[sci.electronics] Television VBI Data transmissions

gil@limbic.UUCP (Gil Kloepfer Jr.) (01/08/90)

As some folks may know, there is an space in the TV video signal called
the vertical blank interval.  There has been recent technology (close
captioning, "Silent Radio" system, etc) which is taking advantage of
some "dead space" in this interval and transmitting signals on it.
Those curious about it can turn the vertical hold control on their TV
set while watching a station transmitting such information, and will
see a little dashed line skipping along in that wide bar which will
roll up or down the screen.

My questions about this, for those in-the-know are:

	1.  Are there any FCC regulations or industry standards
	    which govern the VBI, and how signals are to be
	    superimposed here?

	2.  Specifically, how is closed captioning transmitted?  Is
	    it data, or a slow-scan representation of pixels to be
	    overlayed on the TV picture?

	3.  What is the best way to get at the information on the VBI?
	    Specifically, what is the best way to detect the start
	    and end of the "dead" portion where information would be
	    transmitted?

It seems that the information being stored here is a well-kept secret
much like the SCA subcarrier transmitted with FM broadcasts.  For those
who don't know, SCA, or subsidiary communications authorization, is a
subcarrier transmitted by many FM broadcast stations, and contains
programming such as Muzak, foreign-language broadcasts, medical news,
reading for the blind, etc.  I believe that one is supposed to subscribe
(ie. pay) to use these services for a business, but one can receive the
broadcasts using a box designed to receive the subcarrier (projects
for such boxes have been published in electronics hobby magazines) and
a standard FM radio.  I am looking to do the same kind of thing with VBI
information.

If you e-mail a response, please post it to the net also because I'm
sure that others will be interested in this subject.

------
| Gil Kloepfer, Jr.
| ICUS Software Systems/Bowne Management Systems (depending on where I am)
| ...ames!limbic!gil

markz@ssc.UUCP (Mark Zenier) (01/13/90)

In article <599@limbic.UUCP>, gil@limbic.UUCP (Gil Kloepfer Jr.) writes:
> 
> 	1.  Are there any FCC regulations or industry standards
> 	    which govern the VBI, and how signals are to be
> 	    superimposed here?

The FCC says where.  The what is up to the originator.  There are a couple
of systems used in the US.  NABTS is defined in an EIA standard.  The
version I have is called an interim standard (IS-14).  It may be real
by now.  World Teletext is derived from the UK system.

> 
> 	2.  Specifically, how is closed captioning transmitted?  Is
> 	    it data, or a slow-scan representation of pixels to be
> 	    overlayed on the TV picture?

ASCII.

> 
> 	3.  What is the best way to get at the information on the VBI?
> 	    Specifically, what is the best way to detect the start
> 	    and end of the "dead" portion where information would be
> 	    transmitted?

Most signals have a distinctive header that exists in a fixed relationship
to the horizontal sync.  Just look at every line to see if it fits.

A quick browse thru "Television Engineering" by Benson will give you a lot.
For the WTS system, try the Signetics Linear Data Book volume for Video
products.

markz@ssc.uucp