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ATS/T\ATS07
This website contains an archive of files for the Acorn Electron, BBC Micro, Acorn Archimedes, Commodore 16 and Commodore 64 computers, which Dominic Ford has rescued from his private collection of floppy disks and cassettes.
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| Tape/disk: | Home » CEEFAX disks » telesoftware2.adl |
| Filename: | ATS/T\ATS07 |
| Read OK: | ✔ |
| File size: | 2DB0 bytes |
| Load address: | 0000 |
| Exec address: | 0000 |
File contents
Interactive programming J.J.Brayshaw 1987
Module 7 in our 'interactive programming' series looks at the sub-page
coding system for teletext pages. This extension of the page-numbering
system makes differentiation between elements within a cycling page
sequence a simple matter. However, this was not the idea of sub-page
numbering when it was first conceived. Read on to find what it was first
designed for, and how we can make use of it now ...
__________________________________________________________________________
Teletext Interactive Programming:
Advanced Teletext System. 7
(c) J.J.Brayshaw 1987
Sub-Page coding:
In the early days of teletext, CEEFAX was broadcast using only two of the
spare television picture lines. This meant only a very small quantity of
pages could be made available before the access time became intollerably
slow.
The designers thought about the problem, and came up with the idea of
transmitting different pages using the same page number. This would keep
the overall cycle time (the maximum time taken to receive any transmitted
page number) to a minimum, yet allow an almost unlimited volume of
information to be broadcast. The theory went like this:
Viewers wanting the City share prices can call up page 140. The page
carrying this information will be transmitted every hour, at a
pre-determined time (say 5 minutes past the hour). Other viewers wanting
the Recipe of the day could also call up page 140. The page they would be
interested in would be broadcast every hour, this time at, for example, 15
minutes past the hour.
In order to ensure the City viewers do not inadvertantly study the day's
recipe and end up buying shares in Beef Stew, the two different pages
(shares and recipes) must have a differentiating feature that a decoder
can use to eliminate one yet display the other.
The concept, therefore, was to allow the viewer to select both a
particular page and a particular time of day. Only the page transmitted
with both features matching would be displayed. Anyone intersted in both
subjects could specify just the page number, and all frames broadcast on
that page number would be displayed as received.
It is easy now to see why this coding has been given its present limiting
format. Originally called the 'Time Sub-Code', it is now more aptly known
Page 1
Interactive programming J.J.Brayshaw 1987
as the page sub-code, or sub-page number.
As the sub-coding was designed to reflect the time of day, it was
allocated four hex digits to correspond directly to hours and minutes.
These had to range from 00:00 (midnight) to 23:59 (11.59 p.m.). If we
examine this structure closely, we can see the logic that now limits all
sub-page coding:
Digit 1 (tens of hours) need only ever be 0, 1, or 2. This means all
possibilities are covered using just two bits of a nybble (a nybble being
a four-bit binary number (0000 to 1111), or a single HEX digit (0 to F).
As transmission time and space is critical in teletext broadcasts, the
remaining 2 bits from this nybble could be used for other purposes. Using
two bits actually gives us more possibilites than are needed. The maximum
value for this digit was (and still is) 3 (represented by a binary 11).
Similar logic applied to the second digit (the units of hours) shows that
a range from 0 to 9 is needed. This requires all four bits of the nybble
to be used (the number nine being represented in binary as 1001). Once
again, this gives more possible combinations than are needed, and this
digit can actually represent any figure up to &F (15).
The first digit of the minutes must be able to range from zero to five.
Requiring only 3 of the 4 bits in the nybble, leaving the high bit for
other purposes, gives us a possible maximum in this digit of 7 (binary
111).
Finally, the units of minutes must range from zero to nine (again
requiring all four bits of a nybble). The maximum here, then, is &F (15).
The sub-page code can therefore be a maximum of &3F7F. This is the maximum
for each digit - not all the codes between 0 an &3F7F are possible! (For
example, a sub-page code of &0080 is not possible, as the third digit
cannot exceed 7).
So the sub-page coding is restricted due to this historical legacy. The
sub-page coding is still in use as a 'time-code' on the CEEFAX alarm page
(page 196). The TIME button on a teletext tv's handset will explicitly
select a specific sub-page code to be received. By requesting the alarm
at, for example, 11:25 a.m., you are requesting page 196 sub-page 1125,
which just happens to be broadcast at exactly 11:25 a.m.
