The PD120 mode

18/12/2015

It was interesting to hear that more ISS SSTV activity is planned, this time using PD120 mode. When I invented the PD120 mode in 1997 it was an attempt to provide a high resolution image for HF work with a faster frame time than the three-minute PD180 mode. It works quite well but as expected, image sharpness suffers a little compared to PD180 mode. You really need at least half a cycle of audio to detemine the grey level of a pixel and the PD120 mode uses a little less than that.

Full technical details of the PD modes can be found here: www.classicsstv.com/pdmodes.php

As the PD120 mode is used very little these days, I thought I would try a comparison of PD120 and PD180 modes. I used my WebcamTX (SSTV) program to transmit to MMSSTV using an audio link. Here are the results; please excude the poor quality of the model!

PD120 mode:

PD180 mode:

As you can see, the PD120 image is slightly softer looking than the PD180 version, but still pretty good. By the way, the PD120 mode produces images that are 640 pixels by 496 lines so you must use either the picture viewer tool (the magnifying glass icon) or choose the "Always show RX viewer" option in MMSSTV to see them at full resolution. Just looking at the RX window does not give you the benefit of the extra resolution.

73, Paul G4IJE.

 

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