cx25840 Conexant Systems, Inc., cx25840 Datasheet - Page 59

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

CX25840/1/2/3 Data Sheet
3.4.13
102284B
8/3/05
Fast Channel Switching
Fast channel switching is when you grab a field (or frame) from an input and then
switch to another input and try to grab the first correct field available, and then switch
again to another input, and so on.
To determine if the decoder can grab a field after a channel switch, the decoder must
lock on the signal. To lock on the new input signal, it is necessary to have Horizontal
Lock, Vertical Lock, and Color lock on the video input.
The CX25840/1/2/3 uses optimized hardware to reduce the time needed to switch
between different signal sources. This new technology now achieves much higher
frame rates when switching inputs continuously.
This is achieved through the following:
Fast Horizontal Lock (HLOCK) is a feature that is already optimized in the decoder.
There are no registers to enable. The General Status 2 (0x40E) register has an
HLOCK bit that notifies the application that horizontal lock has been achieved. Even
under line length variations fast horizontal locking is achieved.
Fast Vertical Lock (VLOCK) can be achieved by setting FAST_LOCK_MD bit of the
Video Mode Control 3 (0x402) register to high to enable the fast locking algorithm.
The fast locking algorithm locks onto the first vertical sync it encounters, regardless
of where the internal timing counters expect a vertical sync to occur. It also disables
the hysteresis that is used for the field detection logic. As soon as the even/odd field is
detected, the field signal is updated to reflect that status. Vertical lock can be
determined by reading the VLOCK and VPRES status bits in the General Status 2
(0x40E) register.
To achieve Fast Color Lock (CSC_LOCK), the chroma subcarrier lock speed must be
set to fast mode. To do this, the auto mode of the subcarrier lock must first be disabled
by setting the AUTO_SC_LOCK bit low and then setting the subcarrier lock speed to
“fast” by setting the MAN_SC_FAST_LOCK bits in the Video Mode Control 2
(0x401) and Interrupt Status 1 (0x410) registers, respectively. Status of subcarrier
lock can be determined by reading the CSC_LOCK and
CSC_LOCK_CHANGE_STAT bits in the General Status 2 (0x40E) and Interrupt
Status 1 (0x410) registers, respectively.
To achieve a fast adjustment of gain for the input signal (AGC_LOCK), enable the
AGC_AUTO_EN and VGA_AUTO_EN functions in the Digital Front-End Control
(0x48B) register. By doing so, the decoder has the ability to automatically acquire
gain value for all eight composite inputs. This is particularly useful because the AGC
presents a problem for doing fast channel switching. Since all sources require
different gain values when we switch from one video input to another, it takes time for
the gain to adjust. The adjustment time can prevent how fast the application can
switch between inputs. After enabling the AGC_AUTO_EN and VGA_AUTO_EN
functions, the application would then enable each input, read the AGC_GAIN and
VGA_GAIN values for that source, and save the values for later use. Next, the
application would set AGC_AUTO_EN and VGA_AUTO_EN bits to manual mode,
and write the saved AGC value that is associated with a particular input to the
AGC_GAIN and VGA_GAIN fields. This technique of restoring predetermined gain
values eliminates the amount of time needed to adjust the AGC for the new video
input. The General Status 2 (0x40E) register has an AGC_LOCK bit that notifies the
application that the correct gain has been selected.
Finally, auto format detection should be disabled for fast channel switching. This is
accomplished by setting the VID_FMT_SEL bits in the Video Mode Control 1
register (0x400) to the appropriate video standard to be decoded.
Fast Horizontal Lock
Fast Vertical Lock
Fast Color Lock
Fast adjustment of gain for input signal
Conexant
Detailed Functional Description
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