AK8850 Datasheet, PDF (48/99 Pages) Asahi Kasei Microsystems – NTSC Digital Video Decoder

ASAHI KASEI
[AK8850]
7-4-7 COLOR KILLER
The signal quality of the Chroma signal is evaluated from the Color Burst level of input signal. If this level is lower than a
target level, the Chroma signal decoding process is not performed. When the Standard 40 IRE Color Burst input signal
level becomes lower than the pre-set value (presettable range is from –17 dB to – ( infinite ),-20 dB at default ), the Color
Killer function is activated.The Cb/Cr output signal is fixed to 0x80.
When the Color Killer is activated, the C-data output is 0x80, (Black & White mode). Since the Color Killer function checks
the Color Burst signal, it is disabled when the Component signal is selected.
Operating range of the Color Killer can be set from –17 dB to – ( infinite ). It can be turned on or off by the [ COLOR
KILLER CONTROL REGISTER ].
When the Color Killer process is executed, the Chroma signal output becomes Black level ( 0x80 ) which is a similar level
in the Black & White mode. However, since the C-signal is subtracted from the Composite signal by the Y/C Separation
block, the output result differs from the Black & White mode.
Color Killer Control Register Description :
Color killer function is controlled by the [COLOR KILLER CONTROL REGISTER ].
* [ COLOR KILLER CONTROL REGISTER ]
bit 7
bit 6
bit 5
bit 4
bit 3
CL_KILL Reserved CKLVL5 CKLVL4 CKLVL3
Default Value
1
0
1
0
1
bit 2
CKLVL2
1
bit 1
CKLVL1
0
bit 0
CKLVL0
1
* Turning on and off of the Color Killer function is set by CL-KILL-bit ( bit-0 )
CL_KILL-bit (bit-7)
Color Killer
0
OFF (Disable)
1
ON (Enable) Default
* Turning on and off of the Color Killer function is set by CL-KILL-bit ( bit-0 )
The Color Killer level setting is given in the following equation:
ON / OFF decision level of the Color Killer is made by checking the U level after de-modulating the Color Burst signal. A
proper input signal gives the U-signal a level of 112. A decision is made within the device by multiplying by 4 the U-signal
after de-modulating the input signal. The register set value [ CKLVL5 :CKLVL0 ] is given as:
[ CKLVL5 : CKLVL0 ]=10 exp( clk/20 ) *448
where CKL [ dB ] is the level to enable Color Killer.
In reverse, the Color Killer enable level is calculated from the register set value as :
Input U level = 20 *log ( [ CKLVL5 : CKLVL0 ] / 448 )
i.e., when a lower-than-the input U level signal is input, the Color Killer is activated.
U-Signal Level After Demodulation
112
Color Killer Level (max63)
Set to 63(max), when the calculated value
which is 4 times the demodulated U-signal is
less than 63, Color killer is active.
Rev.0
48
2003/01