Slow Scan Television
What is SSTV ?
Slow Scan Television (SSTV) is a narrow-band (voice spectra) picture transmission method used to receive and transmit static pictures via radio (Source: Wikipedia - 12/7/10). In fact the first pictures of the Apollo 11 landing on the moon were actually transmitted by Slow-Scan techniques beaming a picture back to Earth at the rate of 320 lines at the rate of 10 frames per second.
The techniques for SSTV have improved quite considerably since those early techniques developed for the reception of signals from Space. All that is needed to transmit and receive SSTV is a good, stable sound card and audio lines in and out from your PC and radio. The simplest recommended reception interface would include a 600:600 ohm isolation transformer from the speaker output to the line in on your PC.
Analogue SSTV uses Analogue Frequency Modulation whereby each level of brightness in the source image gets modulated via a shift in frequency between 1500 and 2300Hz depending upon the brightness of the original image. Colour is achieved by sending the brightness of each colour component (red, green or blue) separately (Wikipedia - 12/7/11). A transmission consists of horizontal lines scanned from left to right, building up an image progressively with each scan. A 1200 Hertz synchronization pulse is added so that lines can be correctly assembled at the receiver end. Analogue SSTV is governed by an "encoding scheme" that encodes or decodes the source image for transmission. The main modes of SSTV "encoding" in use include the AVT, Martin, Robot and Scottie modes .
Many digital-mode programs used for modes such as PSK and RTTY (i.e. MixW and DM780) have support built in for receiving SSTV pictures. The most commonly used program for Amateur SSTV transmission and reception is the dedicated SSTV program MMSSTV written by Makoto (Mako) Mori JE3HHT. MMSSTV can be downloaded by clicking the following Link
Analogue SSTV transmissions can be found on HF frequencies around 3.640MHz LSB, 7.170MHz LSB, 14.230 MHz USB, 21.340MHz USB and 28.680MHz USB. On 2m and 70cm transmissions are commonly found around 145.625 MHz (USB and FM) and 433.775 MHz (USB and FM).
Digital SSTV is a recent innovation whereby multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) techniques, based loosely around the Digital Radio Mondiale (DRM) broadcasting protocols as used for commercial Digital Radio Transmissions in Australia, are used to digitally encode and broadcast images and simple data files. The DRM protocol has been adapted to fit within a voice-spectrum bandwidth (up to 2.5KHz) that can easily be modulated and received by standard SSB and FM transceivers.
The most common program in use today is "EasyPal". Development of this program is coordinated by Erik Sunstrup VK4AES. EasyPal can be downloaded by clicking the following Link
Analogue slow scan builds up a picture progressively - line by line - until the image is complete. Digital SSTV works on building up enough received digital data from mathematically compressed images to begin rendering pictures. A huge advantage of Digital SSTV is the ability to request retransmission of data segments that have not been received properly in order to build up a high resolution picture. As a result pictures received by digital transmission tend to be far superior in quality than those received via analogue transmission.
Digital SSTV transmissions are usually conducted between 3 - 6KHz higher on HF bands and on the same frequencies as Analogue Transmissions on VHF and UHF. A considerable number of groups operate across Australia are experimenting with this mode, often using FM on 2m and 70cm.
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