Retro SSTV anyone?

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.

Retro SSTV anyone?

Postby Steve Anderson » Tue Dec 16, 2014 9:46 pm

A thread not really NBTV though some of the differences between the formats may get a few ideas brewing.

This is Slow Scan TV as it was originally conceived by Copthorne McDonald in the early 1950s (no relation to Ronald AFAIK).

A simple analogue system which in use is more like fax rather than TV as we know it. A series of stills where each takes around eight seconds to transmit/receive.

There's tons of stuff on the 'net r.e. SSTV, but almost exclusively devoted to the more modern modes in colour and that use VIS codes. This is monochrome and the waveform is simply a slowed-down version of NBTV or some of the early FSTV modes that used a simple broad pulse for frame sync.

In colour and with VIS codes (a type of header for each frame) you may as well send a jpeg IMHO.

There's a few earlier postings with regard to this in Harry's thread, "Slow Scan Television Experiment", I split it out to separate the two different yet compatible approaches.

************************************************

Over the past couple of months I've built two different SSTV modulators which appear to work well. One uses a micro and thus requires code, the other is hardware based and requires no code.

With a few recordings made with the above it's time to move onto a display device, again in the manner of the 50s and 60s.

Armed with a long persistence CRT and a stripped and cleaned up chassis a start has been made. The front panel (below) is from a previous NBTV monitor and will require replacing for this incarnation. More to come...

Steve A.
Attachments
DP7-5 3.jpg
DP7-5 3.jpg (362.58 KiB) Viewed 11960 times
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4397
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

Re: Retro SSTV anyone?

Postby Harry Dalek » Wed Dec 17, 2014 3:48 pm

Well thats looking fine Steve ,it will be a pleasure to see that little beast come to life .

I will be watching for sure with interest , is it going to be an all valve monitor ?
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: 4565
Joined: Fri Sep 26, 2008 4:58 pm
Location: Australia

Re: Retro SSTV anyone?

Postby Steve Anderson » Wed Dec 17, 2014 8:38 pm

Harry Dalek wrote:...is it going to be an all valve monitor?

Sadly no. I do have a set of tube/valve deflection amps already built but they only have a gain of around 10-11 to the CRT deflection plates. This little CRT being as old as it is needs a lot of drive, the tube amplifiers can deliver it but they require a ramp input of some 25V for full screen deflection, mainly in the horizontal direction. So taking the easy cop-out it'll be sand-state devices apart from the CRT itself.

It is an interim step anyway, and with such a small chassis an all-tube version is not possible. But a 5" version would provide the needed real-estate, now whether I go that route in the future or go the up-conversion route to 625 is yet to be decided, it depends on how this pans out.

One of the problems with using such an early CRT is the focus current. In later CRTs it is almost zero, in something from the early 60s like a DG7-32 it's only 30uA. But this little CRT has a variable focus current of up to 500uA dependent on the intensity. So with the usual resistive focus divider you either have good focus in the highlights and poor focus in the lowlights, or vice-versa. I think I've solved the problem but until I try it out I'm not sure. The simplest answer is brute force and have several milliamps through the resistive divider for focus, but that's a waste of power with costly power resistors and pots. But there is a way...though yet to be proven, at least here...

Steve A.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4397
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

Re: Retro SSTV anyone?

Postby Harry Dalek » Wed Dec 17, 2014 9:25 pm

Steve are you using an old circuit design or doing your own or changing one .

Also since its such an old CRT i suppose care should be taken on the heater ? do you just ease the voltage up on it or something you don't worry about and just apply full working voltage ,that would be a normal 6.3 ?
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: 4565
Joined: Fri Sep 26, 2008 4:58 pm
Location: Australia

Re: Retro SSTV anyone?

