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參數(shù)資料
| 型號(hào)： | TDA938X |
| 廠商： | NXP Semiconductors N.V. |
| 英文描述： | TV signal processor-Teletext decoder with embedded m-Controller |
| 中文描述： | 電視信號(hào)處理器與嵌入式米圖文電視解碼器控制器 |
| 文件頁(yè)數(shù)： | 55/140頁(yè) |
| 文件大小： | 570K |
| 代理商： | TDA938X |
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Philips Semiconductors
Preliminary specification
1999 Sep 28
55
TV signal processor-Teletext decoder with
embedded
μ
-Controller
TDA 935X/6X/8X series
P
ACKET
26 P
ROCESSING
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 over writing the packet 26 data the device incorporates a mechanism which prevents packet 26
data from being overwritten. This 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 over write 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. Packet 26 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.
525 L
INE
W
ORLD
S
YSTEM
T
ELETEXT
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 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 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 by 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 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.
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