VPX 322xE
2. Functional Description
The following sections provide an overview of the differ-
ent functional blocks within the VPX. Most of them are
controlled by the Fast Processor (‘FP’) embedded in the
decoder. For controlling, there are two classes of regis-
ters: I
2
C registers (directly addressable via I
2
C bus) and
FP-RAM registers (RAM memory of the FP; indirectly
addressable via I
2
C bus). For further information, see
section 2.15.1.
2.1. Analog Front-End
This block provides the analog interfaces to all video in-
puts and mainly carries out analog-to-digital conversion
for the following digital video processing. A block dia-
gram is given in Fig. 2–1.
Clamping, AGC, and clock DCO are digitally controlled.
The control loops are closed by the embedded proces-
sor.
2.1.1. Input Selector
Up to four analog inputs can be connected. Three inputs
(VIN1–3) are for input of composite video or S-VHS luma
signal. These inputs are clamped to the sync back porch
and are amplified by a variable gain amplifier. Two in-
puts, one dedicated (CIN) and one shared (VIN1), are
for connection of S-VHS carrier-chrominance signal.
The chrominance input is internally biased and has a
fixed gain amplifier.
2.1.2. Clamping
The composite video input signals are AC coupled to the
IC. The clamping voltage is stored on the coupling ca-
ADVANCE INFORMATION
pacitors and is generated by digitally controlled current
sources. The clamping level is the back porch of the vid-
eo signal. S-VHS chroma is AC coupled. The input pin
is internally biased to the center of the ADC input range.
2.1.3. Automatic Gain Control
A digitally working automatic gain control adjusts the
magnitude of the selected baseband by +6/–4.5 dB in 64
logarithmic steps to the optimal range of the ADC.
2.1.4. Analog-to-Digital Converters
Two ADCs are provided to digitize the input signals.
Each converter runs with 20.25 MHz and has 8-bit reso-
lution. An integrated bandgap circuit generates the re-
quired reference voltages for the converters. The two
ADCs are of a 2-stage subranging type.
2.1.5. ADC Range
The ADC input range for the various input signals and
the digital representation is given in Table 2–1 and Fig.
2–2. The corresponding output signal levels of the
VPX 32xx are also shown.
2.1.6. Digitally Controlled Clock Oscillator
The clock generation is also a part of the analog front
end. The crystal oscillator is controlled digitally by the
FP; the clock frequency can be adjusted within
±150
ppm.
CVBS/Y
CVBS/Y
CVBS/Y/C
VIN3
VIN2
VIN1
clamp
AGC
+6/–4.5 dB
ADC
digital CVBS or Luma
gain
CIN
Chroma
bias
ADC
digital Chroma
input mux
reference
generation
DCVO
±150
ppm
system clocks
frequency
20.25 MHz
Fig. 2–1:
Analog front-end
8
MICRONAS INTERMETALL
MICRONAS [ MICRONAS ]