Postby Steve Anderson » Wed Dec 17, 2014 10:08 pm

Harry Dalek wrote:Steve are you using an old circuit design or doing your own...
It is all of my own offering, albeit some borrowed from others, both old and new...
Harry Dalek wrote:...I suppose care should be taken on the heater? Do you just ease the voltage up on it, which would be a normal 6.3V?
The heater voltage is derived from a 12V winding on a transformer with suitable dropper resistors. It's not easy to get 6.3V secondary transformers these days, and the resistance should limit the inrush current upon cold switch-on, though not one manufacturer I know of has made any reference to this extending tube life or otherwise. You can gauge the correct voltage/current quite often from the colour of the cathode. See avatar* to the right for a guide for most CRTs...

One thing I always do, with any valve/tube, especially a CRT, is actually measure the heater current at nominal voltage...working my way slowly there..all the time keeping an eye on the cathode colour. Tubes/valves improved their efficiency over the years with resulting reduction in heater current. If you plan to use dropper resistors as I do here you have to check the current and calculate the dropper resistors appropriately. These CRTs are becoming rare in Europe as a result of an EU directive that all tubes with 'potentially' lethal phosphors be disposed of in 'a safe manner'. Utter bollocks...ditto Lead-free solder.

Steve A.

* Avatar changed so photo inserted below...
Attachments
CRT Glow 1.jpg
CRT Glow 1.jpg (78.15 KiB) Viewed 11896 times
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4397
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

Re: Retro SSTV anyone?

Postby Klaas Robers » Wed Dec 17, 2014 11:57 pm

Steve, it was in 1973 that I constructed an SSTV monitor using a 7BP7 picture tube. It has the P7 phosfor and magnetic focussing and deflection, which I did with a deflection coil of a 70 degree TV picture tube. This coil has glued on ferroxdure permanent magnetic rings for the focussing. It needed about 5 kV EHT.

I still have it here, but I guess that I need quite some time to get the electrolytic capacitors reformated again, especially the tantalium one for the vertical deflection sawtooth generation.

It was built on a wooden chassis, with aluminium foil underneath. All electronics was transistorised and with a few ICs. It started with an IC limiter followed by a counting FM detector (74121 monoflop) running at the double frequency (2400 - 4600 Hz). That is needed, because you need both the upgoing edges and the down going edges of the FM carrier, otherwise you loose video bandwidth. An FM signal is completely defined by its zero crossings. After the detector I had a 7th order butterworth low pass filter with 1 kHz cut off frequency including some group delay correction and that gave me a fine video signal.

I always had the idea to convert it to an SSTV monitor using a 5FP7 that I still have available. This would fit in the size of the Heathkit SB100 line equipment if I paint it dark green. However then I think I should redo the circuitry in vacume-glass technology.
User avatar
Klaas Robers
"Gomez!", "Oh Morticia."
 
Posts: 1540
Joined: Wed Jan 24, 2007 8:42 pm
Location: Valkenswaard, the Netherlands

Re: Retro SSTV anyone?

Postby Steve Anderson » Thu Dec 18, 2014 12:48 am

Good luck with re-forming the caps, not for those of an impatient nature.

An all tube/valve design is a wonderful concept, but these days, especially here, the power transformers are always a blockade to achieving success. In Europe and probably the US there are still plenty of 'Radiograms' and the like being chucked away. Each with at least a tube/valve friendly power transformer, an 'proper' dual-gang tuning capacitor in the radio part, a small but useful audio output transformer (two if stereo-luxury), valve bases, coils, IFT's and so the list goes on...the 'cabinet', especially from the 60s...scrap...shame November 5th has already passed. Synchronous motors for the turntable too plus any tubes/valves that may be in workable condition.

Oh well...

Steve A.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4397
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

Re: Retro SSTV anyone?

Postby Harry Dalek » Thu Dec 18, 2014 4:38 pm

Do you have a picture of your monitor Klaas ? i think that was great you made a monitor way back then and you have kept it ! i always regret not doing the same .
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: 4565
Joined: Fri Sep 26, 2008 4:58 pm
Location: Australia

Re: Retro SSTV anyone?

