dawsonjon
Loss of synchronization in SC2-120 mode
When receiving in SC2-120 mode and decoding has reached about half the screen, then synchronization is lost. Sync is restored after receiving a few lines, but with obvious image distortion. See photo attached.
Is it possible to fix this?
Greetings by IU4ALH
Maybe, It's an interesting effect, are you able to give me any more details, for example is this a recording, or something received live off air? Does this happen consistently with all images in this mode or just this one? What was the cause of the loss of sync?
The software times everything from the first confirmed sync, so in general if sync is lost for a short period e.g. due to fading the decoder will pick up where it left off without any distortion. This usually results in a line of static, but no distortion.
I need a bit more information to fully understand what is going on here, but it is the sort of effect you are see when a discontinuity occurs and the sync pulse is no-longer where it was expected to be. For example if you aborted an image part way through, then quickly sent a second image before the decoder timed out, there is no timing relationship between the two images so the second image would appear distorted.
This might be one occasion where the automatic slant correction does more harm than good, if the receiver and transmitter are well calibrated you might get better results with the automatic slant correction turned off.
In order to avoid any "fading" problems and to perform safe decoder tests, I opened the MMSSTV program on the PC that I use for my SSTV radio activity (I am IU4ALH Antonino) and I transmitted images by connecting the audio output of the PC with the SSTV decoder input. I tried all the modes provided in the decoder and they work very well, except the SC2-120 mode which, starts very well in decoding, but about halfway through the image what I photographed happens. The test images were always the same, both with all the modes, and with the SC2-120, the transmission is continuous and there is no interruption of the audio signal and this happens with all the images transmitted with the SC2-120 mode, not only with the one in the photo. There was no discontinuity in the image transmission or transmission of overlapping or interrupted and resumed images, so I could not tell you what could be the cause of the loss of synchronism.
It is true that the SC2-120 is a mode not used much among us radio amateurs, and that it would be a good thing to also add the ROBOT 24/36/72 mode. '73 by IU4ALH
Thanks, I will take a look and see if I can reproduce. As you say, sc2-120 doesn't seem to be widely used and hasn't been tested as thoroughly. Robot modes should also be possible, but would require a slight change of approach. Cheers Jon