1.
Introduction
The Hermes2 SSTV Mode is a work-in-progress, at least regarding the
transmission of SSTV images. Regarding reception, the SSTV mode
supports reception of Martin, Scottie, Wrasse, Pasokon and PD
Modes. I did not attempt to support Robot modes because they are
too complicated and I don't see any reason for using those. In
fact, there is little reason to have so many different modes in any
case. Martin and Scottie modes really only differ in the image
transmission protocols an only the Scottie DX mode is significantly
different from Martin modes. I included support for Wrasse SC180
mode because it is comparatively quite straight forward. Pasokon
modes are also not too complicated to support and PD modes are a
true effort to produce and provide open standards for SSTV image
transmission.
Hermes2 SSTV Mode can identify the transmitting station's SSTV mode either by decoding the VIS sequence or by identifying the characteristics of the line sync pulses. During image reception, hermes2 SSTV tries to align the image with the line sync pulses but this is not always possible. However, when the image reception is completed, hermes2 SSTV aligns the complete image by using the averaged parameters of the sync pulses received over the complete image transmission.
2. Operation
Hermes2 SSTV has three separate windows:
The SSTV Mode window has all the control and status display widgets
and it is here that the user controls SSTV operation. In the top
left of this window there is an "oscilloscope" type display and a
"waterfall" type display of the incoming signal's spectrum. Both
are derived from the output of FFT's on the incoming signal
samples.
In the top right there are text entry widgets for the user to enter details of the SSTV QSO, to be saved in an ADIF format and also plain text format file. These files are in the hermes2's working directory in the user's home directory (~/.hermes2/sstv/). Clicking the "New" button results in the entering of the operating frequency and an estimate of the signal report into the QSO record. The user has to enter the Classing of the station, the incoming signal report, the operator's name, the QTH name and the Locator. Clicking on "Save" results in the saving of this information to the ADIF and text format log files. The "Clear" button erases all data in the text entry widgets. The remote operator's classing may be entered by the receiver functions if the FSK signal at the end of image transmission is successfully decoded.
In the bottom half of the SSTV Mode window, the
"Transmit/Receive Control" frame contains various widgets in three
columns:
The left column contains widgets that will be used to control SSTV
image transmission, when this functionality is ready.
The central column contains widgets for the control of SSTV
image reception:
The top button toggles the Receive function ON or OFF.
The "Listen" button stops an ongoing image reception and resets the
receiving functions.
The "Auto" check button to the left (normally active) will enable
the automatic termination of the receiving functions when the
number of image lines received matches the image height appropriate
to the current image format.
The "Mode Auto" button opens a menu to select the current image
format manually, if required. The selected format will be shown in
the text label below it and the button will show "Mode Manual". In
the Auto setting the image format is determined by the receiving
functions either by successfully decoding the VIS signal or
identifying the Line Sync frequency.
Finally, clicking the "Save" button will save any received image to
the ~/hermes2/sstv/images/ directory and if the "Auto" check button
is active, any received images will be saved automatically on
completion.
The right column contains widgets mainly to indicate the status
of the SSTV receiver. The "VIS Leader" "LED" turns green when the
leading tone of the VIS sequence is detected. Unfortunately this
tone is easily mis-detected because it is just a short, unencoded
tone and the detector can be triggered by the tones in the image
transmission etc. However, mis-detection of VIS leader does not
normally cause problems.
The "VIS Mode" indicator turns green when the SSTV Mode (image
format) encoded tone sequence is detected. This sets up the SSTV
receiver for the detected image format if the Mode menu is set for
Auto.
The "Sync Detect" indicator turns green when line sync pulses are
detected. This alone will not start decoding of the image, until
the line sync pulse rate has been determined.
The "Syn Detect" indicator turns green when the line sync pulse
rate has been reliably detected. If the VIS sequence does not
detect the SSTV image format, then this is determined by the Sync
detector and reception is enabled.
Finally the "Sens" slider controls the sensitivity of the VIS
detector as well as the Line Sync detector. It is normally set to a
default of 5 (50%).
And at the bottom of the "Hermes2 SSTV Mode" window there is a
single-line text label widget displaying some information on the
status of the SSTV functions.
The left column will be dedicated to the Transmitter control and to this version, image transmission is still a work-in-progress.
The "SSTV Rx Image" window displays the SSTV image received. This is displayed as it is decoded line-by-line and when it is complete, it is re-decoded to correct the skewing effect of inaccurate line sync detection that occurs with weak and noisy signals. Image decoding ends when the number of lines decoded equals the image height of the current image format in use. Clicking the "Listen" button will terminate image reception and will put the receiver in the "listening for VIS or line sync" mode.
The "SSTV Tx Image" window is dedicated to the Transmitter control and to this version, image transmission is still a work-in-progress.
Hermes2 SSTV has been stable through a few weeks of on the air
tests, decoding various SSTV image modes transmitted on the popular
20m frequency of 14230kHz and the two adjacent frequencies above
and below. The VIS detector is only successful with reasonably
clear signals and occasionally the line sync position is
incorrectly placed near the middle of the image.
The line sync detector is fairly sensitive but some image skew
takes place usually at the beginning of image decoding. But this is
normally corrected at the end of image reception by re-decoding the
image.
4. Copying This software package is released under the GNU Public License. Please see the COPYING file for more details.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details: