IMAGE MODES

ANALOGUE MODES

Before the development of digital modes, analog schemes were developed to transmit images or image-like information via radio.

Hellschreiber was an early transmission mode where text is converted into an image, consisting of a series of scan lines, like an analog black and white TV picture or a fax. The receiver sees a simple image of the text.

SSTV (Slow Scan Television) is used mainly to send photographic images, monochrome or color, using a voice channel, usually with SSB. See below for more information on this mode.

WEFAX (Weather Facsimile) is used to broadcast weather images from satellites or other stations.  WEFAX uses a modulation scheme similar to SSTV.

ATV (Amateur Television) refers to “fast-scan” television transmitted in real time using wideband modulation methods, in a similar fashion to consumer television.

SSTV

Slow Scan TV is used to transmit and receive single images via radio. The most popular program is the dedicated MMSSTV, written by Makoto Mori. The image is scanned via horizontal lines which build up the image progressively. The various levels of brightness of each colour channel – red, green and blue – are modulated by a shift in frequency between 1500 and 2300Hz. In addition, there is a 1200Hz synchronisation pulse to aid in the correct assembly of the scan lines. To get the program, click below to go to the download website. BE CAREFUL, not to click on the big DOWNLOAD button top right, it is not for the SSTV program, its ‘clickbait’ for a driver.

The website above also contains a SSTV Primer, Quick Start Graphics and Slant Adjustment advice. This is an excellent site to explore.

The comprehensive MMSSTV Help Menu has been assembled as a single document by Chris VK2ACD and is available here:

SSTV, being analogue, is quite robust and images will be received even with poor band conditions. A weaker signal-noise ratio just adds “snow” to the image, which can also drop out. This then creates a horizontal noise bar until the signal improves. All that is needed for SSTV is a computer and a good stable sound card to allow audio in and out. Many modern rigs have a soundcard built in, but external sound card devices such as the Signalink are highly recommended.

The MMSSTV software has not been updated since about 2010, but it still works well with modern versions of Windows. I would personally recommend a separate computer or laptop for amateur radio purposes, if possible.

There are a wide variety of modes within MMSSTV, some dedicated to black and white and a large selection of colour modes that support images of varying dimensions (in pixels). For example Scottie 1 takes an image of 320×256 pixels and takes 110 seconds to transmit. MR140 also takes this size but takes 140 seconds. The largest mode appears to be PD290, 800×616 pixels and takes 290 seconds to transmit.

It is possible to receive images transmitted by the International Space Station. These are normally sent by the Russian crew using a Kenwood
TM-D710 amateur radio. The frequency is 145.800 FM and the signals can be good to not so good, depending on the orbit height above your horizon. If the ISS is high up it is possible to receive two or three good images for each pass.

A good website for ISS activity and information is here:
http://ariss-sstv.blogspot.com/

The ISS mainly uses MMSSTV PD120 format for image transmission. Packet mode and voice signals are also available via the ISS. The website above has details of the frequencies and bands used.

In MMSSTV, an image can be “loaded from file” which means navigating to the image’s location on your computer or simply dragging and dropping an image into the TX window. It is preferable for the image proportions be close to the image sizes used by the mode chosen – for example 320×256 pixels. If the dimensions or proportions are different the image is just forced to fit into the frame and the results can look a little odd. Basic cropping tools are available in the program or with programs such as Paint. If you want to experiment with something more professional than these basic ones, then the free Irfanview software can be downloaded, which has cropping tools and much more. Clock calibration within MMSSTV is achieved by tuning to a time station such as WWV and allowing the program to produce a trace line over a period of time. This process is a bit slow, so best to just leave it for a while. This vertical trace line is clicked on at the top and bottom of the trace and it results in a clock offset figure which is then applied. There is a step by step guide to this in the MMSSTV Help Menu referred to above. Slant adjustment during the receive time is automatic plus larger adjustments to slanted received images are also available in the Sync window. Click on the yellow smiley face! Once straight images are received and transmitted, it is unlikely that image slant will be a problem for quite a while.

The image above shows the TX window with an image loaded. It also shows a template on top. The template is simply an overlay that can hold information such as a callsign, greetings, CQ call and grid square position. The options available are numerous. Multiple images can be assembled and boxes can be filled with a wide choice of colours. Type can also be coloured in many ways. It is best to not clutter up templates too much. Templates can, and should, be made up and saved for the various image dimensions used by the modes MMSSTV supports, eg 320×256, 640×296, 512×400 pixels. A template for a larger image will not fit another image size properly.

Templates can be saved and loaded onto the rectangles at the bottom of the screen when the “templates” tab is clicked. The templates that you have pre-saved can be applied to the image by double clicking onto one, with the templates tab above the image window selected. There is a small button in the middle, far left called “S.pix.” This brings up 25 pages with 12 rectangles on each page. Images can be pre-loaded into these rectangles and dragged and dropped into the TX window for transmit.

Information provided by Paul VK2PMG.