cx25840 Conexant Systems, Inc., cx25840 Datasheet - Page 14

• Manufacturer Part Number: cx25840
• Description: Video Decoder And Broadcast Audio
• Manufacturer: Conexant Systems, Inc.

Functional Overview
1.1.1
1.1.1.1
1.1.1.2
1.1.1.3
1.1.1.4
1-2
Video Features
Analog Video Inputs
The CX25840/1/2/3 integrates two high-performance 10-bit Analog-to-Digital
Converters (ADCs) and provides a full 10-bit data path through the video decoder to
maintain optimum end-to-end video quality. Eight analog inputs are provided with
flexible analog muxing that can be configured for one or a combination of the
following audio and video inputs:
Time multiplexing the various inputs to the chroma/sound ADC allows for the
simultaneous digitalization of Pb and Pr inputs in component mode, or chroma with
sound-IF for supporting Y/C sources with broadcast audio. All video inputs have
integrated anti-alias filters, eliminating the need for external filter components.
Integrated Clamping and Automatic Gain Control
DC restoration and Automatic Gain Control (AGC) are provided to compensate for
sources with differing average picture levels. Manual gain control is also supported.
Gain values can be read from and written to the device, allowing for the calibration of
each input and facilitating fast switching from one source to another.
Flexible Decoder Rates
The video data path includes a sample rate converter to enable multiple pixel rates and
to track any timing fluctuations that may be present within the video source. With the
sample rate converter, the user can program the device to decode video at output pixel
rates of either 13.5 MHz for an ITU-R BT.656 compliant output stream or at
12.27 MHz and 14.75 MHz for NTSC and PAL/SECAM square pixel rates,
respectively. The sample rate converter with internal FIFO monitors the horizontal
timing of the input source to create a fixed number of samples per line. It controls a
PLL to slowly adjust the FIFO level such that short-term jitter in the input source is
filtered out of the digitized video stream. This provides stable video data and output
clocks, even with sources like VCRs that can have inherently unstable timing.
High Performance Filtering
Luma/chroma separation of composite video sources is accomplished through a 5-line
adaptive chroma comb filter for NTSC and PAL standards. The adaptive comb filter
looks across five lines of incoming video and determines which of the five lines are
appropriately correlated enough to average together. Depending on the amount of
correlation among the lines, two or three lines are averaged together to form the
resulting combed filtered line. In the case where no correlation exists between lines,
the decoder automatically falls back to chroma band-pass and luma notch filtering.
The output of the chroma comb filter is also remodulated and fed back into the luma
channel. The result is a high quality image with reduced cross-chrominance and cross-
luminance artifacts—such as dot crawl, hanging dots, rainbow effects—that restore
full bandwidth to luminance data from composite sources. Additionally, we have a
Eight composite inputs
Four Y/C inputs
Two composite with one Y/C, one YPbPr, and one sound IF
Four composite with four sound IF
One composite with two YPbPr and one sound IF
Conexant
CX25840/1/2/3 Data Sheet
102284B
8/3/05