See the Wiki for more information: https://github.com/phoyd/avr-teletext/wiki
This is a teletext inserter for AVR microcontroller. It adds teletext to an existing composite signal. It is controlled on I2C and features two modes. In passthrough mode whole teletext packets are received on I2C and inserted into the video signal, allowing the broadcast of an interleaved magazine with multiple pages. In console mode the I2C port receives individual characters and emulates a framebuffer by generating a single page repeatedly. The console supports accelerated scrolling.
The project homepage https://chili.hackerspace-bremen.de/projects/teletext-inserter
A video of the device operating is here: https://plus.google.com/117474986382867317779/posts/eKBn5ePYWbc
A breadboard prototype: http://www.youtube.com/watch?v=iac1NJZw9VQ
A KiCAD Project with the hardware schematic is in the folder schematic. The folder also contains a PDF for quick review.
General parts list:
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AVR microcontroller. I used Atmega 168. Using a part with more RAM would allow for multiple consoles are other fancy stuff.
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LM1881 sync splitter. For generating horizontal and vertical sync pulses. Hard to find in through hole package. I got it at http://www.rapidonline.com
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13.875 MHz crystal. For AVR clock. This is double teletext frequency. Extremely hard to find. I got it from http://quarndon.co.uk/
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A diode to hook up the teletext to the video signal.
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Various resistors and capacitors. Especially you need some filter caps between ground and vcc or the output signal will be too dirty to work.
The AVR implements two I2C addresses: data and control.
0x00 - Data address
Bits Function
0-6 7 bit characters written here will be placed into the output buffer.
7 Unused.
0x01 - Control address
Bits
0 Set mode. 0 = console, 1 = passthrough.
1-7 Unused.
In this mode each character received will be written into a character framebuffer, staring in the top left corner and moving to the right with each character. When the end of the line is reached the cursor position moves to the beginning of the next line. When the bottom of the display is reached the row pointers into the buffer are shifted by 1 line, and the line at the top of the display becomes the line at the bottom. This line is cleared and new characters go here. Essentially it is a scrolling text console. Newlines are supported. Setting the cursor position is not supported. The whole buffer is output on a loop regardless of whether new data is available.
In this mode each received character is written into the output buffer and then outputted only once. The buffer is used with a head and tail pointer here. When a row fills up the head pointer is incremented and when the row is output the tail pointer is incremented. If there are no lines to output a filler line is output instead. This mode is intended to be used to broadcast normal teletext magazines with multiple interleaved pages.
The AVR uses SPI to output the teletext signal. The ISR enables TX in the horizontal interrupt before pushing out the bits at the right moment and disables TX afterwards. This is neccessary, because TX would be high when idle. The original code dealed with it by producing an inverted signal, which would later be inverted/coupled with an 74LS244N.
When the teletext generator is idle or goes low, then the circuit should be disconnected from video, so that the low state does not drain current from the video signal. This is achieved using a diode. In fact a LED could be used which, together with the right resistor value, would produce the right voltage level and even indicate data transmission with a flickering light.
During blanking, video has negative levels agains low, but most diodes have a much larger forwad bias than that difference.