Convert NBTV video to composite?

Forum for discussion of electronic television. Generally, stuff to do with CRTs and not using mechanical displays.

Convert NBTV video to composite?

Postby John » Thu Dec 01, 2016 11:32 pm

With an NBTV signal recored to an audio cassette tape. How should I go about converting an NBTV video signal into composite for NTSC TV display (without a PC involved)?

back story

[
I'm working on a project that basically uses a Sony walkman as a VCR for composite TVs (specifically for a 90s pocket TV). The other day I was looking into the feasibility of it and came a crossed TVDawn doing something similar https://www.youtube.com/watch?v=j3Gvs4GhD3Y I was going to ask him how, but somebody had already asked him and he pointed them to the main NBTV site. I have played around with NBTV using software I downloaded (Video2NBTV and NBTV big picture) thank you Gary. I quite like the formate, I then looked up the history of NBTV and mechanical TVs and I started to geek out a bit. I have encode simple little videos in to wav and put them on my phone to play back through the line in on my PC, which was pretty cool to see for the first time. I did the same thing, but instead of using wav as the output I sent it straight to the sound card with a tape recored hooked up, played it back and it work.
]
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Re: Convert NBTV video to composite?

Postby Klaas Robers » Fri Dec 02, 2016 9:49 pm

To describe short:

- You have a solid state memory RAM chip
- that can contain one NBTV frame.
- This chip is continuously written with the NBTV video information
- and read in the 525 line 30 Hz speed, giving an NTSC video signal.
- Writing and reading is interleaved.

Good luck!! You will need it
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Re: Convert NBTV video to composite?

Postby John » Fri Dec 02, 2016 10:10 pm

Klaas Robers wrote:To describe short:

- You have a solid state memory RAM chip
- that can contain one NBTV frame.
- This chip is continuously written with the NBTV video information
- and read in the 525 line 30 Hz speed, giving an NTSC video signal.
- Writing and reading is interleaved.

Good luck!! You will need it


Thank you. You have bin very helpful on this matter Klaas Robers. That doesn't sound too difficult, I'll order some parts and try it on a breadboard. I'll let you know how it goes.
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Re: Convert NBTV video to composite?

Postby Steve Anderson » Sat Dec 03, 2016 9:48 pm

Hi John, and welcome to the forum.

This is not a task for the faint-hearted and as an example I attach two schematics of a SSTV-625 converter that I built last year. These are just the core of the device, in addition you'll need a DC-restorer and perhaps some gain on the analogue input. But it illustrates what's required. My intention was such that this will handle both NBTV and SSTV.

There's a thread here called "Retro SSTV Anyone?" in the Off-Topic section which may be useful to look through showing the results and progress. This thing isn't complete and I haven't even started on the NBTV side of it due the huge project (real work) I'm involved in currently.

For NBTV the RAM can be vastly smaller, even for SSTV it's overkill but it's what I had at the time. Whatever, the RAM needs to be fast.

The resolution of the gifs isn't good, that's to fit in the limited screen-space on the board, but they can be made into pdfs which are much better. They are some of the interim drawings that were done as the thing developed...certainly not final! Don't even consider building them! The software is in a state of flux too.

There are some simplifications that could be done here and there, but this should give you a feel of what you'll be needing. Items in red are notes of various types, things that need defining or such like...

For 525/60 output the code in the read micro would need changing, and the crystal too.

Photo of the prototype also...

Steve A.
Attachments
SSTV-625 3A-Model.gif
SSTV-625 3A-Model.gif (64.32 KiB) Viewed 3449 times
SSTV-625 3B-Model.gif
SSTV-625 3B-Model.gif (61.7 KiB) Viewed 3449 times
Prototype 01.jpg
Prototype 01.jpg (344.39 KiB) Viewed 3449 times
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Re: Convert NBTV video to composite?

Postby gary » Sat Dec 03, 2016 11:02 pm

When you say "without a PC involved" would that exclude something like a Raspberry Pi? A PI would be smaller than any circuit you are likely to come up with, less expensive, more configurable, and will have (NTSC or PAL) composite AND HDMI - both with audio.
Perfecting an NBTV system is like trying to slam a revolving door...
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Re: Convert NBTV video to composite?

Postby Klaas Robers » Sat Dec 03, 2016 11:23 pm

John,
as you might have seen, Steve is very keen in solving problems in PIC controllers. Of course this can also be done in other types of controllers, Arduino, Raspberry Pi, but then the difference to a PC becomes smaller and smaller. The other way to go is "all in hardware". For that way I made a diagram of the principles, which might help you to orden your mind. I will add that diagram.
In fact years ago I built the right part of the diagram. In stead of the RAM chip I plugged in an EPROM with one NBTV frame programmed in it. That gave a nice picture on the CCIR monitor.
Then something else, more important, came into my mind, so the left part was never completed. 8 bits depth is more than enough. In principle, if you do the A-D in the proper way, 6 bits is enough, but as RAM and EPROM are 8 bits, why not use them all.
The low pass filter of 1.25 MHz is used to remove the 32 line structure of the vertically scanned NBTV picture. The fact that NBTV is scanned vertically helps in removing the visibility of the course line structure of NBTV.
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Blokschema1.GIF
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Re: Convert NBTV video to composite?

Postby Steve Anderson » Sun Dec 04, 2016 2:50 pm

Klaas is correct in that this still could be done in an all hardware manner, in fact Peter Smith's 625-30/32 line converter of the year 2000 (the opposite direction) used an entirely hardware solution. By reversing functional blocks the opposite can be done as Klaas shows above. The downside is an increase in the quantity of ICs and making changes requires hardware knife-and-forking which can get messy after a while.

Which method you choose is entirely your decision, there are pluses and minuses in both.

I chose the software option as although this will be a useful bit of kit when complete it was more a programming exercise for me. I'm fairly new to these modern devices and the last time I wrote code would been in the 1980s. So you could say it was motivation to hone my code skills...though there's much I still need to learn!

Steve A.
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