SSTV-625 Up Converter MkV (Five)...

Where it all started as far as most are concerned and saw heavy use from the 60s through to the 80s. Colour and Hi-res modes have unfortunately pushed this system into the backwaters of SSTV. Time to resurrect interest in this simple analogue system.

Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Tue Mar 30, 2021 8:19 pm

All noted, there's still a lot to do, I need to get back to Bangkok first though...

Steve A.

Bangkok, probably tomorrow morning, Wednesday...

Wednesday...going to postpone my return, some heavy thunderstorms en-route today, I'll avoid them as I'm on two wheels this time...thunderstorms in the tropics can be 'spectacular'...if that's the correct word!
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Fri Apr 02, 2021 2:35 pm

Back home (and dry)...working on the SSTV demodulator, designing the output filter. Currently this initial FIR filter only uses 19 'taps', and produces this step response. Flat (+/-0.25db) to 1kHz, -43db from 2kHz upwards, sampling rate 10kHz, approx. 8% overshoot.

A 'proper' windowed sinc filter arguably slightly better, though somewhat more complex...

X-axis, 100us/sample, Y-axis, arbitrary value.
FIR v1.gif
FIR v1.gif (5.99 KiB) Viewed 202 times


FIR v1 Plot 1.gif


The code for the filter will be within the demod micro [1], its output being a byte-stream to feed the write micro. Possibly an arrangement to feed a D-A to generate a baseband SSTV signal...maybe/optional....though I can't think of a use for it perhaps with the exception of monitoring the signal on a 'scope...which is no bad idea I suppose...and/or some form of 'tuning indicator' for those on SSB.

[1] I hope to be able move the demod code into the write processor in time, thereby eliminating the demod micro...but - one step at a time...

...plus a stand-alone analogue output demodulator could come in useful sometime in the future...


Steve A.

04/04/21 Getting all those 6,000-odd bytes for the look-up table from the spreadsheet into a suitable format for the assembler is proving a challenge! Not something I wish to do again in a hurry!

A while later...I re-did the spreadsheet, now I have the look-up table done...it took a bit of "kicking and screaming", but it's done! Covers the sub-carrier frequencies from 300Hz to 3067Hz...already 'copied & pasted' into the assembler...there's 16 values per line...and there's a few hundred of them! Four example lines below...

Code: Select all
Data   0x04C2,   0x04C4,   0x04C5,   0x04C6,   0x04C8,   0x04C9,   0x04CB,   0x04CD,   0x04CE,   0x04D0,   0x04D1,   0x04D3,   0x04D4,   0x04D6,   0x04D7,   0x04D9

Data   0x04DA,   0x04DC,   0x04DD,   0x04DF,   0x04E0,   0x04E2,   0x04E4,   0x04E5,   0x04E7,   0x04E8,   0x04EA,   0x04EB,   0x04ED,   0x04EF,   0x04F0,   0x04F2

Data   0x04F3,   0x04F5,   0x04F7,   0x04F8,   0x04FA,   0x04FC,   0x04FD,   0x04FF,   0x0500,   0x0502,   0x0504,   0x0505,   0x0507,   0x0509,   0x050A,   0x050C

Data   0x050E,   0x050F,   0x0511,   0x0513,   0x0514,   0x0516,   0x0518,   0x0519,   0x051B,   0x051D,   0x051F,   0x0520,   0x0522,   0x0524,   0x0526,   0x0527
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Steve Anderson
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Mon Apr 05, 2021 5:39 pm

Now that's done, I've started/completed the hardware design of the stand-alone demodulator. There are three simultaneous demodulated outputs...

1) A PWM output, at a relatively high frequency output. If passed through a suitable analogue low-pass filter would yield an analogue output for monitoring on a scope etc..or a direct-view CRT monitor with a version of a P7 phosphor...

2) A UART output (RS232/485 style), again at quite a fast rate...perhaps for saving as a file...though the original FM signal can be saved (and probably would be) as well..

3) A SPI CLK & SDO pair which is what I'll be using to feed the write processor.

In time I hope to be able to 'shoehorn' the above into the write processor as I've mentioned before...though perhaps without all three outputs...I'm hesitant to use 40-pin (or more) devices...I'm trying to keep it simple, yet versatile...

Steve A.
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Mon Apr 05, 2021 6:50 pm

Here's the initial circuit for the stand-alone demodulator - there's every chance it will change...

There are two +5V supplies, +5VA (Analogue) and +5VL (Logic).

09/04/21 RC4 (Pin 6) has become a test point to view the comparator output on a 'scope. Probably only useful during development...

Steve A.
SSTV FIR Demod 1 v1-Model.gif
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Fri Apr 09, 2021 1:15 pm

Demodulator revised to work from 300 to 3300Hz, previously it stopped at 3067Hz.

Also designed analogue PWM filter, simulations below, 1kHz, six-pole Bessel, PWM frequency 39kHz...possibly could be a four-pole version...square-wave input at 200Hz...the very slight/unnoticeable overshoot is the rounding out of calculated values to practical values.

PWM Filter 01.gif
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PWM Filter 02.gif


Steve A.
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Fri Apr 09, 2021 6:47 pm

The above filter corner frequency could be shifted quite a way upwards, its main purpose is to get rid of the 39kHz PWM waveform. If this were done it would have negligible effect on the desired output, yet still suppress the PWM timebase frequency of 39kHz.

Recall that the output has already been filtered by an FIR filter within the micro.

Steve A.
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Sat Apr 10, 2021 2:52 pm

Based on the same figures as the above, a four-pole (simpler) version still does the job...just one TL082...gain can be adjusted upstream/downstream if needed...still over 100db down at the PWM frequency of 39kHz....

Four-Pole Bssal 1-graph WFM2.gif
Four-Pole Bssal 1-graph WFM2.gif (10.64 KiB) Viewed 79 times




Steve A.
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Four-Pole Bssal 1-graph WFM1.gif
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Re: SSTV-625 Up Converter MkV (Five)...

Postby Steve Anderson » Thu Apr 15, 2021 7:10 pm

Beavering away! Look-up table complete, 3033 values, 6066 bytes. (Thank you Excel!). It'll occupy the top-end of program memory leaving around 10kB free for the program, which should be more than enough.

UART output configured for 115,200 or 230,400 baud...though probably not used...except for monitoring during development....

Next...clear workbench and plug in soldering iron...

Steve A.
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