SAA5531 Philips Semiconductors, SAA5531 Datasheet - Page 53

• Manufacturer Part Number: SAA5531
• Description: Enhanced TV microcontrollers with On-Screen Display OSD
• Manufacturer: Philips Semiconductors

Philips Semiconductors
18.1.6.7
One of the uses of packet 26 is to transmit characters
which are not in the basic teletext character set. The family
automatically decodes packet 26 data and, if a character
corresponding to that being transmitted is available in the
character set, automatically writes the appropriate
character code into the correct location in the teletext
memory. This is not a full implementation of the packet 26
specification allowed for in level 2 teletext, and so is often
referred to as level 1.5.
By convention, the packets 26 for a page are transmitted
before the normal packets. To prevent the default
character data overwriting the packet 26 data the device
incorporates a mechanism which prevents packet 26 data
from being overwritten. The mechanism is disabled when
the Spanish national option is detected as the Spanish
transmission system sends even parity (i.e. incorrect)
characters in the basic page locations corresponding to
the characters sent via packet 26 and these will not
overwrite the packet 26 characters anyway. The special
treatment of Spanish national option is prevented if
TXT12.ROM VER R4 is logic 0 or if the TXT8.DISABLE
SPANISH is set.
Packet 26 data is processed regardless of the
TXT1.EXT PKT OFF bit, but setting theTXT1.X26 OFF
disables packet 26 processing.
The TXT8.PKT26 RECEIVED bit is set by the hardware
whenever a character is written into the page memory by
the packet 26 decoding hardware. The flag can be reset by
writing a logic 0 into the SFR bit.
18.1.6.8
The 525-line format is similar to the 625-line format but the
data rate is lower and there are less data bytes per packet
(32 rather than 40). There are still 40 characters per
display row so extra packets are sent each of which
contains the last 8 characters for four rows. These packets
can be identified by looking at the ‘tabulation bit’ (T), which
replaces one of the magazine bits in 525-line teletext.
When an ordinary packet with T = 1 is received, the
decoder puts the data into the four rows starting with that
corresponding to the packet number, but with the 2 LSBs
set to logic 0. For example, a packet 9 with T = 1
(packet X/1/9) contains data for rows 8, 9, 10 and 11.
2000 Feb 23
Enhanced TV microcontrollers with
On-Screen Display (OSD)
Packet 26 processing
525-line World System Teletext
53
The error checking carried out on data from packets with
T = 1 depends on the setting of the TXT1. 8-BIT bit and the
error checking control bits in the page request data and is
the same as that applied to the data written into the same
memory location in the 625-line format.
The rolling time display (the last 8 characters in row 0) is
taken from any packets X/1/1, 2 or 3 received. In parallel
magazine mode only packets in the correct magazine are
used for rolling time. Packet number X/1/0 is ignored.
The tabulation bit is also used with extension packets.
The first 8 data bytes of packet X/1/24 are used to extend
the Fastext prompt row to 40 characters. These characters
are written into whichever part of the memory the
packet 24 is being written into (determined by the
‘X24 POSN’ bit).
Packets X/0/27/0 contain 5 Fastext page links and the link
control byte and are captured, Hamming checked and
stored in the same way as are packets X/27/0 in 625-line
text. Packets X/1/27/0 are not captured.
Because there are only 2 magazine bits in 525-line text,
packets with the magazine bits all set to a logic 0 are
referred to as being in magazine 4. Therefore, the
broadcast service data packet is packet 4/30, rather than
packet 8/30. As in 625-line text, the first 20 bytes of
packet 4/30 contain encoded data which is decoded in the
same way as that in packet 8/30. The last 12 bytes of the
packet contains half of the parity encoded status message.
Packet 4/0/30 contains the first half of the message and
packet 4/1/30 contains the second half. The last 4 bytes of
the message are not written into memory. The first
20 bytes of the each version of the packet are the same so
they are stored whenever either version of the packet is
acquired.
In 525-line text each packet 26 only contains ten 24/18
Hamming encoded data triplets, rather than the 13 found
in 625-line text. The tabulation bit is used as an extra bit
(the MSB) of the designation code, allowing 32 packet 26s
to be transmitted for each page. The last byte of each
packet 26 is ignored.
Preliminary specification
SAA55xx