CX25870/871
1.0 Functional Description
Flicker-Free Video Encoder with Ultrascale Technology
1.3 Device Description
1.3.34 Closed Captioning (CC)
The CX25870/871 encodes NTSC/PAL–M closed captioning on scan line 21, and
NTSC/PAL–M extended data services on scan line 284. Four 8-bit registers
(CCF1B1, CCF1B2, CCF2B1, and CCF2B2) provide the data while bits ECCF1
and ECCF2 enable display of the data. A logical 0 corresponds to the blanking
level of 0 IRE, while a logical 1 corresponds to 50 IRE above the blanking level.
Closed captioning for PAL–B, D, G, H, I, N, Nc is similar to that for NTSC.
Closed-caption (CC) encoding is performed for 625-line systems according to the
system proposed by the National Captioning Institute; clock and data timing is
identical to that of NTSC system, except that encoding is provided on lines 22
and 335, for closed captioning and extended data services, respectively.
The CX25870/871 generates the clock run-in and appropriate timing
automatically. Pixel inputs are ignored during CC encoding. See FCC Code of
Federal Regulations (CFR) 47 Section 15.119 (10/91 edition or later) for
programming information. The EIA608 standard describes ancillary data
applications for Field 2 Line 21 (line 284).
When CCF1B2 is written, CCSTAT_O is set; when CCF2B2 is written,
CCSTAT_E is set. After the CC bytes for the odd field are encoded, CCSTAT_O
is cleared; after the CC bytes for the even field are encoded, CCSTAT_E is
cleared. If the ECCGATE bit is set, no further encoding is performed until the
appropriate registers are written again; a null is transmitted on the appropriate CC
line in that case. If the ECCGATE bit is not set, the user must rewrite the CC
registers prior to reaching the CC line; otherwise the last bytes are re-encoded.
The CC data bytes are double-buffered to prevent loss of data during the encoding
process.
Pseudo-code that can be used to create a software function for Closed Caption
Encoding is included as Appendix D.
100381B
Conexant
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CONEXANT [ CONEXANT SYSTEMS, INC ]
