2004 Hamvention - SSTV Forum Presentation

2004 Hamvention - SSTV Forum Presentation - DIGTRX 1.47

Presentation by Dave Jones - KB4YZ

INTRODUCTION:

My presentation is on using the freeware DIGTRX software version 1.47. I will only cover the basics of operation for pratical use and understansing. Time does not permit me to go into many of the nice features and abilities of this software.
The author is a ham, PY4ZBZ, Roland Zurmely from Brazil. He developed his software using Delphi, a form of Pascal for Windows. DIGTRX requires RDFT programs provided by Barry Sanderson, KB9VAK, to do the encoding and decoding. Roland has stated that DIGTRX is "experimental" and to expect changes in future versions.

INSTALLATION:

You may download the roughly 2 MB executible file from my web site or from Roland's own web site. Installing the shareware software, IrfanView with JP2 plugins are optional, but recommended as it will allow for viewing some image files that have errors. Install IrfanView and plugins first. Note the installation location as you will need to set the path to this location later. Users of Windows 98 and Windows ME, should refer to the link on my web site concerning the WINOLDAP problem and how to resolve it. It runs in a resolution of 640 X 480 or higher. Help is available in three languages.

(SHOW DESKTOP ICON)

START & CLOSE: (Icon on Desktop)

Double click this icon to bring up DIGTRX. DIGTRX is only for Digital SSTV or more exactly, RDFT. That is redundant digital file transfer. It does not support analog SSTV. To close DIGTRX use the "X" button. It is a good idea to close DIGTRX after making many changes to the Setup. This will insure that the changes are stored and won't be lost if something goes wrong.

(SHOW MAIN SCREEN)

SETUP:

Click Setup on the menu bar to set options. DIGTRX has three modes of automatically starting and decoding. Auto RX is normally selected as this will allow full automatic reception. If you wanted to have the program start recording automatically on a start signal but not immediately begin the decode on the stop, then you should select "Record only". I operate with two computers running at the same time. One is dedicated for receiving. The other I use for sending, and on this computer, I normally do not need to have it receiving, so I have it set to "Decode only". This means that I must manually click the "RECORD" button before the program will capture a transmission.

(SHOW OUTLINE)

With the proper hardware connected, selecting "RTS ON while auto-record" will allow an external speaker to be muted while receiving. This makes operating digital SSTV more pleasant and easier operating. Between RDFT transmissions, voice signals will come through normally. I have my mute circuit made so that a tiny bit of audio comes through.

You will want to select "Save RX file" and "Save RX file with time stamp". This will save all successfully received files to the autosave folder. Having the time stamp as part of the filename prevents a file with the same name from overwriting a previously received file.

"Open MapMaker / Bad Block Report" should be selected. This allows files that were received with errors, to be stored in a special folder. When one of these files is repaired, it will be moved to the autosave folder. If your system is going to be left unattended, you may want to select "Open MapMaker" only. This will allow corrupt images to be saved in a special folder, and possibly be corrected if the transmission is repeated. In this mode, the "Bad Block Report" is disabled so there will not be any dialog boxes appearing on the screen concerning "Bad Block Report".

Transmit keying is provided through the serial port using a standard SSTV interface push-to-talk circuit. Select your PTT Com Port as you would for an analog SSTV program. At this time, DIGTRX does not support any form of "Rig Control".

Enter your Callsign.

There is no clock timing to be set.

Select "Save TX file as precoded". This will allow for "Quickload from precoded folder" if you should want to repeat this transmission later without having to encode again.

DIGTRX may be set to start the capture with a start tone that is +/- 150 Hz away from the receive frequency. This is acomplished with a special multi-tone "start signal" and using a cross-correlation method of detection. This system of detection is very sensitive and works well even under noisy conditions. The "abort" and "stop" signals work the same way.

With "RX ON while decode" it allows DIGTRX to capture another transmission while it is still decoding the last transmission. If "RX ON while decode" is not selected, then the decoding must finish before another transmission can be started. One advantage of "RX ON while decode" NOT selected, is that the decoding speed will be slightly improved.

Selecting "Accept bad blocks !" will allow the file to be saved into a special folder and possibly repaired later.

I set my time out timer to 3 minutes. This will cause the capture to abort if a stop tone is not detected within this time. The time out timer is necessary because the start tone will only restart the capture when a capture is NOT in progress. Transmissions rarely run any longer than two minutes.

RX:

Accurate tuning is not necessary but VFO stability is important.

(SHOW MAIN SCREEN)

The best input level varies from system to system. I have good results with a -20 dB showing in the RX bargraph. Adjust the slider for your selected input in Recording Control. You may click the bargraph to access the Recording Control.

DIGTRX supports two forms of creating images that appear in the waterfall. Waterfall images are similar to analog SSTV and are prone to distortion by band conditions. An example here shows the image of a cat.

DIGTRX supports creating messages in the waterfall. An example here shows such a message.

ASCII text files may be sent and received. Here is an example.

Occasionally the waterfall may stop. This does not prevent sending or receiving. To restore the waterfall action, close and restart the program.

If your computer supports hyper-threading, you will want to disable it. A problem exists with the waterfall when running hyper-threading.

(SHOW SCREEN VIEW OF FLOWER)

Images in the RX or TX windows may be viewed in their true size by clicking the image. With the image displayed in the separate window, the file name and resolution of the image is also displayed. Additional options are available such as re-sizing and viewing in full screen.

Do not access the thumbnails while recording. This may cause corruption to the recorded wave file which would result in a failed decode.

A Decode log is available after each transmission received. This log reports shuch things as the number of blocks received, the number of blocks that were bad, the filename, and the frequency offset.

DIGTRX may also be used to record audio off-the-air and play it back.

TX:

(SHOW JP2 Converter)

DIGTRX has a built-in JP2 converter. Help is available fron the menu bar. Any image file type may be converted to JP2. JP2 offers very high compression with little artifacting. Cropping, resizing, adding text, and other tools are available. To use, click on File on the menu bar, Open, and load image. Slide slider to the right to compress more. Reducing the file size and transmit time. When too much distorsion is noticed, slide slider slightly to the left. Clicking on the image will toggle between original and compressed image. When satisfied, click on "Codify for TX". If the transmission time is too long for the quality desired, try one of the resize features to make the image resolution smaller.

I prefer to load my digital camera pictures into Paint Shop Pro. The image resolution starts out at 1600 X 1200. In Paint Shop Pro, I "Resample" the image to 384 X 288. This size makes the images fit the RX and TX windows exactly. Next I paste the image into the DIGTRX JP2 converter, adjust the slider and codify.

(SHOW Precoded folder/THUMBNAILS)

From the menu bar select Thumbnails and Thumbnails of any file folder. If the "Precoded" folder is selected, all the images displayed are available for immediate transmission simply by right clicking on the desired image and left clicking on "Quick load".

Other programs may change the wave output level in Volume Control and fail to return them to the previous setting. DIGTRX has it's own memory setting for the transmit level. This level may be adjusted during any transmission by adjusting the slider.

Volume Control and Wave must be enabled in "Volume Control". All other inputs and devices displayed in "Volume Control" should be muted. Any "Advanced" features of your soundcard should also be disabled.

(SHOW MAIN SCREEN WITH FLOWER)

The Scope/Status window may be selected to view the third order intermodulation distortion of a station sending the Tuning Tones. An IMD of -20 dB is borderline for RDFT. The more negitive, the better. Generally, having the transmitting station lower his audio output will improve the IMD.

The Tuning button puts out two tones that step down through three levels of amplitude. Adjusting your TX Wave out level while transmitting the tuning tone with a properly adjusted system, should allow you to see your output power drop 50% for each change in level.

In the blue area is displayed the information about the file to be transmitted. Filename, file size, and time to transmit. If after starting a transmission, you desire to quit, click the Transmit button a second time. This will send out an abort tone that will reset the stations copying.

(SHOW OUTLINE)

Replay & Repair:

Anyone that successfully receives a picture may replay the file exactly by clicking Replay (and recodify). This type of replay allows someone with bad blocks on the first transmission to be potentially repaired after getting the replay even if the replay has some bad blocks. This type of repair is performed by what is called "Map Maker". "Map Maker" works even if you are not there to click buttons. This works great for net operations where some stations can't hear some of the other stations.

A station running DIGTRX that receives one or more bad blocks on a transmission, may click the BBR button. This creates a special file called a Bad Block Report. When the original sending station running DIGTRX receives this Bad Block Report, he may send the correction blocks as long as the original file is still in his folder of last 10 transmitted pictures.

Anyone that successfully receives a picture may repair others that are running DIGTRX and get one or more bad blocks. See my web page for details. I describe there some examples of repair using the Bad Block Report.

73 and enjoy your digital SSTV.

The above was the Dayton Hamvention SSTV Forum presentation by Dave Jones, KB4YZ on May 15, 2004. Other speakers were Mike Tallent, W6MXV and Yoshi Nishimura, JA6UHL.