"The FAX and SSTV Corner"


SSTV - Fundamentals

The start

SSTV has been developed by radio amateurs inn order to transfer
still pictures within the bandwidth of a voice channel. Slow Scan TV depends
on frame and line sync pulses like all television methods. Because of the
narrow bandwidth (max. 3 KHz) it needs very slow scanning for only still
pictures without additional information like sound or so. In the beginning
SSTV was only black and white at a resolution of 120 pixels on 120 lines,
that needed seven to eight seconfs per frame. By doubling the lines or
the pixels per line or both together the transfer time rose to 16 and 32
seconds.

Color

Trying to transfer colored pictures first there were experiments using
frame sequential procedures with seperated red, green and blue extracts.
These were transmitted in black and white one by one and compiled again
into the color picture at the receiving end. This way the complete
picture color was visible only after transfer of the third extract. The
german slow scan pioneer Volker Wraase, DL2RZ, developed a line sequential
procedure that represents the presently used standard in an adapted form.
Here the color picture is sliced to three extracts that are transmitted
line by line in red, green and blue portions. Assortment of these extracts
to their respective frame store at the receiver is realised by a modified
sync pulse at the start of the red line.

Essential developments were made by the british radio amateurs Eddie ("Scottie")
Murphy, GM3SBC, and Martin Emmerson, G3OQD, who wanted to make
color SSTV less prone to interference. The amount of sync signals was reduced,
so the pictures are transfered nearly running free. These new procedures
were accepted as so-called "new modes" or "free run modes" relatively fast
amoung the slow scanners. Today the mostly used SSTV standard derives from
the work of G3OQD. Out of his four modes the "Martin 1" is prefered around
the world. The color sequence is green - blue - red (GBR) opposite to the
common red - green - blue (RGB). The transfer of a "Martin 1" picture takes
114 seconds and is a well balanced compromise between durance and resolution.

How to get qrv

To realise slow scan operation you can take special SSTV devices
with all functions and interfaces for storing and transfering pictures,
or you take your home computer. With this and various available special
software and different converter types the expenses are relatively small.

Which frequencies are in use for SSTV?

The calling frequencies for SSTV on hf are 3730, 7040, 14230, 21340
and 28680 KHz with daily activities. At the same frequencies there are
meeting FAX friends too, collisions and misunderstandings can be prevented
if you announce your mode before transmitting.

More info

If you want to engage in this attractive mode you can find out
much in the book "Das FAX/SSTV-Praxisbuch fuer Funkamateure" from
DARC-Verlag.
PS: english language book: "Slow Scan Television Explained"
by Mike Wooding, G6IQM (BATC), Great Britain; www.batc.org.uk

73 Klaus, DL4KCK


Facsimile/FAX - Basics

Look back

Facsimile telegraphy is one of the oldest telegraphy modes. Already
before the first use of Morse code in Europe (1848) Alexander Bain invented
a copy telegraph (1843). Practical use of that followed much later.

After 1920 the picture telegraphy was developed further in USA and Europe
by well known companies like RCA, Bell, Muirhead, Siemens and Hell
until today´s photo telegraphy in black and white or even in three color
extracts transfered one after another.

Functioning

With classical style facsimile transmission a paper sheet is scanned
by a light beam generating an electrical signal in amplitude modulation.
The sheet is scanned on a rotating drum line by line similar to television.
The picture signal is modulated onto a sound subcarrier and demodulated
at the receiving end. There the gained signal controls the writing head
in a FAX machine. Fax machines are classified in different groups. The
analog types of CCITT group 1 and 2 are usable in amateur radio FAX traffic
(after some modifications).

The quality of picture transmissions depends upon the number of pixels
per line, number of lines per millimeter and the number of lines per minute
(transfer time). There are international standards like 60, 90, 120 and
240 lines per minute (lpm) and frequency modulation on a 1900 Hz subcarrier
with plus/minus 400 Hz maximum deviation.

Wheather satellite transmissions on VHF/SHF have amplitude modulation
on a 2400 Hz subcarrier that is sent in FM with plus/minus 20 KHz deviation
peaks. Synchronisation of FAX transmitter and receiver is done only at
start of scanning, after that they run free or crystal controlled in sync.
New arrangements have been met after development of modern computer based
FAX techniques.

A color FAX method called "JV-color" with 360 lines per minute by DK8JV
is similar to color SSTV, but gives higher resolution. In a package with
SlowScan and wheather satellite decoding extensions it promoted as "JVFax"
PC program the international spreading of picture modes in amateur radio.

 

How to get on air

To transfer FAX in amateur radio you do not need to modify the
usual transceivers. On hf the mode J3C is used (FAX in SSB). The
picture polarity is set by choice of the sideband.

Following CCITT recommendations picture white should always be at a
higher frequency than the (middle) subcarrier frequency. The normal audio
input and output (micro and loudspeaker) of transceivers can be used to
connect analog fax machines or personal computers.

With using computers and the various software supply together with
different converter types the expenses can stay low.

Which frequencies are used

Calling frequencies for FAX on hf are 3730, 7040, 14230, 18110, 21340,
24930 and 28680 KHz. On 160 and on 30 m no FAX traffic is allowed.
SSTV traffic on the same frequencies dominates the picture modes on the
hf amateur bands.

More info

If you want to engage in this attractive mode you can find out
in the book

"Das FAX/SSTV-Praxisbuch fuer Funkamateure" from DARC- Verlag.

73 Klaus, DL4KCK

update 2013:
Martin Bruchanov, OK2MNM, has written a free e-book for ham radio operators
and radio listeners interested in special communication modes for image
transmission – SSTV, radio facsimile (WEFAX) and digital SSTV (HamDRM).
You can download the entire book (PDF) or view on line...
http://www.sstv-handbook.com/



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