Datasheet

ADV7188
Rev. A | Page 35 of 112
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.
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.
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.
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.
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.
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).
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.
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.
05478-021
A
NALOG VOLT
A
GE
RANGE SUPPORTED BY ADC (1.6V RANGE FOR ADV7188)
DATA
PRE-
PROCESSOR
(DPP)
ADC
SDP
(GAIN SELECTION ONLY)
MAXIMUM
VOLTAGE
MINIMUM
VOLTAGE
CLAMP
LEVEL
GAIN
CONTROL
Figure 21. Gain Control Overview
Table 39. AGC Modes
Input Video Type Luma Gain Chroma Gain
Any Manual luma gain Manual chroma gain
Dependent on color burst amplitude Dependent on horizontal sync depth
Taken from luma path
Dependent on color burst amplitude
CVBS
Peak white
Taken from luma path
Dependent on color burst amplitude Dependent on horizontal sync depth
Taken from luma path
Dependent on color burst amplitude
Y/C
Peak white
Taken from luma path
YPrPb Dependent on horizontal sync depth Taken from luma path