Date: Wed, 18 Mar 1998 08:07:59 +0100 (MET) From: Luigi Rizzo <luigi@labinfo.iet.unipi.it> To: rhh@ct.picker.com (Randall Hopper) Cc: eivind@yes.no, hasty@rah.star-gate.com, multimedia@FreeBSD.ORG Subject: What is teletext (was Re: "Windows 98" program guides) Message-ID: <199803180707.IAA00142@labinfo.iet.unipi.it> In-Reply-To: <19980315112018.47411@ct.picker.com> from "Randall Hopper" at Mar 15, 98 11:19:59 am
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> I think with the answers to some of my questions above, I might be able to > help you more with putting this in. As you can tell, I'm trying to get a > handle on "what is teletext", "what do PAL TVs do with teletext", and "what > would a tuner app like Fxtv do with teletext". Teletext is data transmitted over the "Vertical Blanking Interval" (VBI) of the video signal. This is not PAL-specific, i think the same mechanism and specifications apply to NTSC as well. Splitting the discussion according to the various layers, we have PHYSICAL LAYER These data are encoded with a very simple NRZ modulation (black=0, white=1) of the video signal, with a frequency of approx 6.75 MHz. DATA LINK LAYER Each line of the video signal contains some 40-48 bytes, starting with a couple of 'sync' bytes, then a few address+control bytes, then raw data. There is a small amount of protection on the address+control, and little (parity) or no protection on the data fields. You can think of lines as fragments of larger data units called "pages" which are made of 25 lines each. The address field allows for some 800 different pages (you can think of it as 800 different multicast addresses) to be sent. Decoding is usually done in hardware with a cheap (because of volume production) decoder such as the SAA5246 or others, that assemble pages in memory so that you can download it e.g. using the I2C commands in the bt848 driver. On my web page there is also a software decoder that i used with the meteor and with the brooktree before i got the 5246 data sheets. TELETEXT in EUROPE In most of europe, for the last 10-15 years, broadcast tv channels have been using teletext to carry news and various kind of information. In the last few years, some carriers have also sold "pages" to private companies that use them to broadcast the most disparate info to their clients/branches. E.g. a company used it to distribute updates to a bulletin of italian laws they sold on CD, and i think other companies also used for private purposes (e.g. to send info/databases to their branches spread over the country). In these cases, although some high level protocols exist for carrying data and inserting protection and error recovery capability, TELETEXT and FXTV among the info carried by teletext pages, there are some with program schedules etc. I am not sure if there is any standard for that, but for sure there is the scheduled time written in cleartext etc. There is also some information such the "showview" codes associated to each program, i have no idea if such info include any "program type" data or so. cheers luigi -----------------------------+-------------------------------------- Luigi Rizzo | Dip. di Ingegneria dell'Informazione email: luigi@iet.unipi.it | Universita' di Pisa tel: +39-50-568533 | via Diotisalvi 2, 56126 PISA (Italy) fax: +39-50-568522 | http://www.iet.unipi.it/~luigi/ _____________________________|______________________________________ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-multimedia" in the body of the message
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