Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!sol.ctr.columbia.edu!lll-winken!telecom-request From: jsd@proxima.uucp (Jeremy Druker) Newsgroups: comp.dcom.telecom Subject: Re: Transporting a Bitstream on a Video Channel Message-ID: Date: 15 Jun 91 02:25:05 GMT Sender: Telecom@eecs.nwu.edu Organization: TELECOM Digest Lines: 59 Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 11, Issue 455, Message 2 of 14 In , rmz@ifi.uio.no (Bj|rn Remseth) wrote: > Does anyone know about existing hardware that puts a bitstream on a > video channel and let you extract the same bitstream out of the video > video signal, after it has been transmitted on standard video > transport media such as satellite links, video cassettes and cable TV > networks. > This kind of equipment could use spare capacity on local cable > networks and satellite sattelite links to spread e.g. usenet news > quite inexpensively. > But, the Big Question is: Does this hardware exist? Yep -- it sure does. The Magic Phrase is "Data Broadcast", and it works by sending data during the so-called Vertical Blanking Interval between picture frames. This is how teletext works, and data broadcast is, in fact, a specialized form of teletext. Ordinary teletext works by constantly retransmitting a set of pages of data, where each page comprises up to 25 rows of 40 characters each. Data broadcast, on the other hand, treats the transmitted data as a single continuous stream of bytes. There are one or two companies in the UK that I know of which suuply data broadcast receiving kit ("data decoders"). Only one as far as I know supplies the transmission ("head-end") kit, and that comprises a MicroVax with (hellish expensive) software that they wrote. The data decoder has an antenna socket and an RS-232 socket (or two for the fancy kind that can receive multiple channels simultaneously on different output ports). These cost in the region of 200 Pounds Sterling. In the UK, both BBC and ITV provide data broadcast capacity for sale (the horse racing betting shops use it to broadcast digitized audio commentaries of races, for instance) and in the US, PBS has been using data broadcast for some time too, though for what I don't know. I'm currently working on data broadcast head-end S/W myself (how else could I know all this trivia :-) but, alas, I do not own the rights to it :-( so I cannot publish any on the net. My system runs on a '386 under UNIX or Xenix, and with a DOS PC containing a PC Teletext broadcast card to convert the data to composite video format. The data rate is largely limited to what the data decoder can handle. For 625 line systems with 25 frames (50 fields) per second, the nett data rate is about 15Kbps per VBI line used; in our case we have chosen to use 8 VBI lines, which is as much as our data decoders can accomodate. I could go on and on and on (just say the word :-) but your eyes are surely glazing over by now. If you know of anybody who's interested in actually buying any of this stuff, mail me for names and numbers. Jeremy Druker jsd@proxima.UUCP !uunet!ddsw1!olsa99!proxima!jsd