Audio Level for Digital SSTV


The HDSSTV DOS programs by Barry Sanderson KB9VAK require linear audio reproduction from transmitter to receiver. Impropper audio levels will distort the waveshape and cause the digital SSTV decode to fail. The waterfall display in DIGTRX can be used to check and adjust the soundcard audio levels for the proper setting. For this test, the AGC in the DIGTRX software must be turned off.



Two Tone TX Tuning Tone


Have a station send the Two Tone TX Tuning Tone over the air and watch the waterfall display in DIGTRX for the line produced. You must have the AGC button in the DIGTRX software off. It would be helpful if the sending station has their call ID turned on. The ID should be blue to grey with little or no yellow visable. The first (bottom) part of the Two Tones should be full with solid yellow lines. The next level of the Two Tones may be yellow but not as full or appear as a darker yellow. The lowest level of the Two tone should appear as thin grey lines. See the image below.



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The "X" in the waterfall in the image above was created by a special Two Tone wave file that may be inserted into your program. The 460 Hz to 2300 Hz Sweep Tones start at each end of the spectrum and cross in the center. Barry Sanderson KB9VAK created this file. To download on some browsers, just hold down the "Shift" key before clicking the link.

2tone-12.wav

Two Tone with "X" as a "Precoded waterfall message" (.PMW)

2tone-x.pmw

Download to your DIGTRX\precoded folder.



The levels necessary to produce the image in the waterfall above are very low. You may find that the sending station must also lower the "Wave out level". The receiving station may need to reduce the "Sound card input level". When the levels are correct it is normal to see a very low watt output from the transmitter. An average reading watt meter may show an output of only 25% to 30 % of full duty cycle. (Output above 90% is typical for analog SSTV.)



Perfect Decode


A perfect decode is possible if the levels are just right and the transmission is noise free. Although the decode process is able to correct errors in transmission due to noise and slight misadjustment of levels, faster decodes and more successful decodes are possible if the levels are just right. Once these levels are set, a perfect decode is possible if the transmission is received nearly noise free. By a perfect decode, I mean a decode where no errors were detected so that no error correction was necessary during the decode process.

Each time DIGTRX successfully decodes a file, it generates a new text file, wzbz.txt where the text contents of this file is displayed in the text window of DIGTRX. If you scroll down to the area of this file that shows the block by block results of the decode process, you will see a double vertical line column. To the left you will see the numbers for "outer code changes". Under good conditions, this column should contain all zeros (0). To the right you will see the numbers for the first level of "inner code changes". If the transmission and reception were perfect, then all the numbers in this column would be 306. 306 is the maximum number of error-free bytes per block possible. See the image below.



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Loopback test


On some systems it is possible to connect the line-out of your soundcard to the line-in of the same soundcard in order to copy your own transmissions. Use a cable with stereo minature phone plugs on each end. This will simulate a transmission that is noise free. Create a short wave file with DIGTRX by selecting a file with a size between 300 bytes and 2KB. This will make the test go faster. Locate the file "woutzbz.wav" (in the DIGTRX working folder), with DIGTRX running and ready to copy, play this wave file using Windows Media Player. Perfect decodes are likely if the levels are right. By a perfect decode, I mean a decode where no errors were detected so that no error correction was necessary during the decode process.



Two computer test


Connect the line-out of the soundcard on the "sending computer" to the line-in of the soundcard on the "receiving computer". Use a cable with stereo minature phone plugs on each end. This will simulate a transmission that is noise free. Create a short wave file with DIGTRX (on the "sending computer") by selecting a file with a size between 300 bytes and 2KB. This will make the test go faster. With DIGTRX running and ready to copy on the "receiving computer", Send the wave file from the "sending computer". Perfect decodes are likely if the levels are right. By a perfect decode, I mean a decode where no errors were detected so that no error correction was necessary during the decode process.



Interface test


An interface that is used for analog SSTV may not be suitable for digital SSTV transmissions. My old SSTV interface included capacitors that coupled and filtered the signal coming from the computer. Components such as this can distort the waveform and cause trouble for decoding.

Connect the output of your interface on the "sending computer" to the line-in of the soundcard on the "receiving computer". Use a cable with a stereo minature phone plug on the end that connects to the soundcard. The output signal should connect to the plug tip (left channel). This will simulate a transmission that is going through your interface. Create a short wave file with DIGTRX (on the "sending computer") by selecting a file with a size between 300 bytes and 2KB. This will make the test go faster. With DIGTRX running and ready to copy on the "receiving computer, send the wave file from the "sending computer". Perfect decodes are likely if the levels are right. By a perfect decode, I mean a decode where no errors were detected so that no error correction was necessary during the decode process.


Dave Jones KB4YZ