Postby Klaas Robers » Sat Dec 20, 2014 1:34 am

I should make a photo of it Harry. Hopefully in the coming days.....
User avatar
Klaas Robers
"Gomez!", "Oh Morticia."
 
Posts: 1540
Joined: Wed Jan 24, 2007 8:42 pm
Location: Valkenswaard, the Netherlands

Re: Retro SSTV anyone?

Postby Steve Anderson » Sat Feb 07, 2015 11:33 pm

Continuing with the 3" P7 type SSTV monitor...

Having built two modulators it was time I looked at demodulation. Before attempting any more recent methods it seemed reasonable to establish some form of benchmark from the 'golden period' of SSTV. The Robot 70 and 70A where probably the most common SSTV monitors in use in the 70's/80's so I took that as a reference point.

I have ever-so-slightly brought it up to date by replacing the uA709's with a mixture of TL084 and LM324 devices (saves all those pesky compensation components). And improved the full-wave rectifiers to eliminate the 600mV forward diode drop in the diodes and the I/V non-linearity. The filters apart from the op-amp change, are the same.

In simulating the two filters the 1200Hz filter seemed fine (red trace below), but the 2300Hz filter was quite 'peaky' (green trace). With a few tweaks to some component values I was able to get the characteristic as shown by the blue trace.

Now there may be a very valid reason Robot did this, but until I get a chance to build the thing I have no idea why.

More to come...

Steve A.

P.S. 'Doffing my hat' to the chips of that era I have included a 741...I have a dozen or so of them sitting idle for the last three decades...time to wake at least one of them up! Thank you Bob Widlar.
Attachments
1k0 Robot 70 Filter 1.gif
1k0 Robot 70 Filter 1.gif (12.92 KiB) Viewed 11708 times
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4397
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

Re: Retro SSTV anyone?

Postby Steve Anderson » Tue Feb 17, 2015 9:04 pm

After mucking around with the original Robot 70 SSTV demodulator I have to say that it was very well engineered. They used as few op-amps as possible because they were expensive in the late 60's and 70's when the unit was designed and put into production.

With the benefit of 40+ years of progress in op-amps I have mildly updated the design without altering the aim of the original. First to go were the uA709's, horrid devices and hard to keep stable. Robot themselves replaced them el-pronto when 741's became available. But the original design was well thought out and was workable.

My rebuild of the Robot 70 demodulator largely replaced the early op-amps with modern TL084's, four op-amps in one package with a lot less power wiring to contend with, stable, moderately fast and cheap. Though I have kept at least one 741 in the chain.

Given the fact that op-amps are so cheap now, and we throw them around like confetti, I updated the full-wave rectifiers such that the diodes are included in the feedback loop which eliminates the 0.6V threshold voltage and the non-linear I/V curve. It also results in a lot less passive components, including those diodes.

Apart from the full-wave rectifiers and the general update on semi's the filters are exactly the same as Robot produced in the 70's, in general it is just an update on the design that Robot may have done themselves.

With the 'improved' rectifiers I have little to fault with the original design, what nit-picking I could do is simply down the cost constraints of op-anps at that time, the old cost/performance/sales ratios.

As a (by today's standards) quite simple FM demodulator it performs admirably well, and a few example waveforms are below. There is residual ripple in the syncs and to a lesser extent at black, This could be cleaned up to a degree by a better output filter which would also increase the video bandwidth from some 750Hz to 950Hz resulting in slightly better video resolution. (Op-amp costs remember).

There is a hint of the residual 'S-curve' of the two tuned circuits, but it's so small it may as well be ignored. They spent some time on this, and as it works so well I'm going to leave it as it is.

The 1% resistors can be from a batch of new 5% parts selected with a digital multi-meter within a few percent...actual value not that important. The LM324 in the photo (those that noticed) has been replaced by a TL084, the LM324 simply wasn't up to the task.

Steve A.

