The Level
2.5 Manager enables you to configure services for
the transmission of Level 2.5 teletext. A number of
separate services can be managed. Each service is
configured on a magazine basis, and the configuration
data includes environment information (colours, side
panel configuration and default languages) as well
as details of the assignment of Level 2.5 objects
to pages. The configuration data is maintained on
disk. You can view and amend the data, and update
the transmisson services under manual control as and
when required.
The Manager
is designed to be used in conjunction with the Level
2 .5 Object Editor, which creates and modifies Level
2.5 objects and saves them to disk. The Level 2.5
Object Editor enables you to create and save the set
of instructions making up a Level 2.5 object. It is
designed to be used in conjunction with the Level
2.5 Manager, which assigns objects to the magazines
and pages of Level 2.5 transmission services.
The following
discussion assumes that you already have an understanding
of the Level 1 Teletext standard and some experience
of editing Level 1 pages using an application such
as Flair.
Level 2.5 is a Teletext standard providing new features
such as extra colours and better graphics to enhance
page appearance, whilst maintaining compatibility
with the installed base of level 1 equipment and keeping
page access times within acceptable limits.
The Level 2.5 data is transmitted along with standard
Level 1 Teletext information. Normal TV sets will
just display the Level 1 information as before, but
sets fitted with Level 2.5 decoders will show the
Level 1 page plus the extra level 2.5 information,
which is overlaid on the Level 1 page. To minimise
the amount of extra data to be sent, and hence reduce
the impact on page access times, Level 2.5 data can
be sent once on a per magazine basis, and then applied
to a number of individual pages using simple instructions.
Level 2.5
provides extra colours, non-spacing code, extra character
sets, dynamically redefineable characters, side panels
and objects.
The 8 colours
allowed in Level 1 are extended to 32 colours, organised
into 2 sets of 16 colours each. The first set consists
of fixed colours (the 8 basic colours and the 8 basic
colours in half-intensity). The second set defaults
to 16 pastel shades, but these can be redefined. The
foreground and background colours used on the basic
page can be remapped and any of the 32 available colours
may be used in the level 2.5 enhancements to a page.
In addition, a border colour and a default background
colour may be selected from any of the 32 available
colours.
Unlike
Level 1 Teletext codes, each of which occupies a character
position on the screen. Level 2.5 codes do not take
up any space. This means that several codes may be
inserted in the same position and you may for example
have a change of both foreground and background colour
in the middle of a word.
In addition
to the alphabetic (G0) and block mosaic (G1) character
sets available with Level 1, the Level 2.5 standard
provides supplementary language characters (G2) and
smoothed mosaics and line drawing characters (G3).
If you
cannot achieve the effect you want using the G3 characters,
you may define your own characters. Up to 24 of these
are allowed on one page.
Sixteen
additional columns of information can be attached
to a page. These columns may be placed on the left
to form a left side panel on the right to form a right
side panel, or split between the two sides.
An object
is a group of Level 2.5 Teletext instructions that
are parcelled up together for broadcast as a whole.
An object can be transmitted once and used many times
on different pages thus saving transmission capacity.
To add the object to a particular page, it is merely
necessary to include a simple instruction to invoke
a particular object number, rather than sending all
the instructions which make up the object.
Objects are created in the Level 2.5 Object Editor
and saved as object files with extension .TOB. The
Level 2.5 Manager imports these object files and assigns
them to groups for more efficient transmission. Groups
may in turn be assigned to the magazines and pages
of the current transmission service. Object definitions
are transmitted on GPOP and POP (and indirectly GDCRS
and DRCS) pages. The invocation of objects on particular
pages is achieved through the broadcast of Magazine
Organization Table (MOT) pages.
An object
is a graphical image or special colour effect that
is used to enhance the appearance of a Level 1 page.
A typical object might be a logo, a motif or a page
style.
As an
object is always displayed together with a page it
is important to consider the impact that the object
will have on the underlying page. This depends in
part on the object type. When you create a new object
it is active by default, but you may change its type
to adaptive or passive. The differences between the
three types of objects are explained below.
An active
object might be used to define the page style (eg.
banner headlines or page templates). At the start
(left-hand boundary) of the object the codes in the
Level 1 page remain in effect unless superseded by
codes within the object itself. The codes in effect
at the right-hand boundary of the object affect the
rest of the row in the Level 1 page unless they are
superseded by codes in the level one page.
An adaptive
object could be used to replace block mosaic graphics
with smoothed graphics or DRCS characters, while retaining
the colours in the underlying page. At the start (left-hand
boundary) of the object the codes in the Level 1 page
remain in effect unless superseded by codes within
the object itself. at the right-hand boundary of the
object, however, the settings revert to those of the
underlying page at that position.
A passive
object could typically be used to display a logo over
the Level 1 page. A passive object is not affected
by the codes in the underlying page and does not affect
the page outside its boundaries. At the start (left-hand
boundary) of the object the attributes are reset to
the defaults implied at the beginning of a row. The
codes in the object only affect the column positions
that actually contain an object character.
Invoking
Objects One object can call up another object, thus
enabling a larger object to be built up out of a number
of smaller ones. The rules are as follows:
