Manual Gain Filter Mode; Automatic Gain Control - Analog Devices ADV7850 Hardware Manual

Manual Gain Filter Mode

9.3.5
The ADV7850 provides a special filter option for the manual gain mode. This is functional only when manual gain is enabled. The
purpose of this filter is to be a smoothing mechanism when the manual gain value is updated continuously by an external system based on
either external or readback conditions in the ADV7850. The filter designed is an IIR filter with a transfer function of the form:
The values possible for A can vary from 1 (no filtering) to 1/128K (K = 1024). The value of coefficient A is chosen by programming
cp_gain_filt[3:0].
cp_gain_filt[3:0], Addr 44 (CP), Address 0x84[7:4]
This control is used to set the coefficient A of the IIF filter to filter the gain applied to the video signal when the gain is set manually.
The value set in this control is effective only when manual gain is enabled. The filter is designed as an IIR filter with a transfer function
of the form Y[N]=(1-A)*y[N-1]+A*X[N].
Function
cp_gain_filt[3:0]
0000 
0001
0010
0011
0100
0101
0110

Automatic Gain Control

9.3.6
The AGC of the CP takes measurements of the signal on channel A and determines an appropriate gain value for all three channels. For
the block to operate, it is necessary that a signal with an embedded synchronization pulse is fed through to channel A, for example, Y or
G. The AGC measures the depth of this synchronization pulse and compares it against a target value. The hsd_cha[9:0] readback register
is used to determine if there is a synchronization pulse on the data. If no synchronization pulse is found, AGC cannot work and the
manual gain control should be enabled.
The target value for the AGC can come from three sources. There are two predefined values of 300 mV and 286 mV (the hs_norm bit is
used to decide between the two values) and there is the option of setting an arbitrary target value by setting agc_tar_man, which enables
the usage, and agc_tar[9:0], which sets the arbitrary target level.
In some applications, it is desirable to use the AGC to gain the signal to a smaller range, use the Offset block to preserve the
synchronizations (by lifting the entire video signal up), and thus output the full digitized waveform (including synchronizations) within
the 12-bit output range. For this application, the agc_tar[9:0] value is very important. For more information, refer to Section 9.4.
Note: The 12-bit target code for white is nominally 940, the target code for black is 64.
Examples:
Rev. A May 2012
Y = (1-A)*Y
N
where A is the filter coefficient.
Description
No filtering, i.e. coefficient A = 1
Coefficient A = 1/128 lines
Coefficient A = 1/256 lines
Coefficient A = 1/512 lines
Coefficient A = 1/1024 lines
Coefficient A = 1/2048 lines
Coefficient A = 1/4096 line
=
agc _ tar [ : 9 ] 0 ( Code
Equation 11: CP AGC Target Value
agc _ tar
=
HSync 286 mV
agc _ tar
=
HSync 300 mV
+ A*X
N-1 N
SyncHeight
− •
Code )
White Black
VideoHeigh
286 mV
= − • =
( 940 64 )
714 mV
300 mV
= − • =
( 940 64 )
700 mV
247
mV
t
mV
351
dec
375
dec
ADV7850