[aprssig] APRN status?
Stephen H. Smith
wa8lmf2 at aol.com
Fri Mar 26 13:54:38 CDT 2010
Lee Mushel wrote:
> 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.
> 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
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
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
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:
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:
EasyPal Home Page:
<http://vk4aes.com/> [Scroll down below thumbnails for download
Stephen H. Smith wa8lmf (at) aol.com
EchoLink Node: WA8LMF or 14400 [Think bottom of the 2M band]
Home Page: http://wa8lmf.net
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