images below:
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No compromises! As stated in my
ATV Quarterly article:
"The numerous design trade-offs and performance deficiencies inherent in many of the popular SSTV scan converters, video monitors, and PC interfaces of the past created a false impression that the 8-second SSTV standard was less capable than it really was. Even some of the PC-based SSTV applications available today do a poor job of rendering the 8-second image format, if they support it at all, further reinforcing that false impression. The outstanding performance demonstrated by the TriplePIC SSTV Scan Converter suggests this popular view is overly pessimistic."The TriplePIC separates sync from composite video at the SSTV FM subcarrier, and processes them independently. The video demodulator is optimized for video demodulation exclusively, with all the bandwidth, linearity, and "brick wall" post-detection filtering required to produce outstanding video performance. Furthermore, the TriplePIC oversamples each SSTV video line into 256 8-bit pixels, and produces 256 line FSTV (NTSC) images by averaging adjacent SSTV lines of video to further enhance the quality of the upconverted image.
The TriplePIC employs an 8-pole post detection video filter whose frequency response is down 3 dB at 1300 Hz (and is -30 dB at 2 kHz):
As stated in my ATV Quarterly article:
"An SSTV video resolution that is flat to 128 vertical lines requires a demodulated baseband video bandwidth that is flat to at least 1038 Hz for SSTV scanned at a 15 Hz line rate, and 1164 Hz for video scanned at a 16.666 Hz line rate. Computer aided analysis of the post-detection lowpass filter employed in the Robot 400 scan converter revealed a 3 dB bandwidth of only 766 Hz. A slightly smaller bandwidth of 748 Hz was discovered for the Robot 70A SSTV monitor. At 1000 Hz, response is down nearly 8 dB (with respect to DC) in the Robot 70A, and nearly 10 dB in the Robot 400. These figures do not even take into consideration the further bandwidth restrictions imposed by the slew rate of the limiters, the frequency response of the video demodulators, and the speed and resolution of any A/D conversions (as in the case of the Robot 400). "Not one, but two cascaded sync bandpass filters are dedicated exclusively to sync detection, and provide the optimum bandwidth and selectivity for demodulating sync, while simultaneously rejecting noise, interference, and video sideband energy:
Here are some TriplePIC renderings of vintage SSTV images from recording of SSTV transmissions made on 14.230 MHz during the late 1970s and early 1980s:
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Here is some MPEG-4 video of vintage SSTV rendered by the TriplePIC SSTV Scan Converter:
As stated in my ATV Quarterly article,
"Today, slow-scan television signals are handled almost exclusively in the software domain, and SSTV is now considered to be just one of many "digital modes" of communication. Unfortunately, when specialized communication modes such as SSTV become just another software application, the PC becomes the center of attention, and the mode's relevance in our Internet and cellphone dominated world gets lost in the process. One way of reversing this trend and bringing the fascination and the mystique back to Amateur Radio is to refocus our attention on the electronics that makes our unique modes of communication possible. The TriplePIC SSTV Video Scan Converter was created as a means of doing just that. It was born out of a desire to revisit a time when an 8-second image transmission was considered "long", and to thoroughly assess the capabilities of the original monochrome SSTV standard using signal and image processing techniques superior to any popular SSTV monitor or scan converter available in the past. Not only has this feat been accomplished, but the performance of the TriplePIC has also been shown to surpass that of modern PC-based SSTV software as well."
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John Magliacane Amateur Radio Operator: KD2BD |