IMOGen Test Card & Teletext Generator
 

This is just a small update to last year’s threads, here and here, that detailed the early prototype development of a homebrew Test Card and Teletext Generator that I’d promised myself before the analogue switch off occurs here in London this coming April.

The stripboard prototype has now morphed into what you see in the attached images, and has gained the name "IMOGen" (Incrementing Memory Output Generator).

It can generate various colour or monochrome Test Cards, in differing line standards, and a selection of up to 6 Teletext pages with the current firmware. The VBI signals (Teletext and VITS) can be blanked where necessary for sets with frame blanking circuits never designed to deal with them. Input is 7-12v DC from a wall wart psu and output is 1vpp baseband video.

The front panel controls are –

1 x Rotary switch to select the required Test Card.
2 x Latching push buttons to respectively blank either Teletext or VITS or both.

The best way to think this unit’s capabilities is in terms of memory “slots”. These slots can be used for Test Cards as follows –

PAL Colour 625 = 4 Slots
NTSC Color 525 = 2 slots
Monochrome 405/525/625 = 1 Slot

As the unit has six slots, depending on the firmware loaded, any combination of the above that adds up to a total of 6 can be accommodated. Some possible examples would be –

1 x PAL625 + 1 x Mono625 + 1 x Mono405
or
3 x Mono 625 + 3 x Mono405
or
2 x NTSC525 + 2 x Mono525
etc, etc.

One page of Teletext is possible for every slot designated for storage of a 625 line Test Card. At present 16 reconstructed (re-coded from original screenshots) Teletext pages are available, but hopefully the library will grow as time allows.

Current sample units are loaded with 625 PAL Colour Test card F, Monochrome 625 Test Card C and 405 Test card D + 5 Teletext pages including the Engineering Test Page.

For those interested, the PCB was designed using the free “Design Spark” package from RS and was manufactured by the online PCBTrain service by Newbury Electronics.

I think that’s about it. If anybody is interested in further technical detail, screenshots or reconstructed Teletext pages, then I’ll post more as required :)

Re: IMOGen Test Card & Teletext Generator
 

That looks absolutely fantastic Parabola. I'll be your first customer when you put them on the market. :)

Kind regards.

From Mike.

Re: IMOGen Test Card & Teletext Generator
 

Hi,
I hadn't see the earlier posts, but that looks like a nicely done project. please keep everyone updated. Am I right from reading the threads you mention that UV erasable eproms would not be fast enough to use ?
regards Peter

Re: IMOGen Test Card & Teletext Generator
 

Thanks, both. Mike, the fiver we agreed is in the post :-D

The pixel rate is 12MHz so everything has to happen within 83ns. Finding fast enough UV EPROMs is both much harder, and considerably more expensive, than the 70ns FLASH PROMs I'm using. No reason why they couldn't be used if suitable parts were to turn up in a bargain job lot! It's always possible that 90ns UV EPROMs may work, though that's really hoping the spec is generous enough to just about allow it, being that nearly all the accessed addresses are sequential. I've never tried it.

The alternative would be to use slower UV EPROMs in Odd/Even address pairs to allow slower PROM access times, but the multiplexing and data latching requirements add complication, and one of my aims from the outset was to try to avoid having any further logic between the PROM data lines and the DAC. As certain people on here will know, mainly because I'm not smart enough to design it :-)

Re: IMOGen Test Card & Teletext Generator
 

I already have one on trial and am very impressed indeed. I'll make my way over to you again sometime so that you may update the mono 625 card.

It's a must for keeping those teletext sets going :)

Now to source a system A VHF modulator for the 405 line card....

Re: IMOGen Test Card & Teletext Generator
 

I have seen ImoGen and it's a well thought out design. What makes it special is its teletext support which it better than any previous testcard generator.

For VHF 405 use, there are several published designs for modulators.

