In the absence of certain packets X/28, Level 2.5 and 3.5 decoders may chose to assume that a particular page is not a basic text page and has a different coding scheme from the default defined below. This decision could be made following:
1) inspection of the data in the Magazine Organization Table(MOT); or 2) inspection of the links in packets X/27/4 of other pages; or
3) because a Code of Practice allocates fixed page numbers for particular functions. D.1.1 Missing packets X/1 to X/25
NOTE: Basic text pages and DRCS downloading pages may be transmitted row adaptively. For Object definition pages, packets X/1 to X/25 if required should be used sequentially, starting with packet X/1, and unused rows need not be transmitted. 1) If any decoder assumes the page is a basic text page, any missing rows in the range X/1 to X/23
should be displayed as if they contained 40 character spaces (code 2/0). Where required, packets X/24 and X/25 should be treated in the same way.
2) If a Level 2.5 or 3.5 decoder believes the page to be a DRCS downloading page, the absence of particular packets may be interpreted by a decoder as an error condition if the DRCS character(s) containing the PTUs in those packets is selected for display, or a downloading mode is specified in packets X/28/3 that refers to the PTUs in the missing packets.
3) If a Level 2.5 or 3.5 decoder believes the page to be an Object definition page, the absence of a particular packet prior to the Termination Marker triplet may be interpreted as an error condition. Under these circumstances the response of a decoder is not defined here.
D.1.2 No packets X/26
1) Level 1.5 decoders should assume that the display of the page does not require characters or symbols which cannot be selected via codes in packets X/1 to X/24.
2) Level 2.5 and 3.5 decoders should assume that there are no enhancements to the page other than default objects, if any, or defaults from any packets X/28/0 Format 1, X/28/4, or M/29/0,4.
D.1.3 Packets X/26 with non-sequential designation codes
Packets X/26 should be transmitted with sequential designation codes, starting at 0000. Consequently, the absence of a packet with a particular designation code from a sequence of packets X/26 and prior to a Termination Marker triplet may be interpreted as an error condition by a decoder. Under these circumstances the response of a decoder is not defined here.
D.1.4 No packet X/27/4
Level 2.5 and 3.5 decoders should assume that the Magazine Organizational Table (MOT) contains valid page numbers for the Object and DRCS pages required to display the associated page.
D.1.5 No packet X/28/0 Format 1
1) A Level 2.5 decoder may assume that the associated page (packets X/1 to X/25) is a Level 1 text page, each 8-bit byte coded with 7 data bits and 1 odd parity bit. A Level 3.5 decoder should check for packets X/28/3 and X/28/4 before making the same assumptions.
2) The decoder should check for packet M/29/0 before loading default values for the following: - Primary/Secondary G0/G2 character set designation;
- Side-panels; - CLUTs 2 and 3; - Default screen colour; - Default row colour;
- Black background colour substitution; - Colour Table re-mapping.
D.1.6 No packet X/28/1
A Level 3.5 decoder should check for packet M/29/1 before loading the DCLUTs with default colours. D.1.7 No packet X/28/3
1) A Level 3.5 decoder should check for packets X/28/0 Format 1 and X/28/4 before assuming that the associated page (packets X/1 to X/25) is a Level 1 text page, each 8-bit byte coded with 7 data bits and 1 odd parity bit.
2) If a Level 3.5 decoder believes that the page is a DRCS downloading page, the absence of this packet should be taken as an indication that the page defines 12x10x1 mode DRCS characters only.
D.1.8 No packet X/28/4
1) A Level 3.5 decoder should check for packets X/28/0 Format 1 and X/28/3 before assuming that the associated page (packets X/1 to X/25) is a Level 1 text page, each 8-bit byte coded with 7 data bits and 1 odd parity bit.
2) A Level 3.5 decoder should check for packet M/29/4 before loading CLUTs 0 and 1, with the normal full and reduced intensity colours, including "transparent" at entry 0 in CLUT 1.
D.2
Magazine level
These assumptions are required to be implemented by Level 2.5 and 3.5 decoders only. They should be applied as a set of default conditions to all pages in the magazine in the absence of particular packets M/29 and specific page-related data.
D.2.1 No packet M/29/0
These default conditions apply to both Level 2.5 and 3.5 decoders in the absence of a packet X/28/0 Format 1 in magazine M.
1) In the absence of a local (i.e. country specific) Code of Practice, the default G0 and G2 character sets should be assumed to be the Latin G0 Primary and Latin G2 Supplementary sets shown in tables 35 and 37 respectively. Thus the default value from table 32 to be assumed for the Default
G0 and G2 Character Set Designation and National Option Selection bits is 0000XXX. The C12,
C13 and C14 controls bits select a national option sub-set.
2) A default value for the Second Language Definition bits is not specified here since this is decoder dependent. It may be covered in a local Code of Practice.
3) The side-panel requirements are determined from the data in page's public object link in the MOT. In the absence of a MOT, no side-panels should be displayed.
4) CLUTs 2 and 3 should be filled with the default colours shown in table 30. 5) The screen colour is black, i.e. Colour Map entry 0.
6) The row colour for all rows is black, i.e. Colour Map entry 0.
7) The requirement for black background substitution by full screen/row colour is determined from data in the page's public object link in the MOT. In the absence of a MOT, black background is not substituted.
8) No re-mapping of the spacing attribute colours is required, i.e. CLUT 0 should be used for both foreground and background colours.
D.2.2 No packet M/29/1
In the absence of a packet X/28/1 for a page in magazine M, a Level 3.5 decoder should load the DCLUTs as follows:
1) The DCLUT4 for both global and normal 12x10x2 DRCS modes should be filled, in ascending address order, with the first four full intensity colours, i.e. entries 0 to 3 from CLUT 0.
2) The DCLUT16 for both global and normal 12x10x4 and 6x5x4 DRCS modes should be filled, in ascending address order, with the normal full and reduced intensity colours, i.e. CLUTs 0 and 1, including "transparent".
D.2.3 No packet M/29/4
In the absence of a packet X/28/4 for a page in magazine M, a Level 3.5 decoder should load CLUTs 0 and 1 with the normal full and reduced intensity colours, including "transparent" at entry 0 of CLUT 1.
D.3
Service level
These assumptions should be implemented by all decoders, regardless of their Presentation Level capabilities.
1) If a packet 8/30/1 or a packet 8/30/3 is not transmitted during the VBI period, the Teletext transmission, if present, should be assumed to be confined to the VBI period.
2) In the absence of any packet 8/30/0, 8/30/1, 8/30/2 or 8/30/3 in the Teletext transmission, it can be assumed that the page number 100 is being broadcast and is suitable for use as an introductory page.
NOTE: A decoder may chose to display a user-specified page in preference to the Initial page indicated via packet 8/30, or page number 100.