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[aprssig] APRN status?

Stephen H. Smith wa8lmf2 at aol.com
Fri Mar 26 18:54:38 UTC 2010


Lee Mushel wrote:
> Gentlemen:
>  
> I know nothing about the subject and would appreciate some "link 
> direction" help.    A week ago I was in a National Weather Service 
> office and I asked them if any of the supporting hams (I know that 
> they do help out during Weather Spotting Need" has mentioned SSTV in 
> addition to the small amount of APRS and auto WX reporting that I know 
> is done.   They told me that SSTV had never been brought up and they 
> knew nothing about it.   From some past years experience I know that 
> the word descriptions given by Spotters can be confusing and I would 
> like to know if we can send images in addition to the verbal reports 
> in times of confusion.
>  
> 73
>  
> Lee   K9WRU

SSTV (Slow Scan TV) is an image transmission technique developed in the 
late 1960's and early 1070"s (i.e. before the digital revolution).  SSTV 
is somewhat of a misnomer since it doesn't transmit TV in the normal 
sense (i.e moving pictures).   Instead of sending 15,000-odd lines/sec 
(30 frames/sec) like broadcast TV, SSTV only sends 15-20 lines second 
and takes 30-90 secs to send a single quarter-VGA-resolution frame. By 
sending this slowly, it can send pictures over any normal 
voice-bandwidth radio, rather than requiring the wide bandwidths and 
specialized transmitters required for full-motion broadcast-style 
fast-scan TV.  Note that this is either/or: you talk, or you send a 
picture over the channel, but not at the same time

In it's original incarnation, it was an ANALOG system.  The pictures 
were NOT grids of pixels like digital photos, but rather continuous 
scanning lines just like analog broadcast TV, but thousands of times 
slower.  

In the early years of SSTV, specialized dedicated HARDWARE devices 
(cameras and monitors) were used to capture images, convert them to 
variable-frequency audio tones for transmission, and then convert the 
tones back to images at the receiving end.  Begining in the late 
1980's/early 1990's, computing power and graphics display capability 
available in ham shacks at reasonable prices escalated to the point 
where digital signal processing on generic PCs started replacing 
dedicated hardware.    About 20-odd transmission  formats evolved (it 
was "just" a software change to introduce yet another format) , trading 
off various combinations of resolution, noise immunity and transmission 
time. Several higher-resolution full VGA (480x640) and SVGA (600x800) 
formats were also introduced, although these take even longer (3-5 
minutes) to transmit. 

Today, programs on PCs that use the computer soundcard as a 
analog<-->digital converter for SSTV transmission and reception have 
COMPLETELY replaced the specialized SSTV hardware previously used.     
This presents a problem for in-the-field use, since sending SSTV images 
in the field now definitely requires a  PC connected to the radio.

Probably 90% of all analog SSTV activity is now done using a single 
program, "mmSSTV". This freeware program, developed by a Japanese ham, 
is a standard Windows application that will read any standard graphics 
image file (BMP, JPG, ec), downsize it to SSTV format and then transmit 
it in any of two dozen formats.  At the receiving end, the same program 
will decode the audio tones from the receiver audio-out and display the 
recovered image.   For field, use this means you have to capture the 
scene of interest with a digital camera, transfer a FILE to the PC, and 
have the SSTV application open the FILE for transmission.  

Unfortunately, development on mmSSTV (and any other analog SSTV 
software) seems to have ended about 1995.  Else it would have been very 
useful to have incorporated a TWAIN capability in the program to 
directly acquire an image from a digital camera (or high-res webcam) 
which would have made near-live transmission much more practical. 



The latest development of the last few years is so-called "digital" 
SSTV.  This is actually an error-detecting/error-correcting generic file 
transmission system.  It uses 16 or more QAM audio subcarriers in the 
voice passband (in a manner similar to "56K" modems) to send standard 
JPG or other image files.  The dominant digital SSTV program is a 
freeware application from an Australian ham, VK4AES, called "EasyPal".   

EasyPal is not limited to image files. You can send any file from a PC's 
file system; i.e. Word documents, HTML, MP3 clips, etc.  When 
transmitting images, you can adjust a slider to trade off JPG 
compression level (and thus image degradation) vs transmission time.  A 
standard 640x480 VGA image at medium quality takes about two minutes to 
send. 

EasyPal DOES incorporate a TWAIN driver that can directly acquire images 
from scanners or digital cameras.

EasyPal incorporates an ACK/NAK/ARQ system that can request selective 
retransmissions of missed blocks of sent files, without resend the 
entire file.  These "BSRs" ("Bad Segment Retries") can even be requested 
from stations other than the one that made the original transmission.   
This makes it practical for multiple stations in nets (especially on HF) 
to assist one another in getting a picture from one station to another.   

Further, EasyPal incorporates a built-in FTP client that can 
automatically upload received pics to a specified web server.  On 
first-time run, it actually automatically generates the HTML page 
required and uploads it to the web server. Each subsequent received 
picture automatically pushes previous pics down a list of up to  30 images.

Due to the complex DSP required to process the multiple QAM tones in  
real time, about the minimum PC that will run EasyPal is a 700-800 MHz 
P-III with a true-hardware-based soundcard; i.e. not a "brain-dead" 
software-based motherboard el cheapo sound system.  It MAY be possible 
to get the highest-powered netbooks, now that their processors are 
pushing 1.5 GHz, to run EasyPal, but I haven't had the chance to try 
this yet. 

The TX and RX audio level settings are much more critical than with 
packet or analog SSTV.  (The multiple parallel audio tones will suffer 
massive intermodulation distortion if the audio or the SSB RF stages of 
a transmitter are overdriven even slightly.)  However the results when 
done right are amazing.  The Colorado scenic shots on my EasyPal 
webserver (lower part of the thukmbnail preview page) here:

     <http://wa8lmf.net/livecam>     

were sent over 40-meters SSB from central Colorado over a path of about 
900 miles.  These are TWAIN acquires of images from a Canon PowerShot 
560 digital camera mounted on my dashboard looking out the front window 
of the car.   The pics at the top of the page are scans of 35mm 
Kodachrome slides (stored on the mobile laptop's hard disk) from Viet 
Nam circa 1968-70. They were transmitted over 6-meters FM while I was 
driving around the San Gabriel Valley area of Los Angeles.   (I was 
experimenting with TX audio levels on an FT-857.)


     MMSSTV Home Page:
    <http://mmhamsoft.amateur-radio.ca/pages/mmsstv.php>

     EasyPal Home Page:
    <http://vk4aes.com/>     [Scroll down below thumbnails for download 
links.]


------------------------------------------------------------------------

--

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

NEW!    Universal HF/VHF/UHF Antenna Mounting System
  http://wa8lmf.net/mobile/UniversalAntMountSystem.htm

"APRS 101"  Explanation of APRS Path Selection & Digipeating
  http://wa8lmf.net/DigiPaths

Updated "Rev H" APRS            http://wa8lmf.net/aprs
Symbols Set for UI-View,
UIpoint and APRSplus:


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