Replacing and updating the ZNA234.

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Replacing and updating the ZNA234.

Postby Steve Anderson » Thu Mar 21, 2024 6:48 pm

Not really NBTV or SSTV but standard 625 and maybe 525. The ZNA234 and ZNA134 have been with us many years, and very useful it/they are as sources of stable and simple test signals for the 625/50 and 525/60 systems. Although generally still available like all older chip designs they're destined for obsolecence, if not there already. The same could be said for the IC601.

But, is there a replacemant in the offing or an alternative without an impending demise also? Somehow I doubt it. Or there is but probably not in hobbyist packaging (DIL/DIP). I'm going to attempt a similar clone using a current device.

Before breaking out in yawns, it's an ideal manner to use a simple 20-pin micro to generate maybe 16 different patterns, more possibly. The intention is to make it monochrome, but an RGB or YUV version may come later in a 28-pin package. As mentioned the 525 or dual 525/625 version is a maybe. I'll start with the 625-only version.

An advantage is patterns could be altered, updated and added to over time...memory allowing.

Your thoughts? Or stick with the ZNA234 for now?

Steve A.
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Replacing and updating the ZNA234.

Postby acl » Sat Mar 23, 2024 6:15 pm

Go for it Steve.
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Re: Replacing and updating the ZNA234.

Postby Steve Anderson » Mon Mar 25, 2024 1:45 pm

acl wrote:Go for it Steve.

...that I will Chris!

One of the interesting aspects of the ZNA234 is how some timings are generated from a 2.5MHz clock. The nominal sync pulse width is 4.77us (see datasheet), not really a multiple of the clock period or half-period. Likewise a few other pulse outputs. But in domestic applications it's probably 'near enough'.

Likewise in the version I plan to produce, all timings will have to be a multiple of 100ns which should mean most outputs will (in theory) be closer to spec., with the exception of the narrow pulses used in the Broad pulses and Equalising pulses. Being in the vertical period pulse-train it's unlikely to make any significant difference.

There's two micros I'm considering for this, both in 20-pin DIL/DIP packaging and using 10MHz crystals, the PIC18F14K50 or the PIC18F14K22. They are very similar when applied to this application, the '14K50 has a USB 'Engine' which isn't needed here, the '14K22 doesn't. More to come...

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Re: Replacing and updating the ZNA234.

Postby Steve Anderson » Fri Mar 29, 2024 4:06 pm

Of the two micros mentioned above I prefer the 'PIC18F14K22 but I have plenty of the alternative mentioned in stock. So I'll do the development on the PIC18F14K50 first then port it across when I get some '14K22s which I'll have to import from the UK. Both are available in 20-pin DIL/DIP packaging.

I'm in the process of porting the code from the micro I used in my SSTV-625 up-convertor, the 625 SPG, to the '14K50. I'm sure glad I comment my code, to reverse-engineer it otherwise would be quite some task! I'll also be adding a 'black clamp pulse' output in a similar timing position, duration and blanking as the usual PAL colour burst, though I have no intention of using it for PAL.

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Re: Replacing and updating the ZNA234.

Postby Steve Anderson » Tue Apr 02, 2024 2:24 pm

Keeping this all in a 20-pin package has meant initially using the UART in the micro allowing control from a PC. That's a limitation to me. Having front-panel controls makes it a stand-alone and more useful device, possibly battery powered depending on final power consumption. Surely less than the ZNA234's 135mA just on its own.

Ditching the UART/PC control frees up a total of 4 or 5 pins for control allowing for the choice of up to 16 or 32 patterns potentially, memory allowing. 16 or 32 depends on the micro chosen, the '14K22 being better in this regard. 32 patterns is probably excessive, and I can't think of 32 anyway!

Test Card C or F is beyond the memory capacity of any sane micro without external EPROM/FLASH storage. I'm trying to keep this to two chips plus regulator. A fully-featured version would have to be a step-up in complexity, and of doubtful use in generally what we do here.

Steve A.

Whilst I was just cycling down the road to my local 'corner shop' I wondered if it would be possible to do a 625 SPG in an 8-pin package? Obviously with very limited outputs, say MS (mixed syncs) plus a 3-bit greyscale/stair-step. Or even a 14-pin package. Something to think on maybe....or maybe not!
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Re: Replacing and updating the ZNA234.

Postby Steve Anderson » Thu Apr 04, 2024 3:02 pm

Having thought a little further, an 8-pin micro with a grey-scale or stair-step output isn't quite feasable, lines (both horizontal, vertical and both) OK, plus dots and maybe others. These are signals with just black/white levels. But the stair-step and the like require a ninth pin. So aside from the really simple patterns, it's a non-starter.

A 14-pin micro gets around the pin limitation easily...e.g. a PIC16F688 or a PIC16F684. However, there will be some small compromises in pulse-width due to the low maximum 20MHz clock speed. Though unlikely to be of any consequence in NBTV, SSTV and related applications. This is bourne out in the use of PIC16s in Karen Orton's projects with a 625 output.

The ZNA234 isn't exactly perfect either.

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