UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Test Card / Teletext Generator

Parabola · 25th Jun 2010, 9:33 pm

Hello All,

Business is slow (surprise, surprise!), so I've been spending some time on projects I never normally seem to get around to. One of those being a solid state, multistandard, colour, test card generator with the added ability to also put out a few teletext pages. With analogue teletext on the way out fast, I definitely need something to kick aging teletext decoders into life when running some of my old sets.

So, I envisaged doing this in 3 phases - each one creating something that worked and was useful. Here's the masterplan -

Phase 1 - Single Frame (2 fields), Multistandard (405/441/625/819), Monochrome - Pre-programmed content - changable by chip swap.

Phase 2 - As above, but now in colour (4 frame / 8 fields) and including at least 4 teletext pages.

Phase 3 - As above, but now with added USB connectivity to allow re-programming via a host computer.

The astute will now be saying, "Ah, you've now created the RT Russell generator all over again". Well, yes - but you can't buy an RT Russell anymore! There is also a similar (405 line) device in the projects section of Duke_Nukem's site. However, that's a little under spec'd for 625 colour and teletext. My point is, I make no claim to the operational concept of this device - this is my attempt at creating a practical unit to my own design.

Today I've got Phase 1 completed - please see the attached pictures.

The unit is shown generating 625 Test Card F in monochrome, feeding video directly into a very el-cheapo 80's mono monitor intended for low end home computer (with TV out) use.

Continuing with 625 as an example; The output is full bandwidth as the pixel clock is running at 12MHz. That equates to 720 pixels per active horizontal line. As the syncs are also coded as pixels, the total line, including syncs and portches, comes out at 768 pixels in total. Therefore, the total byte count for one frame (2 interlaced fields) is 768 x 625 - 480,000. This 'image' is stored in a 4M Bit Flash EPROM (70ns) to meet the required clock speed (U/V erasable EPROMs are, largely, too slow).

Because the entire frame, including syncs, is contained in the ROM 'image', it's a small matter to switch standards to 405, for example, or any other required format. In the same way, teletext coding can be inserted into the vertical blanking interval lines.

As you will see from the pics, I managed to achieve Phase 1 using less chips than I first envisaged, hence the 3 empty sockets! The empty area in the middle of the board is for the addition of more PROMs for adding colour and increasing the number of text pages. I also need to stick a buffer (op-amp or transistors) on the output of the DAC, but overall I'm pleased with progress so far.

Comments / suggestions welcomed from the smarter folk than me on here (you know who you are!).

Parabola

25th Jun 2010, 9:33 pm	#1
Parabola Retired Dormant Member Join Date: Sep 2008 Location: London, UK. Posts: 117	Test Card / Teletext Generator - Phase 1 Hello All, Business is slow (surprise, surprise!), so I've been spending some time on projects I never normally seem to get around to. One of those being a solid state, multistandard, colour, test card generator with the added ability to also put out a few teletext pages. With analogue teletext on the way out fast, I definitely need something to kick aging teletext decoders into life when running some of my old sets. So, I envisaged doing this in 3 phases - each one creating something that worked and was useful. Here's the masterplan - Phase 1 - Single Frame (2 fields), Multistandard (405/441/625/819), Monochrome - Pre-programmed content - changable by chip swap. Phase 2 - As above, but now in colour (4 frame / 8 fields) and including at least 4 teletext pages. Phase 3 - As above, but now with added USB connectivity to allow re-programming via a host computer. The astute will now be saying, "Ah, you've now created the RT Russell generator all over again". Well, yes - but you can't buy an RT Russell anymore! There is also a similar (405 line) device in the projects section of Duke_Nukem's site. However, that's a little under spec'd for 625 colour and teletext. My point is, I make no claim to the operational concept of this device - this is my attempt at creating a practical unit to my own design. Today I've got Phase 1 completed - please see the attached pictures. The unit is shown generating 625 Test Card F in monochrome, feeding video directly into a very el-cheapo 80's mono monitor intended for low end home computer (with TV out) use. Continuing with 625 as an example; The output is full bandwidth as the pixel clock is running at 12MHz. That equates to 720 pixels per active horizontal line. As the syncs are also coded as pixels, the total line, including syncs and portches, comes out at 768 pixels in total. Therefore, the total byte count for one frame (2 interlaced fields) is 768 x 625 - 480,000. This 'image' is stored in a 4M Bit Flash EPROM (70ns) to meet the required clock speed (U/V erasable EPROMs are, largely, too slow). Because the entire frame, including syncs, is contained in the ROM 'image', it's a small matter to switch standards to 405, for example, or any other required format. In the same way, teletext coding can be inserted into the vertical blanking interval lines. As you will see from the pics, I managed to achieve Phase 1 using less chips than I first envisaged, hence the 3 empty sockets! The empty area in the middle of the board is for the addition of more PROMs for adding colour and increasing the number of text pages. I also need to stick a buffer (op-amp or transistors) on the output of the DAC, but overall I'm pleased with progress so far. Comments / suggestions welcomed from the smarter folk than me on here (you know who you are!). Parabola Attached Thumbnails