This software generates a teletext signal in software. No hardware mods are needed.
This support both PAL (Teletext) and NTSC (CEA608 format captions).
(The demo mode should no longer crash TVs!)
Usage:
Have a Raspberry Pi connected to a TV by composite video. It doesn't matter if you are running X or not. Dispmanx will draw over anything else.
At least 32MB of GPU memory is required.
Raspbian defaults to NTSC composite output. To permanently change it
to PAL, put sdtv=2 in config.txt and reboot.
Build the programs:
make
Twiddle the registers:
sudo ./tvctl on
Ensure you see the message "Teletext output is now on."
If you're connected via PAL, run the demo:
./teletext
and press the text button on your TV remote.
If you're connected using NTSC, run the demo:
./cea608
and enable the TV's closed caption controls to show CC1.
tvctl on|off
This tool prepares the composite out for teletext transmission by shifting the output picture into the VBI area. It will check the registers are in a known state before doing anything. "on" and "off" commands will have no effect if the state is already on or off, or if the registers are in an unknown state.
teletext [-m even field line mask] [-o odd field line mask] \
[-l white level] [-f] [-]
Optional line mask arguments are a 16 bit mask to create quiet lines in vbi output, first line is LSB, last is MSB. For example running "teletext -m 0xFFF0 -o 0x0FFF" will output teletext packets on the first four lines of even fields and last four lines of odd fields. If only one mask is provided the same value will be used for both fields.
The white level is specified as a number between 0 and 100. This is the "brightness" of the high bits of the teletext signal. Default is 100.
The '-f' argument activates full-field mode, where every available ine on the display is used for teletext, allowing much greater bandwidth. Most decoders do not support this mode.
Running with no arguments will show a demo. Running "teletext -" will read packets from stdin and display them. You can therefore pipe packets from another tool, possibly over the network with ssh or netcat.
The packet format is 42 byte raw binary packets, without the clock run-in. Each packet received will be transmitted once, so you must send packets endlessly. See
http://www.etsi.org/deliver/etsi_i_ets/300700_300799/300706/01_60/ets_300706e01p.pdf
for details of the teletext protocol.
cea608 <->
Running with no arguments will show a demo. Running "cea608 -" will read data from stdin and display. The data is in binary format, with each field represented as two bytes of parity-encoded data. This is the "raw" format some capture tools can output. Data is output at 59.97 fields per second. See
https://en.wikipedia.org/wiki/EIA-608
for details on the protocol and links to the specifications.