Cameron KS21TK305A, Руководство по обслуживанию

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SERVICE MANUAL
34
1.8.9 Chroma, luminance and feature processing
Some versions contain a 4H/2H (2D) adaptive PAL/NTSC comb filter. The comb filter is
automatically activated when standard CVBS signals are received. A signal is considered as
“
standard signal
”
when a PAL or NTSC signal is identified and when the vertical divider is in the
modes
‘
standard narrow window
’
or
‘
standard TV norm
’
.For non-standard signals and for SECAM
signals the comb filter is bypassed and the signal is filtered by means of bandpass and trap filters.
The chroma band-pass and trap circuits (including the SECAM cloche filter) are realized by
means of internal filters and are tuned to the right frequency by comparing the tuning frequency
with the reference frequency of the colour decoder.
The circuit contains the following picture improvement features:
•
Peaking control circuit. The peaking function can be activated for all incoming CVBS, Y/C and
RGB/YPBPR signals. Various parameters of the peaking circuit can be adapted by means of the
I2C-bus. The main parameters are:
–
Peaking centre frequency (via the PF1/PF0 bits in subaddress 19H).
–
Ratio of positive and negative peaks (via the RPO1/RPO0 bits in subaddress 47H). The peaks in
the direction
“
white
”
are the positive peaks.
–
Ratio of pre- and aftershoots (via the RPA1/RPA0 bits in subaddress 47H).
•
Video dependent coring in the peaking circuit. The coring can be activated only in the low-light
parts of the screen. This effectively reduces noise while having maximum peaking in the bright parts
of the picture.
•
Black stretch. This function corrects the black level for incoming signals which have a difference
between the black level and the blanking level. The amount of stretching (A-A in Fig. 105) and the
minimum required back ground to activate the stretching can be set by means of the I2C-bus
(BSD/AAS in subaddress 45H).
•
Gamma control. When this function is active the transfer characteristic of the luminance amplifier
is made non-linear. The control curve can be adapted by means of I2C-bus settings (see Fig. 107). It
is possible to make the gamma control function dependent on the picture content (Average Picture
Level, APL). The effect is illustrated in Fig. 108. Previously this function was mentioned under the
name
“
white stretch function
”
.
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