The Alarm page is the only page that is being transmitted which uses the
sub-page coding as the original designers expected. Other cycling
sub-pages use this time-code as a simple method for decoders to
differentiate between consecutive pages in a rolling sequence. The fact
that CEEFAX is now broadcast on three times as many television picture
lines as the original transmissions used, means the original purpose for
Page 2
Interactive programming J.J.Brayshaw 1987
the time code is now largely obsolete.
As the sub-page numbering was a part of the original Teletext concept, it
is included in the first row of the page (row zero). This was the only
packet of the original transmission format that contained some
non-displayable bytes (other than row and magazine addresses). Packet 27
was introduced some years later!
Studying packet zero, we find, as with all other packets, there are 45
bytes. The first three bytes are clock run-in and framing codes which
allow the decoder to judge the exact speed of the incoming bits and to
tell the start and end position of each byte. The next two bytes are the
magazine number and row number. So far, all this is identical to all other
packets in the page. The next two bytes (bytes 5 and 6 counting fom zero)
are the page number in Hamming coded form. Packet zero is the only row to
contain the page number - all packets received after packet zero are
assumed to be in the same page, until a different packet zero is received.
Bytes 7, 8, 9 and 10 contain the sub-page coding in Hamming coded form.
This means each byte decodes to a four-bit nybble representing each digit
of the sub-page coding. Two control bytes follow (these will be explained
in future modules) and the packet is completed with 32 bytes of data.
When a page is requested using ATS, the Hamming-coded bytes are decoded
before being placed in computer memory. This makes our task of identifying
the sub-page code a very simple one. Packet zero takes on a different
format after being processed by the ATS. The clock run-in and framing
codes are not transferred. Each pair of eight-bit Hamming coded bytes have
been translated to two four-bit nybbles which have been concatenated to
form a single byte. This, of course, creates a few gaps in the row of
data! The ATS fills these with a zero to maintain packet length
compatibility with all the other received packets. The format of row zero,
then, after ATS decoding, is:
--------------------------------------------------------------/ /-----
| Byte: 0 1 2 3 4 5 6 7 8 9 10 /..../ 42 |
|-------------------------------------------------------------/ /----|
| Meaning: Mag Row Page Sub-page Control 0 0 0 data .... data|
| No. No. No. High Low Bytes Pad chrs |
--------------------------------------------------------------/ /-----
So, to find the sub-page code, we simply use the *TRANSFER command (or
*DATA if we also wish to perform a CRC to test the integrity of the
received page) then examine the appropriate bytes directly. All the
de-Hamming and exclusion of the unwanted bits have been carried out for
us! Furthermore, packet zero is the only one of the received packets whose
precise position in memory we already know. When a page is received, the
packets can be in any order with the exception of row zero. This row must
always be received first, as it is used to identify the start of the
Page 3
Interactive programming J.J.Brayshaw 1987
transmission of the page, and so it will always occupy the beginning of
our allocated page memory store.
So, after a *TRANSFER or *DATA to, for example, location &7000, packet
zero must start at location &7000. The sub-page data is in bytes 3 and
4, so a simple routine to read them is possible:
sub_page%=((&7000?3)*256)+(&7000?4)
If we need to use this information to describe the page (or, for example,
to request the next sequential sub-page provided it is being transmitted)
we need to ensure the number is complete with all leading zeros. This can
be achieved easily by turning the sub-page number into a string, adding a
string of four zeros at the beginning, then taking the last four digits of
the result:
sub_page$=RIGHT$("0000"+STR$~sub_page%,4)
This can then be added to the magazine and page numbers in string form to
create a suitable OSCLI command if required:
pn$=STR$~(mag%)+RIGHT$("00"+STR$~(page%),2)+"s"+sub_page$
PROCoscli("TRANSFER "+pn$)
In practice, however, not all the possible sub-code numbers in the
sequence are always used. It is normal to transmit pages with sub-page
coding from 1 to 9, then miss sub-pages &A to &F and start again at
sub-page &10. This makes the logical progression of rolling sub-pages in a
sequence appear to be numbered in decimal, from 1 to 9, then 10 to 19,
etc. However, it is possible to have sub-page sequences that include the
hex numbers also! There are times when sub-page numbering is quite
erratic, and it cannot be relied upon that sub-page numbers will always be
sequential in any given rolling sequence.
There are two special cases to sub-page coding:
When a sub-page code is given as zero, it signifies it is the only one on
that page number, and is therefore a 'static' page;
If the sub-page code is given as &3F7F, this also means it is a static
page and should be treated as sub-page 0.
It is not always reliable to depend on the displayed page numbering on a
set of rolling sub-pages (as given at the right-hand end of packet 1).