Re: IMOGen Test Card & Teletext Generator
 

Hello Jeffrey,

Although both are no longer available, Richard Russell and Dave Grant's testcard generators also supported teletext!

Regards,

Dave.

Re: IMOGen Test Card & Teletext Generator
 

Dave, I'm well aware that both Dinosaur and RTR generators had some teletext capability. ImoGen has more extensive teletext support. Exactly how much I'm not sure.

Re: IMOGen Test Card & Teletext Generator
 

Thanks Tas and Jeffrey, more fivers to post. That's the entire marketing budget spent now. I knew I should've allowed at least £20, but I spent it all on PROMs :D

Tas, I still haven't fixed the high frequency grating in the 625 TCC source image, but the version I have now is nicer than what you currently have in that slot. I can always post you a PROM so as to avoid the Christmas traffic.

As far as I'm concerned, although I've never owned one or even held one, Richard Russell's generator is the gold standard in this class of device and nearly everything I have done is to try to go some way to emulating what is possible with his design. I'm getting there, but IMOGen still needs a "next generation" to become easily user programmable, and I've not even attempted to add some of the extra features that his unit can offer but are not of so much interest to the vintage tv user (widescreen signalling and fast text, for instance).

I think that Jeffrey was being kind to me here by promoting the fact that, in it's current form, IMOGen can store up to six text pages. This is more than the four that RTR's generator stores at one time. This is related to the available PROM space and the number of complete frames stored. The other feature that I incorporated is the front panel selectability of the VBI signals. This allows the instant blanking of the Teletext data lines and VITS if required. Of course, you can choose whether to program them, or not, with RTR's generator, but that does involve a trip to a PC to achieve the same result.

As far as I know, and I know very little about what was offered with Dave Grant's Dinosaur TCGen, it only produces one page of text which contains some design details and credits about the generator itself. I'd be interested to hear differently because it should certainly be able to do at least four if correctly coded to. I don't believe the VBI signals would be blankable without reprogramming the PROMs.

Also what Jeffrey may be thinking of is my intention to try to double the available text pages with a new version of firmware in future. This will involve some address swapping manouvures during the relevant data lines and should be possible as long as I don't run out of macrocells in the CPLD to implement the logic. There is plenty of space in the PROMs. I need to get my VHDL head on (watch out!) to look at this seriously in the near future.

As you say, the main problem with the two excellent designs that you mentioned is that, nowadays, you cannot buy them anymore. Hence my efforts with IMOGen, mainly to satisfy my own need.

Jeffrey has also been kind by not mentioning that our recent comparative tests showed that the reconstruction filter in IMOGen is not quite as good as the Dinosaur design. This isn't all that surprising, because the Dinosaur filter is more complex and looks like it's been designed by someone who knew what they were doing, whereas mine is relatively simple and was designed by some freeware analogue filter software. This is the most tricky and infuriating element of the design, as trying to make a half decent reconstruction filter, I've learnt, at a sample rate as low as 12MHz is always going to be a series of compromises. I'm still working to improve this at the moment, even though the differences we are talking about can only really be discerened on a professional monitor and are, mostly, not an issue with real world vintage TVs.

Re: IMOGen Test Card & Teletext Generator
 

If we were looking at a wish list for an ideal design it would include any or all of:

• SRAM or DRAM to hold the current image or even an image sequence
• Flash memory interface (USB or SD card) to hold lots of images
• USB interface for PC control, perhaps even for dynamic teletext
• Higher sample rate (13.5MHz), with oversampling probably computed at picture load time, to solve output filter problems

As it is, ImoGen fills a gap in the market and does it well.

Re: IMOGen Test Card & Teletext Generator
 

Hello Jeffrey

Fair enough, though the impression I got from what you wrote, was that you weren't aware of their teletext capability!

Regards,

Dave.

