AK8850 Datasheet, PDF (28/99 Pages) Asahi Kasei Microsystems – NTSC Digital Video Decoder

ASAHI KASEI
[AK8850]
7-2-2. Clamp
The CLAMP function stabilizes the AC-coupled Video input signal level. The AK8850 utilizes both an Analog Clamp
circuit and a Digital Clamp circuit.
When the Composite and Y signals (S-video,Component Y signal ) are input, the SYNC-TIP level or PEDESTAL level of
Composite signal and LUMINANCE signal ( Y ) are clamped. The digitized input signal data is further signal-processed
and clamped to the PEDESTAL level( digital pedestal clamp ).
Clamp functions take place during the clamp pulse, as described below, in (1) SYNC SEPARATION FUNCTION.
The Clamp interval is equal to the HSYNC interval (approximately 15 KHz ).
Both the C and Pb/Pr signals are clamped at the SYNC-TIP timing of the Luminance signal.
(1) SYNC SEPARATION FUNCTION( Clamp timing pulse generation )
In order to separate the SYNC signal of the target signal, the Clamp Timing Pulse is used to clamp the input signal to a
pre-determined level which is fed back via the Ain pin. To generate a Clamp timing pulse ,select one of the SYNC1 /
SYNC2 / SYNC3 input signals by setting [ SYNCIN1 : SYNCIN0] of the ( INPUT SIGNAL SELECT REGISTER).
The selected signal must be a signal which corresponds to AIN1~ AIN6 signals.
The separated SYNC signal is used for the clamp timing.
Sync-separation is done by slicing approximately 70 mV positive point ( default value ) from the SYNC-TIP level.
As described above, the Clamp timing pulse is generated based on the separated signal ( SYNC DET ).The slice point is
adjustable by register settings.
Rev.0
28
2002/01