AK8851 Datasheet, PDF (49/80 Pages) Asahi Kasei Microsystems – NTSC/PAL/SECAM Digital Video Decoder

ASAHI KASEI
[AK8851]
(5) Overflow Indication of Input Signal Level
The AK8851 has 2 types of overflow indicators,” Luminance signal overflow indication “ and “ Chroma signal overflow
indication “ to indicate that the ADC input signal has exceeded its acceptable range on its way through the AGC
(Automatic Gain Control) and ACC (Automatic Color Control) blocks.
[PKWHITE]-bit :
When Luminance signal overflow is detected, this bit is set to “1” which indicates that an overflow has occurred in the
Luminance signal processing path.
[PKWHITE]-bit
0
1
Status of Signal
Input signal is normal level
Input signal is overflowed
Note
(6) REALFLD-bit :
This resides at [Status 2 Register]. It indicates if an EVEN or/ ODD Field is being decoded.
[REALFLD]-bit
real field status
Note
0
Field is EVEN field
1
Field is ODD field
[OVCOL]-bit :
This bit is set to “1” when a Chroma signal overflow is detected in the Chroma signal data path.
[OVCOL]-bit
Over color status
Note
0
Color signal is not overflowed
1
Color signal is overflowed
(footnote)
When an input overflow occurs, the following causes of overflow are likely.
It is assumed that input signal with following characteristics is applied when [PKWHITE]-bit or [OVCOL]-bit is set to “1” :
(a) Effective Video signal level is appropriate but SYNC-signal level is not large enough. In this case, the AGC adjusts the
amplitude of the SYNC-signal level to be equal to 40-IRE / 300 mV. This pushes the effective Video signal level higher and
causes an overflow to occur in the data processing block.
(b) Effective C-signal level is optimum but Color-Burst signal is not large enough. In this case, the AGC amplifies the
Color-Burst signal to be equal to 40-IRE / 300 mV. This pushes the effective C-signal level higher and causes an overflow
to occur in the data processing block.
MS0244-E-03
49
Confidential
2005 / 07