RGB/Mono 625 SPG

[Steve Anderson]

Somewhat off-topic but someone may find this of use.

It's a SPG (Sync Pulse Generator) for 625 50Hz TV. It's not for composite colour 625 but monochrome or component colour. As such it generates all the required 5V TTL pulses along with simple RGB patterns. At this stage just RGB bars, full-screen white, full-screen black and reversing bars. (Top half of frame normal, bottom half reversed..)

It uses a two-quid PIC and one buffer chip. Although the PIC in theory can deliver 25mA from the outputs, the voltage does drop a bit, hence the buffer. The output of the RGB is at the usual 700mV level at 75Ω.

These days most TVs don 't have RGB inputs for component anaogue video but Y,Pr,Pb inputs so in the works is a simple matrix to perform this conversion. There are other advantages in processing colour component video in this format too.

It produces the correct four-field sequence so the clamp output could be used as a burst-gate if contemplating PAL colour. But I'm not going to attempt the correct eight-field sequence for PAL (Mathematical PAL), I'll leave that up to others.

All timings are within spec., the worst being an error of 100ns. As the patterns are quite simple the program fitted in the 2K-words available in the 16F628, the '648 wasn't needed. The blue items in the diagram are just for programming and normally wouldn't be required.

It also produces a 12.5Hz NBTV frame pulse coincident with field one of the 625 pulses.

Other patterns could be produced but only the colour primaries and their compliments are available, plus black & white.

I did build something back in the late 70s that performed the same task. It used around 30 TTL chips and guzzled about an amp! How things have changed.

If anyone is interested...

Steve A.

I've just remembered, it doesn't output the PAL squarewave so it probably couldn't be used for PAL without changed and/or additional hardware.

[AncientBrit]

Very neat Steve, wondered why you'd gone fairly quiet. Midnight oil and all that.

I'm interested in the code, do you want to PM me?

Cheers,

Graham

[Steve Anderson]

Hi Graham, it's been very busy with 'real work' here for a few months so not much NBTV time.

This was done as a source for the 625 video to NBTV converter I started a thread on some time back, I can also use a DVD player but I have to borrow it from the family room and it's not ideal.

I'm still tweaking the code and as a result of realizing the PAL squarewave is missing I may modify it a little...we'll see...though I have no intention of using PAL. Once I'm happy with it I'll send on the .asm file. If it makes any difference I'm using MPLAB IDE Version 8.73a. Or I can send the .hex file.

Good to hear from you again.

Steve A.

[AncientBrit]

Hi Steve,

If you can send me the asm file when finalised I'd be grateful.

I'll 'port' it to a PIC18F2525 that I have to hand.

Kind regards,

Graham

[Steve Anderson]

...I'll 'port' it to a PIC18F2525 that I have to hand. Graham

To be honest if switching to an 18F2525 I would start over. I have a few 18F2620s here, exactly the same but with a bit more program memory. I did consider using one for this and in some ways I'm still tempted.

It would allow you to get 50ns accuracy by using the x4 PLL and a 10MHz xtal. With all that additional memory (24/32k words as opposed to only 2k) more patterns could be stored and they could be a bit more complex.

With the extra I/O pins more patterns could be selected and/or simple grey-scales could be generated, say 2 or 3 bits per primary.

I guess it depends on what you intend to do with it. If you only need the pulses then the '628 is adequate, a true SPG. If you need patterns then really it becomes a TSG (Test Signal Generator).

But I will send on the .asm file when ready, but it won't be tomorrow!

Cheers,

Steve A.

[AncientBrit]

Thanks Steve.

Okay, an SPG, not TSG.
Always useful to have though.

When you are ready. No pressure,

Cheers,

Graham

[Panrock]

I too noticed you were quiet Steve_A! I'm still hoping to 'plug in' to your electronic expertise later in the year - when my 120-line scanner reaches that stage.

In the meantime there are a few pictures of how it's going so far HERE.

Steve O

[Steve Anderson]

...I'm still hoping to 'plug in' to your electronic expertise later in the year... Steve O

Steve, assuming I have the time (I usually do) then I'm happy to do what I can.

I'm sure I have visited that forum before and I thought I had signed up to it. But any combination of user name and password that I would most likely use failed. I can sign up again, I just don't want to duplicate myself if you see what I mean...and I'm certain no-one else wants that too!

It wouldn't be much of a hassle to turn this little device into a 405 SPG too, with RGB simple colour too. The timing may not quite be as accurate but I would think perfectly adequate. The line frequency of 10125Hz isn't very easy to derive from standard crystals, but it was done decades ago so I'm sure it can be done today.

Instead of RGB colour bars an eight-step greyscale could be produced using the same three outputs.

Progress on the 120-line mirror screw looks good...and I can see the amount of work that has already gone into it.

Steve A.

Stabbing a few buttons on the calculator reveals that the 405 line frequency would be -0.035% at 10121.5Hz.

[Panrock]

Steve,

Thanks for the usual unstinting kindness. I'll of course be more than happy to fit in with your convenience. The main priority then will be the wideband video driver for the LED stick.

The BHTG board merely possesses a legacy 'NBTV' section that I'm using for occasional announcements on this project. You only need to log in if you want to comment there - so it's not essential. Where we're talking right now is the real NBTV meeting place. But if you wish I can delete your registration on the other board so you can re-register there... that should restore your ability to log in.

Your device is clearly very versatile if it can also become a 405 SPG! I'm quite sure the chaps at the VRR forum and the VRAT forum would be very interested in this, especially the two of them that possess 405 colour receivers.

Cheers and thanks.

Steve O

[Steve Anderson]

Steve,

So I guess you're saying I have signed up for the 405 forum in the past....I did a search on my own name that I may have used as a member log-in, but only yourself and the other two Steves came up.