P.S. You have to download the circuit diagram as it's too wide for most screen resolutions, click link below...
Attachments
Robot 70 Demod 1.jpg
Robot 70 Demod 1.jpg (78.9 KiB) Viewed 11655 times
Robot 70 Demod WFM 4.gif
Robot 70 Demod WFM 4.gif (44.43 KiB) Viewed 11655 times
Robot 70 Demod WFM 5.gif
Robot 70 Demod WFM 5.gif (44.09 KiB) Viewed 11655 times
Robot 70 Linearity 1.gif
Robot 70 Linearity 1.gif (5.67 KiB) Viewed 11655 times
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4397
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

Re: Retro SSTV anyone?

Postby Steve Anderson » Wed Feb 18, 2015 9:13 pm

For the heck of it - and to check if the simulator was correct, I traced out the response of the demodulator beyond the normal 1200-2300Hz range. Good news is the simulator was spot-on.

Steve A.
Attachments
Robot 70 Linearity 2.gif
Robot 70 Linearity 2.gif (4.88 KiB) Viewed 11639 times
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4397
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

Re: Retro SSTV anyone?

Postby Klaas Robers » Thu Feb 19, 2015 7:55 pm

Steve, how would the green and the blue curve look if you plotted them on a frequency linear scale? An FM-detector should give an output voltage that is linear on a linear frequency scale. I got the impression that the logarithmic scale distorted the form of the 2400 Hz resonance. The blue one could have given a worse demodulation characteristic.....

The guys at Robot were keen people to my opinion. The very basic FM-detector features automatic white and black clipping, which on a noisy HF-channel was very welcome. I realise that my counting FM-detector (sawtooth characteristic) did this too, but much sharper. I should have used a retriggerable monoflop (74123) instead of the non retriggerable 74121. That would have given me automatic horizontal white clipping.
User avatar
Klaas Robers
"Gomez!", "Oh Morticia."
 
Posts: 1540
Joined: Wed Jan 24, 2007 8:42 pm
Location: Valkenswaard, the Netherlands

Re: Retro SSTV anyone?

Postby Steve Anderson » Thu Feb 19, 2015 8:39 pm

Klaas, I assume you're referring to the chart posted on the 7th February, a few posts above. I just tried a linear scale and it looked far worse and somewhat lop-sided - no surprise really. The two Excel sweeps above are on a linear scale and show just how good it is, this also shown in the resultant staircase output waveforms above. These are measurements from a real built version, not a simulation. However the demodulated video bandwidth could be better, something to work on downstream.

The Robot filter arrangements were good - even if a little 'odd'. What wasn't so good were those original full-wave rectifiers, but that was a cost constraint at the time.

Steve A.
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4397
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

Re: Retro SSTV anyone?

Postby Steve Anderson » Thu Feb 19, 2015 10:41 pm

Further to the above...

A test conducted with NO filter prior to the modulator was conducted...direct from the waveform generator into the DDS modulator (magenta trace below in #6). I was expecting all sorts of artifacts but they didn't surface.

The two other traces are (in yellow) the original Robot 70 post rectification filter waveform, and (in cyan) a slightly improved design of my own making. Now apart from the very slight gain variations there's little to choose between them.

Overlaying all three in #7 results in little difference and the modulator-demodulator linearity is fairly good in both cases. The bandwidth is a bit better with the four-pole dual-stage filter, but not overly so...rise/fall times show a slight improvement.

There is of course the time delay from source to demod output, mostly due to the filters but partly the A-D process in the modulator.

Steve A.
Attachments
Robot 70 Demod WFM 6.gif
Robot 70 Demod WFM 6.gif (51.88 KiB) Viewed 11606 times
Robot 70 Demod WFM 7.gif
Robot 70 Demod WFM 7.gif (44.51 KiB) Viewed 11606 times
User avatar
Steve Anderson
"Fester! Don't do that to 'Thing'"
 
Posts: 4397
Joined: Fri Mar 30, 2007 10:54 pm
Location: Bangkok, Thailand

Next

Return to Cop McDonald's FM SSTV System

Who is online

Users browsing this forum: No registered users and 1 guest