NBTV/SSTV-625 Up Converter MkIII

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: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Aug 18, 2020 4:07 pm

...and here's Part 2, March 1983...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Aug 18, 2020 4:17 pm

I'm thinking about a down-converter for the future, i.e. 625-SSTV, the hardest component to specify is the 625 A-D which needs to sample at about 3.3MHz (305ns/sample). Now the good old (very old) CA3306 will do the job, A) If you can find them, and, B) if 6-bits is good, enough which I'm informed it is. Although I have a number of CA3306 chips I'd prefer to use something that is still available from the usual outlets, even if we ignore two bits of an 8-bit A-D. But memory and everything else is is in multiples of 8-bits so we may as well use them.

This should follow on when the SSTV-625 up-converter is complete. I'll start it in a different thread though...

I can get 50 or so CA3306's here for around US$3.00 ea. plus post (to you), so that's a temporary solution. The company that has them doesn't do mail-order, so I'd have to go and visit them and pay cash, no credit cards.

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Tue Aug 18, 2020 6:23 pm

One other thing to consider, I've noticed over the past few years that many retailers are either running down their stocks, or discontinuing the LM1881 sync separator. The major sources still carry them, but for how much longer? With the demise of 625/525 they'll vanish in time...stock up with them while you can if you can foresee a need for them in the future...my local supplier has 335 in stock but with no intention of getting any more...I have asked...currently around US$2.80.

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NBTV/SSTV-625 Up Converter MkIII

Postby acl » Tue Aug 18, 2020 8:27 pm

Hi Steve,

Thanks for your time helping me out. I have built the one in Radcom . I will still keep the unit and see if anything turns up in the future. With respect to the CA3306
and synch seperator I think they are still around although sourced from China. See link below.

https://www.ebay.co.uk/sch/i.html?_from ... &_osacat=0

Regards Chris Lewis
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby acl » Tue Aug 18, 2020 8:32 pm

And this one for the synch seperator


https://www.ebay.co.uk/sch/i.html?_from ... 1&_sacat=0

Stay safe


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NBTV/SSTV-625 Up Converter MkIII

Postby acl » Tue Aug 18, 2020 8:51 pm

Just an idea Steve,

As the main stumbling block of the the 625 line section .The late Graham Lewis designed a NBTV to VGA 15 pin computer monitor convertor using PICs, I don't know how easy it is to replicate a HDMI output signal from such hardware or fit a VGA to HDMI convertor on the output.This would negate the need for 625 monitors which in time will become obsolete.


On the upconvertor how about using readily availible USB HD security type cameras as inputs ? and possibly an additional microSD card reader so images could be loaded from a PC.

Regards Chris Lewis
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Aug 19, 2020 1:11 pm

acl wrote:...This would negate the need for 625 monitors which in time will become obsolete.

Regards Chris Lewis

True, however we bought a new flat-panel TV about six months ago and it still has analogue baseband 625/525 inputs in PAL, NTSC and SECAM as well as component (YUV). Also an RF (antenna) input for where there is still analogue RF TV transmissions (here). But as you say, for how much longer?

Even some of the simpler PC mode monitors are quite a challenge, say 640x480 VGA @ 60Hz has timings which are substantially faster than 625/525, (approximately double). I wonder if recent PC monitors will still support such an old standard? I'll have a look at the specs for my one, though it is five years old. It has a 15-pin VGA connector on the back, whether more recent ones do I don't know.

I'll see if I can find that item by Graham, I've seen it somewhere...you're correct, using PC monitors is probably the way to go in the future.

Steve A.

Added later...My PC monitor does accept 640 x 480 VGA @ 60Hz, and I've found Graham's item in Vol 31 No. 3 of the newsletter. It's an overview not a detailed item with circuit diagrams etc.. That could have taken a whole newsletter in itself to publish!
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Aug 19, 2020 6:22 pm

I'll continue on with the SSTV-625 version, upon completion I'll look into SSTV-VGA, also NBTV to both output standards. Because the VGA standards are quite tight the output processor will require a 14.31818MHz crystal, that shouldn't be hard to find in the UK, but here...only SMD.

The output processor will probably need changing to a faster device and the output stage beefed up to provide drive for three (RGB) signals, though the picture would still be monochrome. One of the reasons I prefer YUV over RGB is monochrome requires only one signal, not three. The matrixing between the two is quite simple if working with colour.

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Aug 21, 2020 5:33 pm

One of the things I was concerned about during the development of the original MkI SSTV-625 up-converter was the settling time of the DAC08 which is quoted anywhere between 85 and 150ns depending on whose datasheet you read. As it turned out it didn't seem to be an issue. But here using a VGA display it might be. There are few D-As that are faster - at least those that are hobbyist friendly (DIL packaging and at a sane price).

I have thought about using a discrete R-2R array which might perform better at VGA rates than the DAC08. I guess there's only one way to find out! As the output stage has been changed to a common-base arrangement this may help in both cases...my posting of August 13th 2020.

Another concern may be the RAM speed, I'll look into this next...

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NBTV/SSTV-625 Up Converter MkIII

Postby acl » Fri Aug 21, 2020 6:31 pm

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Hi Steve,

These may help, they are part circuit diagrams for Grahams NBTV to 625 line converter,
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Aug 21, 2020 7:29 pm

acl wrote:These may help, they are part circuit diagrams for Grahams NBTV to 625 line converter,

Thanks Chris. However it is a NBTV-624 up converter, not 625, i.e. non-interlaced....see the output stage of Graham's unit. In the days of analogue, generally CRT monitors/TVs this was fine, but now much of the signal is internally processed I'm not sure if a modern TV/monitor will accept it. Probably yes, but I don't fancy finding out the hard way!

Without the interlacing the display is effectively 312 lines at 50Hz instead of 625 at 25Hz...which would be fine in this application IF the TV/monitor accepts it.

Again, IF the TV/monitor works OK with a VHS source it may be OK...timing from a VHS machine was erratic/wobbly to say the least!..unless the VHS machine had a TBC (Time Base Corrector), the vast majority didn't.

So without further investigation I'm reluctant to go the 624 route, though potentially easier - it may work on my TV/monitor, but yours?

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Sat Aug 22, 2020 4:08 pm

The other thing perhaps worthy of note is the MkI SSTV-625 converter produces a full vertical field signal with broad and equalizing pulses rather than the simple 2.5 line 'solid' field sync of lower cost items, particularly early TV games. I plan to do the same with the MkIII.

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