NBTV/SSTV-625 Up Converter MkIV

[Steve Anderson]

Here's an update to the processor section of the up-converter, as it's quite a significant update it's now the MkIV. The RAM and output section is currently unchanged.

I just looked back and the last update I did to this part was on the 1/1/2020! Oops..

Steve A.

The final versions of the schematics will be of a better quality than this example.

Footnote, if not interested in the SSTV function the two RAM chips can be omitted (IC201 & IC202). There's enough RAM in the read processor to handle NBTV.

[acl]

Looking good Steve, I wish I had your programming and design skills.

Regards Chris Lewis

[Steve Anderson]

Thanks Chris,

I'm looking for a crystal/oscillator for this project, it's not essential but would tidy things up a bit. Something similar to the photo, 16MHz but 3.3V not 5V. Can't find any from local suppliers here and didn't find any on the major UK supplier sites.

I could use a 5V version with a level translator, or even a couple of resistors, but it means adding another supply of 5V. The first photo is a half-size version, the pins equate to pins 1,4,5 & 8 of an 8-pin DIL package. The full size version mimics a 14-pin DIL at pins 1,7,8 & 14. Either would be fine.

There are plenty in SMD packages but I'm trying to avoid them.

Steve A.

[Steve Anderson]

MAJOR CHANGE...

I'm changing the logic supply to 5V, since I changed the micros to PIC18F26K22s everything can handle 5V, so those 5V crystal/oscillators will be fine! Why I thought the RAM was for only 3.3V is beyond me! I must have had a bad day that day!!

I will update the schematics to reflect this change.

Steve A.

[acl]

Hi Steve,

Looking forward to getting my soldering lance out.


As I understand it only one of the PIC chips requires a crystal reference. As shown it is a passive device. What are the advantages of selecting an active one?

Cheers Chris Lewis

[Steve Anderson]

As I understand it only one of the PIC chips requires a crystal reference. As shown it is a passive device. What are the advantages of selecting an active one? Cheers Chris Lewis

The solo crystal used in the MkI was to ensure the 625 video output was within specification in terms of timing, although older CRT TVs/monitors could handle a small amount of error of 1-2% or more (though not the colour sub-carrier), many modern flat-panel versions can't.

In the days of analogue PAL the colour sub-carrier was specified as 4.43361875MHz +/-1Hz within the TV station and transmitted. Often TV stations locked their SPGs (Sync Pulse Generators) to GPS which made it even more accurate. Some articles were published which used the off-air PAL sub-carrier at home as a frequency standard before GPS receivers were affordable to the general public. All other vision signals, line/frame times were derived from the sub-carrier so they too were of the same accuracy. Not that we need that here, just a bit of background...

In the MkI I did notice a few minor artifacts due to the two micros running at slightly different frequencies, the internal HFOSC of the write processor is specified as 16MHz +/-2% from 0 to +60 Celcius at >2.5V supply. So the idea is to use the same oscillator to externally feed both micros. I have seen the oscillator in one micro feeding another micro, but this strikes me as a 'fudge', hence the active oscillator feeding both. They're only a quid or two..it also frees up an I/O pin on each micro if needed.

Steve A.

[Steve Anderson]

One thing I should emphasise is the very similar named D-A from Texas Instruments (and possibly others), the DAC0808, is NOT the same device for IC203, a DA-08 or DAC0800. It has no 'lc' pin (pin 1) and is a 'NC', no connect. Also it has no complimentary current output (pin 2) which is also needed here, it's marked as a ground.

Steve A.

[Steve Anderson]

A small change required in the power supply due to the logic voltage change to +5V. IC802 becomes a good old 7805, two resistors deleted and R802 changed to 120 Ohms. No heatsinks should be needed but space should be provided for a small heatsink for IC802 for those in warmer climates...err, that's not the UK!

Steve A.

SSTV-625 MkIIV PSU 2.gif (51.94 KiB) Viewed 8913 times

[Steve Anderson]

One other item to note, looking back at the circuit I posted on the 14th Dec. (which needs an update), the micros have a coloured stripe each. If I supply these pre-programmed the physical chips will have similar coloured stripes on the underside. (Red or green nail-varnish). As they're now the same device but loaded with different software there needs to be a way of identifying which is which.

I have no idea what the outcome would be if they did get mixed up, but it's no more difficult than ensuring pin 1 is where it should be on the board.

Remember:- WRITE=RED, READ=GREEN.

It's also worth considering good quality sockets (Turned-pin) for the two micros for possible future software updates. I will issue the .ASM & .HEX files to anyone who requests them so they can program the micros themselves if they have the means. Alternatively Chris Lewis has suggested he could be able to program/re-program chips in the UK.

Steve A.

[Steve Anderson]

Updated main processor circuit incorporating recent changes noted above...

Steve A.

[Steve Anderson]

A few revisions today...it WILL settle down soon!!

Steve A.

[Steve Anderson]

Right! I think that's gonna be it! Time to plug in the soldering iron, order the few bits missing and get on with porting and updating the code from the MkI. Then adding the NBTV bit.

I may issue a beta release prior to adding the NBTV part.

Steve A.

[Steve Anderson]

I've added a 'Freeze/Thaw' button whereby in SSTV mode the frame stops on detection of the 30ms frame pulse. Another press continues on with the update of the 625 output.

It's a very minor update, I don't see any reason to post the circuit yet again.

I'm also considering adding a baseband SSTV input, i.e. it's the SSTV signal prior to the FM modulation. It would show what the mod-demod process does the the picture.

Added later... With the 'Freeze' function added there's only one I/O pin free on the write micro...I think all the required features have been met...just gotta get 'em working now!!

I've now placed a 'Design Freeze' on the device, unless I've c**ked up somewhere....quite possible!!

Steve A.

[Steve Anderson]

From this point on I'll be posting circuits, drawings, etc. in PDF format. This should remove the blurred/fuzzy appearance of the GIFs I've previously posted. I think it's due to the software I use to convert Autocad output (TIFF) to GIF. Sometimes it can be perfect, other times not.

A sample PDF is attached, let me know if you have any problems with it. Next time though I'll remember to rotate it! PDF viewers are a free download and the free Adobe PDF Reader can produce a printed output.

Do not use the attached drawing, it's just a sample...

Steve A.

SSTV-625 MkIII Main 1 v4 .pdf

(94.85 KiB) Downloaded 329 times

[acl]

Steve,

The PDF file looks 'Fine and Dandy'. I have now ordered most of the components to build. I will be using fibreglass stripboard as it is stronger than the paper based one. I will order a fine soldering iron using the standard 'non-eco friendly' lead based solder as it works fine, doesn't cook the components and eat through your soldering bits. Looking forward to you releasing the beta software.

Have a Brilliant Christmas

Chris Lewis