The main focus of this file is on SSTV because information on this mode is so scarce. I keep trying to ignore WEFAX and ATV because they are even larger topics already well covered by various handbooks and magazines. However, some WEFAX and ATV information sneaks in when it is closely related to SSTV, for instance, some software that does both SSTV and WEFAX.
SSTV stands for slow-scan television
SSTV and related modes
There are several different ways to send images over ham radio:
1. RTTY art – Remember when people were amused by making pictures
out of characters?
2. FSTV (Fast Scan TV – Also called ATV) – Similar to broadcast TV.
Full motion, color, sound, etc. Restricted to UHF and up because
a signal requires several MHz of bandwidth.
3. WEFAX (weather facsimile) – Very high-resolution grayscale images
sent by audio tones over a period of minutes.
4. SSTV (Slow Scan TV) – Low to medium resolution still images sent
through audio channels over a period of several seconds to a few
minutes. Mostly color these days.
5. Digital SSTV – I haven’t seen any proposals yet. You could always
compress images using an international standard, such as JPEG, and
send the files over existing packet radio networks.
Transmission Modes
——————
The original 8 second SSTV transmission mode (around 1958) had the following
characteristics:
black = 1500 Hz
white = 2300 Hz
gray levels in between
120 scan lines
15 lines per second
5 mS of 1200 Hz for horizontal sync
30 mS of 1200 Hz for vertical sync
Since that time many more modes have been invented, sometimes to add new capabilities, sometimes the result of Not Invented Here syndrome.
The major groups are:
Robot – Introduced with the Robot scan converters (California).
Wraase – Introduced with the Wraase scan converters (Germany).
Martin – Developed by Martin Emmerson (England).
First available as replacement PROMs for Robot 1200C.
Scottie – Developed by Eddie Murphy (Scotland).
First available as replacement PROMs for Robot 1200C.
AVT – Developed by Ben Blish/Williams (Montana).
First available in the AVT system.
The Robot, Wraase, Martin, and Scottie modes are all closely related.
They all use the tones above for black, white, and gray levels.
They all have 1200 Hz horizontal sync, although some Martin and Scottie
implementations rely on accurate crystal oscillators and ignore the
horizontal sync once synchronized. Color is generally transmitted by sending
each scan line 3 times, once each for red, green, and blue components.
Robot is different from the rest in that it encodes colors with
luminance (Y) and chrominance (R-Y and B-Y) instead of R,G,B.
Each of these modes has a few different speeds, usually providing 120 or 240
scan lines and varying degrees of horizontal resolution. (Actually, some send
128 or 256 lines but the top 8 or 16 are always a fixed grayscale,
leaving 120 or 240 usable lines.)
The Robot modes have a much longer vertical sync (called VIS) containing
7 bits of information and a parity bit. This identifies the format of the
following image so manual selection is not required on systems that
recognize it. Everyone else has adopted the Robot VIS coding and assigned
themselves unused codes in the original specification.
The AVT mode is radically different from the rest. It has no horizontal
sync at all; very accurate crystal oscillators are required to prevent
slanted pictures. Rather than a vertical sync pulse of about 1/3 second,
it has a digital header with 32 repetitions of the transmission mode and
a sequence number. It is only necessary to receive one of the 32 groups
correctly to achieve synchronization. For more details see the AVT article
in CQ-TV mentioned in the bibliography.
Frequencies
———–
By convention, SSTV operation is generally found on only a few frequencies.
>From a brochure from the International Visual Communication Association:
3.845
7.171
14.230
14.233
21.340
28.680
144.5
Nets
—-
Two different Slow Scan nets meet on Saturdays at 15:00 and 18:00 UTC
on 14.230 MHz.
