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Old 16th Dec 2011, 10:54 am   #1
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Join Date: Sep 2008
Location: London, UK.
Posts: 117
Default 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
3 x Mono 625 + 3 x Mono405
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
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