adv7188 Analog Devices, Inc., adv7188 Datasheet - Page 35

• Manufacturer Part Number: adv7188
• Description: Multiformat Sdtv Video Decoder With Fast Switch Overlay Support
• Manufacturer: Analog Devices, Inc.

GAIN OPERATION
The gain control within the ADV7188 is done on a purely
digital basis. The input ADCs support a 12-bit range, mapped
into a 1.6 V analog voltage range. Gain correction takes place
after the digitization in the form of a digital multiplier.
This architecture is advantageous because, unlike the commonly
used programmable gain amplifier (PGA) placed in front of the
ADCs, the gain is completely independent of supply, temperature,
and process variations.
As shown in Figure 21, the ADV7188 can decode a video signal
as long as it fits into the ADC window.
The two components of decoding a video signal are the amplitude
of the input signal and the dc level on which it resides. The dc level
is set by the clamping circuitry (see the Clamp Operation section).
If the amplitude of the analog video signal is too high, clipping
can occur, resulting in visual artifacts. The analog input range
of the ADC, together with the clamp level, determines the
maximum supported amplitude of the video signal.
Table 39. AGC Modes
Input Video Type
Any
CVBS
Y/C
YPrPb
MINIMUM
VOLTAGE
Luma Gain
Manual luma gain
Dependent on horizontal sync depth
Peak white
Dependent on horizontal sync depth
Peak white
Dependent on horizontal sync depth
MAXIMUM
VOLTAGE
CLAMP
LEVEL
Figure 21. Gain Control Overview
Rev. A | Page 35 of 112
ANALOG VOLTAGE
RANGE SUPPORTED BY ADC (1.6V RANGE FOR ADV7188)
ADC
The minimum supported amplitude of the input video is
determined by the ability of the ADV7188 to retrieve horizontal
and vertical timing and to lock to the color burst, if present.
There are separate gain control units for luma and chroma data.
Both can operate independently of each other. The chroma unit,
however, can also take its gain value from the luma path.
The possible AGC modes are summarized in Table 39.
It is possible to freeze the automatic gain control loops. This
causes the loops to stop updating and maintains the AGC-
determined gain that is active at the time of the freeze until the
loop is either unfrozen or the gain mode of operation is changed.
The currently active gain from any of the modes can be read
back. Refer to the description of the dual-function manual gain
bits, LMG [11:0] luma manual gain and CMG [11:0] chroma
manual gain, in the Luma Gain and the Chroma Gain sections.
PROCESSOR
(DPP)
DATA
PRE-
Chroma Gain
Manual chroma gain
Dependent on color burst amplitude
Taken from luma path
Dependent on color burst amplitude
Taken from luma path
Dependent on color burst amplitude
Taken from luma path
Dependent on color burst amplitude
Taken from luma path
Taken from luma path
SDP
(GAIN SELECTION ONLY)
CONTROL
GAIN
ADV7188