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Circuit Descriptions, Abbreviation List, and IC Data Sheets

9.6

Video: TV-board (diagrams A4 and A5)

Figure 9-3 Block diagram video processing

The video processing is completely handled by a one-chip
video processor: the TDA888x. This IC is called BOCMA
(Bimos One Chip Mid-end Architecture) and contains:

•

IF demodulator.

•

Chrominance decoder

•

Sync separator.

•

Horizontal & vertical drive.

•

RGB processing.

•

CVBS and SVHS source select.

It has also build in features like:

•

CTI.

•

Black stretch.

•

Blue stretch.

•

White stretch.

•

Slow start up.

•

Dynamic skin tone correction etc.

Further, it also incorporates sound IF traps and filters, and
requires only one crystal for all systems.

9.6.1

Source selection

The BOCMA has an input for the internal CVBS signal and two
inputs for external CVBS or Y signals. The circuit has only one
chroma input so that it is not possible to apply two separate Y/
C inputs.
The selection of the various sources is made via the I2C-bus.
The used IC version has two independently switchable outputs:

•

The CVBS1 output (pin 54) is identical to the selected
signal that is supplied to the internal video processing
circuit and is used as source signal for the teletext decoder
(Painter). Both CVBS outputs have an amplitude of 2.0
V_pp.

•

The CVBS2 output (pin 26) is fed to pin 19 of SCART2 for
WYSIWYR (What You See Is What You Record).

If the Y3/C3 signal is selected for one of the outputs, the
luminance and chrominance signals are added so that a CVBS
signal is obtained again.

9.6.2

Analogue Comb filter

After the selection of the external signals (EXT1, EXT2 or Side
I/O) is made, the Y/CVBS signal is fed to the TDA888x (pin 29)
along with the Front-end signal (pin 24). The selection between
the two is made in the BOCMA and the output (pin 26) is fed to
the (optional) 2-line comb filter (to separate the luminance from
the chroma). The comb filter output is again fed to the BOCMA
(pin 21) for further processing.
The external colour signals are also fed to the comb filter. In
SVHS mode the comb filter is bypassed and the external
signals are directly fed to the BOCMA.
Switching the comb filter is done via pen 49 of IC7301. The
"video standard" selection is done via the SYS1 and SYS2
signals from the microprocessor. If the comb filter is not used,
jumpers are present and the external Y/C signals are directly
fed to the pins 20, 21 of IC7301.
After this stage, the external RGB signals (from SCART) are
added to pins 40 - 44.

9.6.3

Histogram (YUV picture improvement) IC

The demodulated video-signal can be checked on pins 40, 45,
and 46 of IC7301 and is fed to pins 39, 47, and 48. In this path,
the Histogram IC TDA9171 can be inserted. Without this IC,
jumpers are used.
This TDA9178 can control various picture improvements:
histogram processing, colour transient improvement and
luminance transient improvement.

•

Sets without TDA9178; for sets without TDA9178, the
Dynamic Skin Tone Control, Blue Stretch, and Green
Enhancement are controlled by the BOCMA.

CL 36532023_068.eps

140403

7301
TDA888X-B

CVBS_TER_OUT

CVBS2OUT

CVBS1OUT

CVBSINT

CVBSY2

CVBSY3

CVBS2OUT