Re: IMOGen Test Card & Teletext Generator
 

Hello Parabola,

In the version of Dave Grant's TC Generator that I have, images are stored in pairs of EPROMs, one pair can store one colour or four monochrome images, a total of four frames. It is, therefore, possible to have four teletext pages per EPROM pair, which represents four pages for a colour image and one teletext page per monochrome image per EPROM pair.

There was no easy way to produce images for Dave's generator before Richard Russell produced his TC Generator. The programming software it uses can also be used to produce files that can, with some manipulation of the data locations, be programmed into a pair of EPROMs that will work in Dave's generator!

As for the reconstruction filter, I was told (I forget by whom) that the one in Richard's generator is better than the one in Dave's generator!

Regards,

Dave.

Re: IMOGen Test Card & Teletext Generator
 

Thanks, Dave.

That's pretty much my understanding. The fields are stored as 8 x 256K gapped EPROM images within two larger 8MB EPROMs, so 4 frames per PROM pair.

I accept that you can reprogram Dinosaur if you have the means to manipulate the data how it's required. What I don't know, and it's only a point of passing interest, is if Dinosaur ever came pre-programmed with a selection of Teletext pages.

I would be very surprised if RTR's reconstruction filter wasn't the best - without doubt the man certainly knew what he was doing.

No to number 2, but it's more than possible that the next version will be a USB device containing an SRAM frame buffer and a DAC - all else done in software. That would allow for all the other "wishes", and I'm starting to enjoy writing software more than straining my eyes with soldering SMDs!

Re: IMOGen Test Card & Teletext Generator
 

Hi Parabola, many thanks for the reply,
I only asked about the uv eproms as I have some second hand 27c4001 eproms here.
again the project looks most impressive,
sorry for asking another question, but how easy would it be to add in another TV standard ? Of course having the 625, 525 and 405 options should cover most requirements ( certainly in the UK ).
regards Peter

Re: IMOGen Test Card & Teletext Generator
 

Hello Parabola,

You're welcome.

Indeed. I am working on some software to 'automate' the process, using another of Richard's products, his BB4W - BBC Basic For(4) Windows.

I don't believe so, based on what Dave said about getting them programmed! I think they only ever came with the teletext page(s) you mentioned, certainly, that was all that was present when I got mine.

Regards,

Dave.

Re: IMOGen Test Card & Teletext Generator
 

Assuming it's not beyond the mathematical restraints of the coding regime, then producing differing standards is the function of the various rag-bag collection of software apps that I've written, or hacked, and use to create suitable PROM images for my hardware (all written in BB4W, Dave ;) )

What is the standard you have in mind?

Re: IMOGen Test Card & Teletext Generator
 

:)....

Re: IMOGen Test Card & Teletext Generator
 

As mentioned previously, I've re-coded a selection of original Teletext pages for use with IMOGen - currently 16 in all.

There is also a small, self-contained Windows viewer application that allows them to be viewed on a PC for selection purposes prior to programming.

If anyone would like to take a trip down the Teletext memory lane themselves, then this application can be download from here -

http://www.vintagepixels.co.uk/imoge...View_v1-07.zip

Just extract the two files contained in the zip file to a new folder - double click on the executable to run it, then load Demo.car from the "File" menu. The upper menu bar navigation options will then become active and are fairly self-explanatory.

Hope you enjoy the nostalgia!

PS: For those who already have an earlier copy, this version (1.07) has correct implementation of double height control code emulation and so displays the Engineering Test Page correctly. It's probably worth replacing your previous version :)

Re: IMOGen Test Card & Teletext Generator
 

Hi,

I downloaded the zip and Symantec said it would not allow it because it was infected with "bloodhound.sonar.9" whatever that was....

Otherwise very impressed with youe construction work....Professional.

Regards

Steve.

Re: IMOGen Test Card & Teletext Generator
 

When Parabola demo'ed his software to me he said ignore any warnings. Apparently he hasn't put in some bit of code that certifies it as a genuine window prog. I don't claim any understanding of this but I know him well enough to say that he's not going to be distributing malware.