AK8850 Datasheet, PDF (45/99 Pages) Asahi Kasei Microsystems – NTSC Digital Video Decoder

ASAHI KASEI
[AK8850]
* Non-Sensing Bandwidth of the Pedestal Clamp is set as follows.
[DPCC1:DPCC0]
286mV SYNC
Pedestal Level 240
00
Pedestal level is not 240, digital pedestal
Default
clamp function works.
Non-Sensing Bandwidth : +/- 1LSB
01
Pedestal Level is not in the range of 238 -
241, digital pedestal clamp function works
Non-Sensing Bandwidth : +/- 2LSB
10
Pedestal Level is not in the range of 236 -
243, digital pedestal clamp function works
Non-Sensing Bandwidth : +/- 3LSB
11
Pedestal Level is not in the range of 232 -
247, digital pedestal clamp function works
300mV Sync
Pedestal Level 252
Pedestal level is not 252, digital pedestal
clamp function works.
Non-Sensing Bandwidth : +/- 1LSB
Pedestal Level is not in the range of 250 -
253, digital pedestal clamp function works
Non-Sensing Bandwidth : +/- 2LSB
Pedestal Level is not in the range of 248 -
255, digital pedestal clamp function works
Non-Sensing Bandwidth : +/- 3LSB
Pedestal Level is not in the range of 244 -
259, digital pedestal clamp function works
7-4-4 AGC ( Automatic Gain Control )
This function sets the PGA gain so that input level is properly set. The PGA gain is calculated so that the SYNC level is
equal to 40 IRE / 300 mV ( 286 mV-type Sync / 300 mV-type Sync ) and its value is set to PGA ( classification of 286
mV-type Sync and 300 mV-type Sync is listed in Table 7-2-1-1 ).
Gain-calculation and Gain-set are performed during the VBI interval of each Field.
The AGC time constant is programmable in 3 steps using the [ CONTROL 1 REGISTER ]. The PGA gain is derived from
the SYNC level information only.
AGC is performed only on the Composite signal and the Y portion of the YC input and Component input signals. Auto
gain is not performanced on the C-signal or Pb / Pr signals.
It is assumed that correction of the C-signal is done using the ACC of the digital portion. Correction of the Component Pb
/Pr signals should be manually done using the Color Saturation Control function.
When the AGC function is disabled by the [ CONTROL 1 REGISTER ], the gain of each channel can be manually set via
the [ PGA1 / PGA 2 / PGA 3 GAIN CONTROL REGISTER ].
AGC Control Register Description:
AGC is controlled by the [ CONTROL 1 REGISTER ].
* [ CONTROL 1 REGISTER ]
bit 7
bit 6
bit 5
bit 4
bit 3
FRCSYNC
Reserved
INTPOL[1]
INTPOL[0]
AGCC1
Default Value
1
0
0
0
0
bit 2
AGCC0
0
bit 1
AGCT1
0
bit 0
AGCT0
0
AGC set is defined as follows.
Enable / Disable and the Time-Constant ( T ) of the AGC are set .
[AGCT1:AGCT0](bit-1:bit-0)
AGC Function
[0,0]
Disable (Default)
[0,1]
T=1Field
[1,0]
T=7Field
[1,1]
T=29Field
The AK8850 sets the PGA value so that the SYNC-TIP level is equal to 40 IRE / 300 mV ( 286 mV-type Sync / 300
mV-type Sync ) while the AGC is enabled.
The 286 mV-type Sync here means NTSC Composite, NTSC YC input and Betacam Component signals, while the 300
mV-type Sync means 625 Component and MII Component signals.
40 IRE / 300 mV ( 286 mV-type Sync / 300 mV-type Sync ) are equivalent to 240 / 252 ( 10-bit ) in digital code. As the
SYNC-TIP is set to 16 ( 10-bit ), Sync sizes are equivalent to 224 / 236 ( 10-bit ) respectively.
286mV SYNC
300mV SYNC
Sampled Digital Code
240
252
Sync level
224
236
While the AGC is enabled, manual gain adjustment is not possible by the PGA1 / PGA2 / PGA3 GAIN CONTROL
Registers ( AGC has higher priority than manual gain adjustments ).
Rev.0
45
2003/01