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The development of the PD modes

During 1996, I started experimenting with SSTV modes capable of higher resolution than the popular Martin and Scottie modes. My initial plan was to use 640 pixels by 512 lines and send RGB information in a similar way to the Martin modes. However, to reduce the frame period to a reasonable time, I opted to send the green information at full resolution but to take an average of two lines of red and blue information. This worked quite well, giving a three minute transmission time, but sharp horizontal transitions showed some colour fringing. When I demonstrated these modes to Don Rotier, K0HEO (the author of WinPix Pro - now a silent key) he suggested using R-Y and B-Y colour encoding, similar to that used in the Robot 36 and 72 second modes. I was reluctant at first, due to the extra complexity of the encoding/decoding process, but when I gave in and tried it I was impressed by the quality and lack of colour fringing. Thus, the "PD" modes were born; PD standing for Paul and Don and also for "Public Domain", as we intended to publish full specifications for our new SSTV modes in the hope that other SSTV software developers would include them in their programs. 

Here is a packet radio bulletin I sent out in 1997 giving details of the PD modes:

The PD SSTV modes - part 1 of 2

Hello and happy new year! Don Rotier, K0HEO and I have recently made a number of changes to the specifications of the PD range of SSTV modes. The main changes are as follows:-

A full specification for all five of the PD modes can be found in part two of this bulletin. Versions of WinPix Pro and Pscan (for the Acorn Risc PC) which incorporate the new specifications are due in January 1997.

Hopefully the PD modes will now be implemented by authors of other SSTV software. Auto-tuning completely eliminates the need for precise tuning, and the addition of a 2 minute mode should also help to make the modes more usable on the HF bands.

Whether the PD modes really take off probably depends a great deal on the users of the various SSTV software packages. Would you like to send and receive high quality 640 x 480 images in just 2 or 3 minutes? If so, why not ask the author of your software to add the PD modes. Most authors are only too pleased to receive feedback from users of their programs, and if enough ask for a new feature to be added, they will often oblige. You could perhaps enclose a copy of this bulletin (particularly part two) with your request.

The PD SSTV modes - part 2 of 2

The PD SSTV modes were developed by G4IJE and K0HEO to provide a range of high resolution SSTV modes with reasonable transmission times. A 640 x 480 picture can be transmitted in 3 minutes with better quality than a 4.5 minute colour FAX transmission. The original PD mode timings have recently been revised and some new modes added.

All the PD modes use Y, R-Y and B-Y encoding, similar to that used in the original Robot 1200c colour modes. The chrominance components (R-Y and B-Y) use reduced vertical resolution to minimise transmission time. Each line starts with a long sync pulse (more on this later) which is then followed by Y0, R-Y, B-Y and Y1 components. Each component is of identical duration. Assuming that line numbering starts from 0, Y0 is the luminance information from line 0, the R-Y and B-Y are the AVERAGE of the R-Y and B-Y from lines 0 and 1, and Y1 is the luminance from line 1. Thus each "line" of SSTV sent is actually encoded from TWO picture lines. The receive software uses Y0, R-Y and B-Y to decode the first picture line and Y1, R-Y and B-Y to decode the second line.

The PD modes are intended for synchronous (ie "free running") reception. The line sync pulses serve to mark the start of each line "pair" to allow easy starting if the VIS is missed.

A range of modes/resolutions are available:-

Mode Pixel Line   Sync Porch Frame(secs) Resolution VIS VIS+P
PD 90 532 703040  20000 2080 89.989120   320 x 256   99  99
PD120 190 508480  20000 2080 126.103040  640 x 496   95  95
PD160 382 804416  20000 2080 160.883200  512 x 400   98  226
PD180 286 754240  20000 2080 187.051520  640 x 496   96  96
PD240 382 1000000 20000 2080 248.000000  640 x 496   97  225

All figures are in microseconds except the frame times. The VIS codes are in decimal and the VIS+P codes includes the even parity bit. The line times are for a complete "double" line, comprising sync + porch + Y0 + R-Y + B-Y + Y1. Please note that there are no pulses or porches between the four components.

Here is a graph (showing time against frequency) of one line of PD mode:-

The PD120, 180 and 240 modes allow the user to trade-off image quality against transmission time. PD180 provides a very good quality image in 3 minutes, but if this is considered a bit too long for HF use, the PD120 mode can be used instead. This takes just over 2 minutes but image quality is slightly reduced.

The long line sync pulses make it easy to implement an "auto-tune" feature, which is highly desirable to prevent tuning errors (on SSB) causing colour changes. By sampling the sync pulse and averaging the result over several lines, a correction factor can be easily applied to the decoding calculations. This technique is used by WinPix Pro and Pscan and works very well indeed.

73 de Paul G4IJE.

That was written a long time ago and the WinPix Pro and Pscan software packages mentioned are no longer available. However, the PD modes are still very much alive, thanks in part to the ISS! MMSSTV includes the modes but not all users realise they must use the RX viewer or the zoom tool to get the benefit of the extra resolution. If your PD180 mode images are not displayed at 640 x 496 then you are doing something wrong!


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