AK8851 Datasheet, PDF (19/80 Pages) Asahi Kasei Microsystems – NTSC/PAL/SECAM Digital Video Decoder

ASAHI KASEI
[AK8851]
8.PGA ( Programmable Gain Amp.)
The Ak8851 has 2 PGAs (Programmable Gain Amps), PGA1 and PGA2 on the input stage. The gain range of each PGA
is from 0dB to 12dB with a gain step of approx. 0.1 dB/step. Input signals to the AK8851 are attenuated to 50 % level by an
external resistor-divider.
PGA1 adjusts the gains of Composite and Y signals for discrete Y/C signals, and PGA2 handles the C signal gain of the
discrete Y/C signal (refer to Fig.2).
PGA1 set is done by [PGA1 Control Register](R/W)[Sub Address 0x0A].
PGA2 set is done by [PGA2 Control Register](R/W)[Sub Address 0x0B].
When the AGC function is enabled by [AGC and ACC Control Register], the PGA registers are disabled. When the AGC
function is disabled, the gains of PGA1 and PGA2 can be adjusted independently.
Bit Allocation of [PGA1/PGA2 Control Register] is shown below.
*[PGA1 Control Register]
Sub Address 0x0A
bit 7
bit 6
Reserved
PGA1_6
0
1
bit 5
PGA1_5
0
bit 4
bit 3
PGA1_4
PGA1_3
Default Value
0
0
bit 2
PGA1_2
0
Default Value : 0x46
bit 1
bit 0
PGA1_1
PGA1_0
0
0
*[PGA2 Control Register]
Sub Address 0x0B
bit 7
bit 6
Reserved
PGA2_6
0
1
bit 5
PGA2_5
0
bit 4
bit 3
PGA2_4
PGA2_3
Default Value
0
0
bit 2
PGA2_2
0
Default Value : 0x46
bit 1
bit 0
PGA2_1
PGA2_0
0
0
9.AGC
The AK8851 measures the input signal’s SYNC signal level (the difference between SYNC-Tip level and Pedestal level)
and then judges the input signal level. The AGC function controls PGA gain so that the SYNC signal level equals 286
[mV]/300 [mV].
This function ensures a proper level to the ADC by amplifying the incoming input signal.
For Y/C inputs, the C signal is either amplified or attenuated by the same gain value as the SYNC level of the Y signal
adjustment to be 286 [mV]/300 [mV] (a fine tuning of the C signal is done by the Auto Color Control (ACC) function in the
digital block).
AGC Adjustable levels expressed in 10-bit code are shown in the following table.
Input Signal
NTSC-M
NTSC-4.43
PAL-M
PAL-B, D, G, H, I, N, Nc
SECAM
PAL-60
Target ADC value(decimal)
224
236
Note
286[mV]
300[mV]
AGC value can be frozen via a register setting. When it is frozen, the pre-set gain constant is held in the AGC.
When AGC function is disabled, PGA1 and PGA2 gains can be independently set.
Set the AGC parameter by programming [AGC and ACC Control Register](R/W)[Sub Address 0x06].
Sub Address 0x06
Default Value : 0x00
bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
ACCFRZ
ACC1
ACC0
AGCFRZ
Reserved
AGCC
AGCT1
AGCT0
Default Value
0
0
0
0
0
0
0
0
MS0244-E-03
19
Confidential
2005 / 07