bt865a Conexant Systems, Inc., bt865a Datasheet - Page 35

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

Bt864A/Bt865A Data Sheet
1.9
100138C
02/17/03
Teletext
Closed-caption will override EVBI inserted data on lines 21 and 284 for 525-line
formats, and lines 22 and 335 for 625-line formats. Closed-caption will be overridden
by teletext if teletext is enabled on these lines.
Closed caption data registers are double buffered and can be loaded without the risk of
corrupting data as it is being encoded onto the appropriate video line (line 21 or 284
for 525-line formats, line 22 or 335 for 625-line formats).
Teletext encoding is accomplished via a two-wire interface, TTXDAT and TTXREQ,
and internal registers that are programmed through the I
encoding in the Bt864A/Bt865A conforms to Teletext B for 625/50 television
systems. See “Recommendation 653-1 Teletext Systems” for further information
about the standard. Teletext should be disabled for 525-line television systems.
The internal registers allow for the enable/disable of teletext and the programming of
the start and stop of the TTXREQ signal, the active teletext lines in an FIELD 1, and
the active teletext lines in FIELD 2. Active teletext lines override closed caption,
Macrovision, the BLANK* input, and active video. See the “Internal Registers”
section for more details.
The TTXREQ signal is generated by the encoder to indicate to external devices when
teletext data is required. The start and end of the TTXREQ signal waveform is
determined by the value of registers TXHS and TXHE, respectively. The values of
registers TXHS and TXHE correspond to the internal clock (CLK) counter (see
Figure
propagation delay of the teletext source, so that the teletext data is provided to the
TTXDAT pin at the proper time. If the falling edge of TTXREQ does not occur by the
end of the video line, the beginning of the new video line will automatically reset
TTXREQ. Legal values for these registers are given in
The data to TTXDAT is sampled on every rising input clock edge and must meet the
following protocol for proper teletext data insertion. The protocol demands that the
teletext data bit duration is the required number of CLKs.
Internal to the chip is a sequencer and a data shaper to minimize the jitter. Using the
midpoint of the falling edge of the horizontal sync pulse as it appears at the output Y/
CVBS or CVBS/G, the teletext data protocol must begin 262 to 264 CLKs later for
CCIR601(13.5 MHz pixel rate) or 286 to 288 CLKs later for Square Pixel Operation
(14.75 MHz pixel rate). Relative to the internally generated teletext window, the
protocol must start 5 to 7 clocks earlier. The teletext window begins at 10.2 µsec from
the horizontal sync pulse's falling edge and the data rate is the specified 6.9375 Mbits/
sec.
1-19). The proper selection of these edges will allow the user to adjust for the
Conexant
Table
2
C interface. Teletext
1-5.
Circuit Description
1-25