This is only added as an editorial feature, and may, from time to time, be
inaccurate, or in fact not be present at all. There is, unfortunately, no
100% reliable method for a program to determine how many sub-pages make up
a particular rolling sequence. This information is simply not transmitted!
Page 4
Interactive programming J.J.Brayshaw 1987
The best we can do is to rely on a combination of sub-page numbering and
the displayed page numbers in packet one.
__________________________________________________________________________
The program accompanying this module will receive a page from a rolling
sequence (actually from CEEFAX page 711, but this can easily be altered),
perform a CRC (cyclic redundancy check) on the page data, then decode its
sub-page number. The results of both checks are given on row 25 of the
display.
The telesoftware program "CEE/Vid" will receive a complete cycling
sequence of rolling sub-pages on any given page number, using some of the
methods described above, and store the sequence as a VidiEd magazine for
use with the VidiEd package recently broadcast.
Page 5
00000000 0d 0d 0d 0d 49 6e 74 65 72 61 63 74 69 76 65 20 |....Interactive |
00000010 70 72 6f 67 72 61 6d 6d 69 6e 67 20 20 20 20 20 |programming |
00000020 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
00000030 20 20 20 20 20 20 20 20 20 20 20 20 20 4a 2e 4a | J.J|
00000040 2e 42 72 61 79 73 68 61 77 20 31 39 38 37 0d 0d |.Brayshaw 1987..|
00000050 0d 0d 0d 4d 6f 64 75 6c 65 20 37 20 69 6e 20 6f |...Module 7 in o|
00000060 75 72 20 27 69 6e 74 65 72 61 63 74 69 76 65 20 |ur 'interactive |
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00000080 69 65 73 20 6c 6f 6f 6b 73 20 61 74 20 74 68 65 |ies looks at the|
00000090 20 73 75 62 2d 70 61 67 65 0d 63 6f 64 69 6e 67 | sub-page.coding|
000000a0 20 73 79 73 74 65 6d 20 66 6f 72 20 74 65 6c 65 | system for tele|
000000b0 74 65 78 74 20 70 61 67 65 73 2e 20 54 68 69 73 |text pages. This|
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00000130 73 69 6d 70 6c 65 20 6d 61 74 74 65 72 2e 20 48 |simple matter. H|
00000140 6f 77 65 76 65 72 2c 20 74 68 69 73 20 77 61 73 |owever, this was|
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000001e0 6f 77 20 2e 2e 2e 0d 5f 5f 5f 5f 5f 5f 5f 5f 5f |ow ...._________|
000001f0 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f |________________|
*
00000230 5f 0d 0d 0d 54 65 6c 65 74 65 78 74 20 49 6e 74 |_...Teletext Int|
00000240 65 72 61 63 74 69 76 65 20 50 72 6f 67 72 61 6d |eractive Program|
00000250 6d 69 6e 67 3a 0d 0d 41 64 76 61 6e 63 65 64 20 |ming:..Advanced |
00000260 54 65 6c 65 74 65 78 74 20 53 79 73 74 65 6d 2e |Teletext System.|
00000270 20 20 37 0d 0d 28 63 29 20 4a 2e 4a 2e 42 72 61 | 7..(c) J.J.Bra|
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00000900 79 20 63 61 6c 6c 65 64 20 74 68 65 20 27 54 69 |y called the 'Ti|
00000910 6d 65 20 53 75 62 2d 43 6f 64 65 27 2c 20 69 74 |me Sub-Code', it|
00000920 20 69 73 20 6e 6f 77 20 6d 6f 72 65 20 61 70 74 | is now more apt|
00000930 6c 79 20 6b 6e 6f 77 6e 0d 0d 0d 0d 0d 20 20 20 |ly known..... |
00000940 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
00000950 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 50 | P|
00000960 61 67 65 20 31 0d 0d 0d 0d 0d 0d 0d 0d 0d 49 6e |age 1.........In|
00000970 74 65 72 61 63 74 69 76 65 20 70 72 6f 67 72 61 |teractive progra|
00000980 6d 6d 69 6e 67 20 20 20 20 20 20 20 20 20 20 20 |mming |