I wouldn't expect the NBTV section to be too active, my interest would be the 405 part. Not that 405 was ever used in this region, the closest place I know that did use 405 is Hong Kong and much further away, New Zealand.

I assume the 405 colour system is/was the experimental NTSC version with a sub-carrier of 2-odd MHz...I wonder how good/bad it was? Producing a coder for 405 NTSC wouldn't be too difficult.

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

I spent quite some time on this 625 version yesterday...boy, I tell you, getting the vertical interval correct is doing my head in! I'm tempted to drop the four-field bit and make it two-field. After all it was never meant to be a PAL SPG, just a 625 component version.

Getting timing spot-on involves a lot of calculations (mainly in ones head), it's simple addition and subtraction, just there's a heck of a lot of 'em!

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

Correct me if I'm wrong...your 120-line system is at 25fps with an aspect ratio of 4:3? As I recall that requires a bandwidth of about 250kHz.

Steve A.

405 NTSC sub-carrier was 2.6578125MHz, 262.5 x line frequency of 10125Hz.

[Panrock]

Hi Steve,

So I guess you're saying I have signed up for the 405 forum in the past....I did a search on my own name that I may have used as a member log-in, but only yourself and the other two Steves came up.

You're still there - as 'Steve A', joined on 15th October 2005, and you penned this interesting introductory spiel: http://www.405-line.tv/phpBB3/viewtopic.php?t=40

I wouldn't expect the NBTV section to be too active, my interest would be the 405 part. Not that 405 was ever used in this region, the closest place I know that did use 405 is Hong Kong and much further away, New Zealand.

The NBTV section activity there is zero. I simply use it as a convenient place to record my own day-to-day activity and thoughts. I kind of feel such a thing would soon get very tedious and boring to read on here...

As for the 405 aspect, that forum is little more than a notice board for the project to get 405 on the air again. This has involved a few of us in significant time and expense, and most of the forum activity is in private areas hidden from public view.

I assume the 405 colour system is/was the experimental NTSC version with a sub-carrier of 2-odd MHz...I wonder how good/bad it was? Producing a coder for 405 NTSC wouldn't be too difficult.

According to David Boynes, very good indeed! Subjectively as good as 625, probably partly because the shadow mask occludes the line structure - to some extent.

I spent quite some time on this 625 version yesterday...boy, I tell you, getting the vertical interval correct is doing my head in! I'm tempted to drop the four-field bit and make it two-field. After all it was never meant to be a PAL SPG, just a 625 component version.
Getting timing spot-on involves a lot of calculations (mainly in ones head), it's simple addition and subtraction, just there's a heck of a lot of 'em!

This sounds like a real labour of love! You'll find more kindred spirits on the VRR and VRAT fora (google these if interested).

Correct me if I'm wrong...your 120-line system is at 25fps with an aspect ratio of 4:3? As I recall that requires a bandwidth of about 250kHz.

All spot-on. You'll see on the other forum that my latest concern has been the whole thing flying apart and throwing stainless steel daggers at 45mph into the hearts of the viewers!

Steve O

[Steve Anderson]

Hi Steve,

I've had a wander around all three forums this afternoon for a few hours and all three are of considerable interest. And congrats on getting 405 radiated again, if only intermittently.

I know this is OT, but I assume the 60-odd MHz carrier frequency was allocated by Ofcom and the old 'London frequency' wasn't available.(45MHz). Has anyone designed a converter to do this for TVs with only the one reception (fixed) frequency? I would think a talented Radio Amateur should be able to do this...but that's not me!

I'm taking a break from the programming today as I think I over-stressed my limited neuron supply yesterday.

Steve A.

[Panrock]

I know this is OT, but I assume the 60-odd MHz carrier frequency was allocated by Ofcom and the old 'London frequency' wasn't available.(45MHz).

This is correct - and was something of a disappointment to us. The MoD was laying claim to it, but were very cagey about why. It's possible this band is used as part of an exotic spread-spectrum application.

When I came to construct the transmitting aerial though, I thanked my lucky stars. It made everything that much smaller and more manageable!

Has anyone designed a converter to do this for TVs with only the one reception (fixed) frequency? I would think a talented Radio Amateur should be able to do this...but that's not me!

Well, our signal is bog-standard System A vestigial-sideband, so in principle this shouldn't be too hard to do. A low-side oscillator at 19.5MHz and suitable buffering and filtering circuitry would be all that would be required.

I'm taking a break from the programming today as I think I over-stressed my limited neuron supply yesterday.

It's the weekend in the UK. Dunno about Thailand though...

Steve O

[Steve Anderson]

Well, I've sorted out all the timing errors, mostly due to faulty mental arithmetic I think. All I have to do now is add the 12.5Hz NBTV frame flag and knock up a RGB-Y Pb Pr matrix

The software could do with a severe tidy-up and pruning in certain areas but I'm going to leave that for another day.

Some scope screen-scrapes below...

Steve A.

[Steve Anderson]

A week has passed, so an update...

This RGB-YPbPr matrix is proving much more difficult than I expected. It's getting there and it's nearing completion, but what a headache!

First problem was to source some PNP transistors of suitable bandwidth locally, I know others have done this with the likes of BC557s and so on, but they do limit the bandwidth somewhat. So it has become an all NPN design which I can get locally.

Second, I initially decided to build the matrix on the same board as the PIC but the fast logic edges kept coupling into the matrix, so it has now become two separate boards with a screen between them.

The matrix is an all-discrete arrangement with not a chip in sight and should use around 8-9 transistors when finalized. It's also single-supply, +12V at the moment, but that may change to +15 or +18V in the end.

Steve A.