TiVo Teletext Decoder

It can select individual packets or full pages, displaying them on the terminal as plain text, or in colour with limited graphics. This requires ANSI colour support for the terminal program (eg. HyperTerminal on Windows or XTerm on Unix), and cannot display teletext graphics, except as full-character blocks - hence the unreadable "BBC CEEFAX" at the top of this screenshot.

It can also show subtitles from the input stream (ie. current broadcast, not recording).

So far this is just a toy, and I doubt it will be much real use to anyone on its own (most people with TiVos have a Teletext-capable TV, and anyway who wants to view teletext from a terminal window?).

However one potential development that could be very interesting is the use of the decoded PDC signals to adjust the start and end of a recording for schedule changes. This probably wouldn't be able to handle all possibilities (unlike a PDC-capable VCR, it may not be practicable for TiVo to switch channels just to check for PDC signals on another channel), but it should be possible to extend a recording where there are no conflicts in a similar way to what EndPad does, but only actually recording the extra if required. PDC signals are only available over aerial on BBC1, BBC2, some ITV regions (I understand), and Channel 5 (though most last-minute schedule changes seem to be on BBC anyway). This program currently just displays decoded PDC packets.

bash-2.02# teletext -zTC X8/30.1
31-Aug-2005 00:09:23 X8/30.1 | BBC ONE | index:P100 30-Aug 22:35 BBC1 UK
31-Aug-2005 00:38:00 X8/30.1 | BBC ONE | index:P100 recording inhibit/terminate BBC1 UK
31-Aug-2005 00:42:26 X8/30.1 | BBC ONE | index:P100 31-Aug 00:40 BBC1 UK LCI:1

Download

• Program binary
• Source code

Usage

teletext [OPTIONS] [PACKET-SELECTIONS]

-f      show full teletext packet information. Parity errors shown as ~
-p      show only full pages for packets 0-24
-P      like -p, but also displays other packets
-z      show only changes in X8/30.1 (PDC) packets
-c      show teletext formatting using ANSI colour codes
-o      use existing buffer contents
-O      only use existing buffer contents
-C      continuous running
-v      verbose running for debugging (-vv for extra verbose)
-q      quiet: don't display packets (useful to get statistics)
-e      show errors
-n NUM  without -p: stop after displaying NUM packets (default=25 if no -N)
-n NUM  with    -p: stop after displaying NUM pages   (default=1  if no -N)
-N NUM  stop after reading NUM packets
-m      check for missing scan-lines
-M LIST check for missing scan-lines, apart from the ones listed, eg -M 22e,23
-b      decode blank lines
-s      show statistics summary at the end
-t SECS time to wait between each read (default=0.5s, data loss if >5s)
-T      display local timestamps based on X8/30.0 (BSDP) packets

With the -f option, full details of each packet are displayed, eg.

bash-2.02# teletext -f P199 X8 P100:1/0

018419 80265D50 L09o/2B P100:0001/00 |  P100  CEEFAX 1 100 Wed 31 Aug 01:00/38| English, update, out of seq

018419	Packet number read by the program
80265D50	frame address in the fpga7114 module's buffer (hex)
L09o/2B	Scan line 9, odd frame. 2B is the SAA7118's SAV code, bits 0-7 (hex; SAV=Start of Active Video line). Why do I see different values for this?
P100:0001/00	Page 100, subpage 1, packet 0.

For header packets (/00), the language (only the correct decoding for certain regions) and flags (update, out of seq in this case) are also shown.

The packet text is displayed using 7-bit ASCII only between |s. Special characters (eg. accents) are displayed with their closest ASCII equivalents. Graphics are shown as "#". If -c is specified, the text output is displayed in colour and graphics characters are shown as solid foreground colour.

As well as text packets 0 to 25, packet 30 is also fully decoded (if selected).

bash-2.02# teletext -f X8/30
000039 80242500 L07e/6C X8/30.0 | BBC TWO | index:P100 9-Sep-2005 23:51:58 +0100 BBC2 UK (A2FE)
000399 80246880 L07e/6C X8/30.1 | BBC TWO | index:P100 9-Sep 23:35 BBC2 UK (2C3E) LCI:1

Other packets types (26-29 and 31), if selected, are displayed in hexadecimal, verbatim.

My UK 2.5.5 TiVo has a 197 kB VBI buffer. That can hold 4206 48-byte frames (~168 pages), or about 5 seconds of UK BBC1 transmission (you can see the actual number of packets in the buffer (including blanks) with teletext -qOCs; and time teletext -qN 4206 times how long it takes to receive). In contrast, ThomsonTiVo noted that on the US system it takes 81 seconds to process 137 pages via one pass through the VBI buffer.

In most cases, the program spends >95% of the time sleeping, so the load on the TiVo's processor isn't too high.

One oddity that I haven't understood yet is that packets 8/0 to 8/24 can have a variable data count byte (2nd byte of the frame), instead of the expected 0x4B which other packets have (11 4-byte words=44 data bytes in the frame, with bit 6 set for even parity). Perhaps TiVo somehow uses this to flag subtitles (the only ones it cares about). Anyway, this prevented earlier versions of this program from displaying subtitles.

Compilation

bash-2.02# powerpc-TiVo-linux-gcc -Wall -O99 -s -o teletext teletext.c

Add -DSMALL to remove some of the larger tables and full help text. This reduces the binary size from 90k to 55k, but shows some values (like the PDC channel names) as numeric codes.

See Also

• OzTiVo page on extracting teletext
• PDC Explained
• Teletext and PDC information
• Philips SAA7118 video decoder datasheet (from Philips Semiconductors; UK TiVos use the SAA7118, while US TiVos use the SAA7114)
• Teletext standard (ETSI EN 300 706 v1.2.1 (2003-04-16)) (from ETSI)
• Programme Delivery Control (PDC) standard (ETSI EN 300 231 v1.3.1 (2003-04-16)) (from ETSI)
• Country and Network Identification codes (ETSI TR 101 231 (2005-06a)) (from EBU)
• ANSI terminal codes to display colours on the terminal, with details in Control Functions for Coded Character Sets (ECMA-48 (5th ed, June 1991)) (from ECMA)

Authors