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<font size="2"><font face="Arial,sans-serif">I've been gradually
refining the APRN (Automatic Picture Reporting Network) concept that
combines SSTV and APRS for the last couple of years. The idea is to
follow an SSTV transmission with an APRS position burst so that a
receiving station can both see the picture and the location it was sent
from. The original approach to this was to use a Kenwood TH-D7 or
D700 to automatically add a Mic-E burst at unkey from it's internal
APRS hardware following the end of the SSTV video transmission. <br>
<br>
The FREEware mmSSTV soundcard program is overwhelmingly the most
popular application for sending and receiving SSTV in use today. I've
discovered a way to get mmSSTV to automatically send APRS position
reports at the end of each transmission without having a GPS, or any
APRS packet hardware or software at all. [At least if you are
operating from a known fixed location...]<br>
<br>
mmSSTV has the capability to send a CW id at the end of each
transmission. As an alternative, it can automatically playback an
audio recording previously captured in a proprietary format by mmSSTV
itself. If the audio recording happens to be of an APRS
position-reporting packet burst, you get a perfect posit-after-image
APRN transmission. The option to make the recording is located in the
mmSSTV pulldown menus under "File, Record sound to the file.." <br>
<br>
This recording process creates an audio file with the wierd extension "
.MMV ". This is a format peculiar to mmSSTV, and is not a standard
Windows .WAV file. After spending two nights experimenting with audio
editors and viewing hex dumps in UltraEdit, I discovered this kind of
file is actually a standard raw PCM audio file with a screwball 4-byte
header proprietary to mmSSTV attached.<br>
<br>
To use the recorded file in the mmSSTV program, pull down "Options,
Setup MMSSTV..." . Click on the TX tab. In the lower right corner
is a box for the CW ID configuration. Click the radio button "MMV" to
use the sound file instead of a CW id. Click the button "REF" to
navigate to, and select, the .MMV file with the desired posit in it.
Click OK and you're done. Now every mmSSTV transmission will have an
APRN posit automatically appended at the end of the picture.<br>
<br>
<br>
<br>
One can record the .MMV file directly, including the neccesary header,
with mmSSTV's built-in audio record function but there are major
advantages to using an external audio editor application to do the job.
<br>
<br>
1) There is no easy way to start and stop the mmSSTV recorder
really rapidly, and no provision for editing the resulting recordings.
. As a result, you get a 1/3rd second Mic-E burst surrounded by a
minimum of almost two seconds of dead air, noise and a squelch crash. <br>
<br>
2) There is a very slight pause between the end of the SSTV
transmission and the beginning of the audio playback. If you use a
tone-keyed sound card interface, the transmitter may not stay keyed
between the two. The beginning of the packet burst may get cut off as
the transmitter rekeys, especially if there is any more-or-less silence
at the beginning of the .MMV file. [If you use the typical serial-port
handshaking line PTT keying for the soundcard interface, this won't be
a problem since mmSSTV continues to assert the PTT until the tail-end
audio file has played out.]<br>
<br>
By producing the file in an audio editor application, I can copy the
packet-burst-only from the original recording to a new file where it is
preceeded, without gaps, by an exact one-half-second burst of a 1KHz
sinewave tone for a total file length of about .9 seconds. . This
ensures the sound card interface, transmitter and any repeaters are
fully keyed before the packet burst begins.<br>
<br>
<br>
How I did it: <br>
<br>
I set up my Kenwood TH-D7 in manual PTT-activated mode to send a
callsign and lat/long in an APRS burst each time I unkeyed. I then
connected the demodulator output of my IFR1500 service monitor (which
has a very high quality low-distortion FM discriminator) to the line
input of a computer's sound card. I captured the 1/3rd-second Mic-E
packet squawk using the CoolEdit audio editor (a.k.a. Adobe Audition
after their acquisition of Syntrillium Software) on my PC. I then saved
the file as a "raw PCM" format at 11025 samples/sec 16-bit mono but
with the extension ".MMV" instead of the default ".PCM ". To make the
file work with MMSSTV, insert the 4 "magic" hex bytes 55 AA 00 00
to the front of the file with a hex editor. The resulting audio
recording file is only 20 to 22K. <br>
<br>
To make this process simpler, I made a one-line DOS batch file that
combines a tiny file containing the four magic bytes above with the
file created by CoolEdit to create the mmSSTV compatible file. The
entire contents of this batch file, "AddHeader.BAT", are:<br>
copy /b MMVheader.mmv+%1.pcm APRN_%1.mmv<br>
<br>
This has to be run from a command line (i.e. from a Windows DOS box
a.k.a. "Command Prompt" in Windows 2K or XP) logged into the mmSSTV
directory, with the syntax<br>
<br>
" ADDHEADER callsign " <br>
<br>
The command concatinates the magic header file with a
default-extension .PCM file created by CoolEdit saved with the users's
call sign (for example. "WA8LMF.pcm"), into a new file (
"APRN_WA8LMF.mmv" ) with the mmSSTV friendly " .MMV " extension. <br>
<br>
<br>
<br>
<br>
<br>
Stephen H. Smith wa8lmf (at) aol.com <br>
<br>
Home Page: <a class="moz-txt-link-freetext" href="http://wa8lmf.com">http://wa8lmf.com</a><br>
<br>
New/Updated Symbols for <a class="moz-txt-link-freetext" href="http://members.aol.com/wa8lmf/ham">http://members.aol.com/wa8lmf/ham</a><br>
UI-View and APRSplus:<br>
<br>
<br>
<br>
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