
2004 Sep 03
63
Philips Semiconductors
Product specication
Enhanced TV microcontrollers with
On-Screen Display (OSD)
SAA56xx
handbook, full pagewidth
x00
x01
x02
x03
x04
x05
x06
x07
x08
x09
x0a
x0b
x0c
x0d
x0e
x0f
x10
x11
x12
x13
x14
x15
x16
x17
x18
x19
x1a
x1b
x1c
x1d
x1e
x1f
x20
x21
x22
x23
x24
x25
x26
x27
x28
x29
x2a
x2b
x2c
x2d
x2e
x2f
x30
x31
x32
x33
x34
x35
x36
x37
x38
x39
x3a
x3b
x3c
x3d
x3e
x3f
xc0
xc1
xc2
xc3
xc4
xc5
xc6
xc7
xc8
xc9
xca
xcb
xcc
xcd
xce
xcf
xd0
xd1
xd2
xd3
xd4
xd5
xd6
xd7
xd8
xd9
xda
xdb
xdc
xdd
xde
xdf
xe0
xe1
xe2
xe3
xe4
xe5
xe6
xe7
xe8
xe9
xea
xeb
xec
xed
xee
xfef
xf0
xf1
xf2
xf3
xf4
xf5
xf6
xf7
xf8
xf9
xfa
xfb
xfc
xfd
xfe
xff
7xx
bit
7
Bytes in the table
bits in each byte
column
row n
n + 1
n + 6
n + 7
0
MGD160
816
24
32
39
6xx
5xx
4xx
3xx
2xx
1xx
8xx
Fig.22 Transmitted/subtitle page organisation.
22.1.6.7
Packet 26 processing
One of the uses of packet 26 is to transmit characters that
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, there is a
mechanism which prevents packet 26 data from being
overwritten. The mechanism is disabled when the Spanish
national option is detected because 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 the Spanish national option is disabled if bit
TXT12.SPANISH is cleared (logic 0) or if bit
TXT8.DISABLE SPANISH is set (logic 1).
Packet 26 data is processed regardless of bit
TXT1.EXT PKT OFF, but setting bit TXT1.X26 OFF
disables packet 26 processing.
Bit TXT8.PKT26 RECEIVED is set by the hardware
whenever the packet 26 decoding hardware writes a
character into the page memory. The flag can be reset by
writing a logic 0 into the SFR bit.
22.1.6.8
525-line World System Teletext
format but the data rate is lower and there are fewer data
bytes per packet (32 rather than 40). There are still
40 characters per display row so extra packets are sent,
each containing the last eight characters for four rows.
These packets can be identified by 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 two
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.
The error checking carried out on data from packets with
T = 1 depends on the setting of bit TXT1.8-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 eight 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 the rolling time. Packet number
X/1/0 is ignored.