Date: Wed, 18 Mar 1998 09:23:21 -0300 (EST) From: Joao Carlos Mendes Luis <jonny@coppe.ufrj.br> To: luigi@labinfo.iet.unipi.it (Luigi Rizzo) Cc: rhh@ct.picker.com, eivind@yes.no, hasty@rah.star-gate.com, multimedia@FreeBSD.ORG Subject: Re: What is teletext (was Re: "Windows 98" program guides) Message-ID: <199803181223.JAA06747@gaia.coppe.ufrj.br> In-Reply-To: <199803180707.IAA00142@labinfo.iet.unipi.it> from Luigi Rizzo at "Mar 18, 98 08:07:59 am"
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#define quoting(Luigi Rizzo) // > 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. Well, it applies to PAL/M, and PAL/M is more NTSC than PAL. Even worse, one channel here in Brazil shows some moving bars (like changin binary code) on the first and second lines of fxtv. Most analog viewer will simply not be able to see that, but Unix has always been known for seeing farthest. :) // PHYSICAL LAYER Oh no, here comes OSI, again... // 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. This is the dot freq, right ? So it must be less on NTSC and/or PAL/M. // 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. Do you have some soft algorith for this ? I'd like to peek at those digital lines I see. Maybe there is a Station to Station command to inform about commercial beginning and end. I could use it to mute fxtv. :) // 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, I've seen this kind of thing ehre in brasil using the third channel of FM radio stations. On a job I have worked on, Rio de Janeiro's Stock Exchange rates were broadcast/received using this method over a very popular station. // 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. Do you know if Closed Caption info is sent by this way ? If so, generic Hauppauge model 400 can also receive this, as the windows driver seems to be able to do that (but I could not test yet). Jonny -- Joao Carlos Mendes Luis jonny@gta.ufrj.br +55 21 290-4698 jonny@coppe.ufrj.br Universidade Federal do Rio de Janeiro UFRJ/COPPE/CISI PGP fingerprint: 29 C0 50 B9 B6 3E 58 F2 83 5F E3 26 BF 0F EA 67 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-multimedia" in the body of the message
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