00000990 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
000009a0 20 20 20 20 20 20 20 4a 2e 4a 2e 42 72 61 79 73 | J.J.Brays|
000009b0 68 61 77 20 31 39 38 37 0d 0d 0d 0d 0d 61 73 20 |haw 1987.....as |
000009c0 74 68 65 20 70 61 67 65 20 73 75 62 2d 63 6f 64 |the page sub-cod|
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000009e0 75 6d 62 65 72 2e 0d 0d 41 73 20 74 68 65 20 73 |umber...As the s|
000009f0 75 62 2d 63 6f 64 69 6e 67 20 77 61 73 20 64 65 |ub-coding was de|
00000a00 73 69 67 6e 65 64 20 74 6f 20 72 65 66 6c 65 63 |signed to reflec|
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00000a20 79 2c 20 69 74 20 77 61 73 0d 61 6c 6c 6f 63 61 |y, it was.alloca|
00000a30 74 65 64 20 66 6f 75 72 20 68 65 78 20 64 69 67 |ted four hex dig|
00000a40 69 74 73 20 74 6f 20 63 6f 72 72 65 73 70 6f 6e |its to correspon|
00000a50 64 20 64 69 72 65 63 74 6c 79 20 74 6f 20 68 6f |d directly to ho|
00000a60 75 72 73 20 61 6e 64 20 6d 69 6e 75 74 65 73 2e |urs and minutes.|
00000a70 0d 54 68 65 73 65 20 68 61 64 20 74 6f 20 72 61 |.These had to ra|
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00002bc0 66 6f 72 6d 20 61 20 43 52 43 20 28 63 79 63 6c |form a CRC (cycl|
00002bd0 69 63 20 72 65 64 75 6e 64 61 6e 63 79 20 63 68 |ic redundancy ch|
00002be0 65 63 6b 29 20 6f 6e 20 74 68 65 20 70 61 67 65 |eck) on the page|
00002bf0 20 64 61 74 61 2c 20 74 68 65 6e 20 64 65 63 6f | data, then deco|
00002c00 64 65 20 69 74 73 0d 73 75 62 2d 70 61 67 65 20 |de its.sub-page |
00002c10 6e 75 6d 62 65 72 2e 20 54 68 65 20 72 65 73 75 |number. The resu|
00002c20 6c 74 73 20 6f 66 20 62 6f 74 68 20 63 68 65 63 |lts of both chec|
00002c30 6b 73 20 61 72 65 20 67 69 76 65 6e 20 6f 6e 20 |ks are given on |
00002c40 72 6f 77 20 32 35 20 6f 66 20 74 68 65 0d 64 69 |row 25 of the.di|
00002c50 73 70 6c 61 79 2e 0d 0d 54 68 65 20 74 65 6c 65 |splay...The tele|
00002c60 73 6f 66 74 77 61 72 65 20 70 72 6f 67 72 61 6d |software program|
00002c70 20 22 43 45 45 2f 56 69 64 22 20 77 69 6c 6c 20 | "CEE/Vid" will |
00002c80 72 65 63 65 69 76 65 20 61 20 63 6f 6d 70 6c 65 |receive a comple|
00002c90 74 65 20 63 79 63 6c 69 6e 67 0d 73 65 71 75 65 |te cycling.seque|
00002ca0 6e 63 65 20 6f 66 20 72 6f 6c 6c 69 6e 67 20 73 |nce of rolling s|
00002cb0 75 62 2d 70 61 67 65 73 20 6f 6e 20 61 6e 79 20 |ub-pages on any |
00002cc0 67 69 76 65 6e 20 70 61 67 65 20 6e 75 6d 62 65 |given page numbe|
00002cd0 72 2c 20 75 73 69 6e 67 20 73 6f 6d 65 20 6f 66 |r, using some of|
00002ce0 20 74 68 65 0d 6d 65 74 68 6f 64 73 20 64 65 73 | the.methods des|
00002cf0 63 72 69 62 65 64 20 61 62 6f 76 65 2c 20 61 6e |cribed above, an|
00002d00 64 20 73 74 6f 72 65 20 74 68 65 20 73 65 71 75 |d store the sequ|
00002d10 65 6e 63 65 20 61 73 20 61 20 56 69 64 69 45 64 |ence as a VidiEd|
00002d20 20 6d 61 67 61 7a 69 6e 65 20 66 6f 72 0d 75 73 | magazine for.us|
00002d30 65 20 77 69 74 68 20 74 68 65 20 56 69 64 69 45 |e with the VidiE|
00002d40 64 20 70 61 63 6b 61 67 65 20 72 65 63 65 6e 74 |d package recent|
00002d50 6c 79 20 62 72 6f 61 64 63 61 73 74 2e 0d 0d 0d |ly broadcast....|
00002d60 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d |................|
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00002d80 0d 0d 0d 20 20 20 20 20 20 20 20 20 20 20 20 20 |... |
00002d90 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
00002da0 20 20 20 20 20 50 61 67 65 20 35 0d 0d 0d 0d 0d | Page 5.....|
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