bt865a Conexant Systems, Inc., bt865a Datasheet - Page 38

• Manufacturer Part Number: bt865a
• Description: Ycrcb To Ntsc/pal Digital Video Encoder
• Manufacturer: Conexant Systems, Inc.

Circuit Description
1.9.4
1.10
Figure 1-21. Typical CGMS Waveform
1-28
Teletext Clock Output
Copy Generation Management System (CGMS)
Encoding
0.786
0.286
1.0
V
0
The Bt864A/Bt865A can output the teletext clock from the TTXREQ pin by setting
TXRM = 1. In this mode, this teletext clock would only be output on active teletext lines
and each line would have exactly 360 clocks to be used to synchronize the teletext data to
the Bt864A/Bt865A. The rising edge of clock could be used to latch the data on the output
of the device providing the teletext data. The falling of the clock indicates that the
Bt864A/Bt865A has received the teletext data.
Copy Generation Management System (CGMS) is used in 525-line systems on lines
20 and 283 (a.k.a. line 20, field 2). The CGMS data is 20 bits long and is entered on
register bits WSDAT[20:1].
Although there is no clock run-in in CGMS, a reference pulse is provided
automatically.
CRC data is not calculated and must be provided by the user.
typical CGMS signal. Note that bit 1 is closest to the HSYNC pulse and bit 20 is
farthest. The amplitude of the CGMS pulses are 70 IRE when high, and 0 IRE when
low. For further CGMS details, see specifications EIA-J CPR-1202, EIA-J CPR-1204,
and IEC 61880.
100
70
0
–40
IRE
Set register bit EWSF1 to 1 to enable CGMS on field 1, line 20.
Set register bit EWSF2 to 1 to enable CGMS on field 2, line 283.
11.2 µs ± 0.3 µs
Reference
Conexant
Line 20/283
Bit No.
49.1 µs ± 0.44 µs
1
2.235 µs ± 50 ns
2
3
.. .. ..
20
Figure 1-21
Bt864A/Bt865A Data Sheet
100138_036
illustrates a
100138C
02/17/03