SAA4978 View Datasheet(PDF) - Philips Electronics
Part Name
Description
MFG CO.
SAA4978 Datasheet PDF : 56 Pages
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Philips Semiconductors
Picture Improved Combined Network
(PICNIC)
Product specification
SAA4978H
handbook, halfpage
1
40
120
SAA4978H
81
MHB174
Fig.3 Pin configuration.
7 FUNCTIONAL DESCRIPTION
The SAA4978H consists of the following main functional
blocks:
• Analog preprocessing and analog-to-digital conversion
• Digital processing at 1fH level
• Digital processing at 2fH level
• Digital-to-analog conversion
• Line-locked clock generation
• Crystal oscillator
• Control interfacing I2C-bus and SNERT
• Register I/O
• Programmable Signal Positioner (PSP)
• 80C51 microcontroller core
• Board level testability provisions.
7.1 Analog input blocks
7.1.1
GAIN ELEMENTS FOR AUTOMATIC GAIN CONTROL
(9 dB RANGE)
A variable amplifier is used to map the possible YUV input
range to the analog-to-digital converter range e.g. as
defined for SCART signals.
According to this specification, a lift of 6 dB up to a drop of
3 dB may be necessary with respect to the nominal values.
The gain setting within the required minimum 9 dB range
is performed digitally via the internal microcontroller.
For this purpose a gain setting digital-to-analog converter
is incorporated. The smallest step in the gain setting
should be hardly visible on the picture, this can be met with
smaller steps of 0.4%/step.
Luminance and chrominance gain settings can be
separately controlled. The reason for this split is that
U and V may have already been gain adjusted by an
Automatic Chrominance Control (ACC), whereas
luminance is to be adjusted by the SAA4978H AGC.
However, for RGB originated sources, Y, U and V should
be adjusted with the same AGC gain.
7.1.2
CLAMP CIRCUIT, CLAMPING Y TO DIGITAL LEVEL 32
AND UV TO 0 (TWOS COMPLEMENT)
A clamp circuit is applied to each input channel, to map the
colourless black level in each video line (on the sync back
porch) to level 32 at 9 bits for Y and to the centre level of
the converters for U and V. During the clamp period, an
internally generated clamp pulse is used to switch-on the
clamp action.
1999 May 03
10
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