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!
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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 612 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
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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
SSTV FIR Demod 1 v1-Model.gif (20.42 KiB) Viewed 559 times
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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
PWM Filter 01.gif (23.88 KiB) Viewed 514 times


PWM Filter 02.gif


Steve A.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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 489 times




Steve A.
Attachments
Four-Pole Bssal 1-graph WFM1.gif
Four-Pole Bssal 1-graph WFM1.gif (10.14 KiB) Viewed 489 times
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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

Postby Steve Anderson » Sat Apr 17, 2021 1:50 pm

...and for my next trick...how to rationalise that variable quantity of samples within a line to a constant number per line?

As the sampling is being done at half-wave sub-carrier timing that means between 3000-4600 samples/sec, or 165-253 samples within an active video line of 55ms (EU standard), not including syncs...and not including sidebands which could extend those numbers in both directions. The idea is to end up with (say) 128 samples within the active video period per line.

Time to sit outside with pen, paper, a calculator and a few beers for inspiration...(any excuse)...

Steve A.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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

Postby Steve Anderson » Sun Apr 18, 2021 2:51 pm

...well, the beers didn't help yesterday, but I may have had an idea this morning...

We know the intensity and the duration of each incoming half-waveform of the SSTV sub-carrier (via the demodulator). So why not have variable-duration and a variable quantity of pixels per line? The signal to the VGA display is analogue so the display won't know or care.

So a black line (55ms active video) would be chopped up into 165 pixels of 333us, a white line would be comprised of 253 pixels of 217us. Worst case memory requirement would be an all-white screen of 32,384 bytes (128 lines), there's plenty of RAM left in the devices used.

It also means no interpolation is required which effectively is a low-pass filter - the last thing we need!

I've yet to check if the read processor is up to the task, mainly in terms of speed. This also may allow the demodulator to be within the write micro instead of separate. I'm still cogitating on all of this and it may not be possible, I'm not sure either way yet.

If anyone can see 'holes' in the concept I'd like to hear from you...I don't mind being shot down in flames...it wouldn't be the first time...

Steve A.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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

Postby Harry Dalek » Sun Apr 18, 2021 3:17 pm

I can see this one takes a lot of thought Steve i am watching but can't help at all its all above my head ; (
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
User avatar
Harry Dalek
"Fester! Don't do that to 'Thing'"
 
Posts: 4607
Joined: Fri Sep 26, 2008 4:58 pm
Location: Australia

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

Postby Steve Anderson » Sun Apr 18, 2021 3:22 pm

Harry Dalek wrote:I can see this one takes a lot of thought...

To quote a friend of mine, "It's doin' me 'ead in"...translation..."It's giving me a bit of a headache"...

Steve A.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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

Postby Steve Anderson » Tue Apr 20, 2021 4:02 pm

One thing that occurred to me a long time ago regarding 'that' overshoot at the start of the line in the MkI version, was part or all of it down to the filter in the modulator? It is a four-pole Bessel arrangement which should have virtually none, but I don't recall checking or even simulating it...though I may have done. So to as a start, a simulation....

...nothing to worry about it seems...it was designed to have a gain of four...

The slight overshoot is probably down to using practical component values rather than 'exact'...not forgetting tolerances, 1% resistors, 5% capacitors in a real-world situation. Simulators assume component values are exact unless you do a "Monti-Carlo" simulation which simulates variations with component tolerances.

Steve A.
Attachments
Four-Pole Bssal 1-graph.gif
Four-Pole Bssal 1-graph.gif (10.09 KiB) Viewed 265 times
Four-Pole Bssal 2-graph.gif
Four-Pole Bssal 2-graph.gif (10.01 KiB) Viewed 265 times
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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

Postby Steve Anderson » Tue Apr 20, 2021 4:44 pm

...for example, here's a Monti-Carlo simulation of the above with all resistors at 1% and all capacitors at 5%...run 50 times...i.e. 50 throws of the dice...the red trace is the input (perfect), the green trace the output variation as a result of those random resistor and capacitor tolerances...you can add in temperature and gain variations, particularly relevant with discrete (transistor/diode) circuits, supply voltage variations and others...

Four-Pole Bssal 3-graph.gif
Four-Pole Bssal 3-graph.gif (10.09 KiB) Viewed 263 times


Steve A.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

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

Postby Steve Anderson » Tue Apr 20, 2021 5:28 pm

Here's the difference with 5% resistors, caps still 5%, 50 runs...forget the time info...the outputs are all the same +/-

I also tried it with 5,000 iterations/runs (throws of the dice), no practical difference...it took 15 minutes!

Steve A.

Four-Pole Bssal 4-graph.gif
Four-Pole Bssal 4-graph.gif (10.25 KiB) Viewed 263 times
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4463
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

PreviousNext

Return to Cop McDonald's FM SSTV System

Who is online

Users browsing this forum: No registered users and 2 guests