
Philips Semiconductors
Preliminary specification
1999 Sep 28
49
TV signal processor-Teletext decoder with
embedded
μ
-Controller
TDA 935X/6X/8X series
into the display block and automatically writes an alphanumeric green character into column 7 of row 0 of the
display block every TV line.
When a requested page header is acquired for the first time, rows 1 to 23 of the relevant memory block are
cleared to space, i.e. have 20H written into every column, before the rest of the page arrives. Row 24 is also
cleared if the TXT0.X24 POSN bit is set. If the TXT1.EXT PKT OFF bit is set the extension packets
corresponding to the page are also cleared.
The last 8 characters of the page header are used to provide a time display and are always extracted from every
valid page header as it arrives and written into the display block.
The TXT0.DISABLE HEADER ROLL bit prevents any data being written into row 0 of the page memory except
when a page is acquired off air i.e. rolling headers and time are not written into memory. The TXT1.ACQ OFF
bit prevents any data being written into the memory by the teletext acquisition section.
When a parallel magazine mode transmission is being received only headers in the magazine of the page
requested are considered valid for the purposes of rolling headers and time. Only one magazine is used even
if a don’t care magazine is requested. When a serial magazine mode transmission is being received all page
headers are considered to be valid.
E
RROR
C
HECKING
Before teletext packets are written in to the page memory they are error checked. The error checking carried
out depends on the packet number, the byte number, the error check mode bits in the page request data and
the TXT1.8-BIT bit.
If an uncorrectable error occurs in one of the Hamming checked addressing and control bytes in the page
header or in the Hamming checked bytes in packet 8/30, bit 4 of the byte written into the memory is set, to act
as an error flag to the software. If uncorrectable errors are detected in any other Hamming checked data the
byte is not written into the memory.