> -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Kirk Northrop > Sent: 15 January 2007 23:44 > To: [email protected] > Subject: Re: [backstage] Five Live Partnership - get your > idea commissioned. > > Brian Butterworth wrote: > > Any page, therefore, that has dropped out of the "Ceefax" > list could > > comprise of a single line (with the 'double height' > character in front of > > it) with the message "see news.bbc.co.uk/NNN". So even > if there was 300 > > 'pages' that hit (for Ceefax and Digital TV red button) users this > > would take the space of 12 Ceefax pages in terms of bandwidth and a > > few bytes each for digital TV. > > Do you not have to transmit the whole page, including blank > characters?
Each line of teletext is broadcast on a line of the TV screen. They comprise of 40 characters (7 bit) with a 16-bit header which contains a 'row address' and the 'magazine number'. The magazine number is the first digit of the page number (1 for 100-1FF, 2 for 200-2FF, 7 for 700-7FF, 0 for 800-8FF). The header also contains various status codes, such as the ability to 'clear the screen' or leave the existing data there. If the page contains blank lines (such as after a line where the 'double height' character is used) then this row is simply not transmitted. Before each page a 'row 0' line is broadcast, again a single line. This has the 'magazine number' and the 16 bit (ie, 00 to FF) page number, and a 32 bit sub-page number which is usually referred to as the time, (0000-FFFF) because it was used for 'alarm clock' pages. As TV set controls only have 0 to 9 buttons the page numbers are usually used as binary coded decimal (BCD). Pages from different magazines can be interleaved because the data rows contain only the 'magazine number'. So you can broadcast different 'magazines' at different rates. So, the upshot is that when a page only contains two lines (such as 888) only three lines of data are broadcast for it, one header row which says 'magazine 8 page 88' and two rows of data 'magazine 8 row 20' and 'magazine 8 row 22'. The colours and graphics codes are implemented with values 01-1F, which occupy a character space. Sorry this is from Oracle, but here is a graphic: http://upload.wikimedia.org/wikipedia/en/0/07/Oracle_data_organisation.jpg > > -- > From the North, this is Kirk > - > Sent via the backstage.bbc.co.uk discussion group. To > unsubscribe, please visit > http://backstage.bbc.co.uk/archives/2005/01/mailing_list.html. > Unofficial list archive: > http://www.mail-archive.com/[email protected]/ > > -- > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.5.432 / Virus Database: 268.16.10/626 - Release > Date: 14/01/2007 20:29 > > -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.432 / Virus Database: 268.16.10/626 - Release Date: 14/01/2007 20:29 - Sent via the backstage.bbc.co.uk discussion group. To unsubscribe, please visit http://backstage.bbc.co.uk/archives/2005/01/mailing_list.html. Unofficial list archive: http://www.mail-archive.com/[email protected]/
