CS4953 datasheet(12/44 Pages) CIRRUS

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CS4952/53

DS223PP2

Color Subcarrier Synthesizer

The subcarrier synthesizer is a digital frequency

synthesizer that produces the correct subcarrier fre-

quency for NTSC or PAL. The CS4952/3 generates

the color burst frequency based on the input CLK

(27 MHz). Color burst accuracy and stability are

limited by the accuracy of the 27 MHz input. If the

frequency varies then the color burst frequency will

also vary accordingly.

In order to handle situations in which the CLK var-

ies unacceptably, a local crystal frequency refer-

ence may be used on the ADDR & XTAL device

pins. In this instance the input CLK is continuously

compared with the external crystal reference input

and the internal timing of the CS4952/3 is automati-

cally adjusted so that the color burst frequency re-

mains close to the requirements.

Controls are provided for phase adjustment of the

burst to permit color adjustment and phase com-

pensation. Chroma hue control is provided by the

CS4952/3 via a 10-bit Hue Control Register

(HUE_LSB and H_MSB). Burst amplitude control

is also made available to the host via the 8-bit burst

amplitude register (SC_AMP).

Chroma Path

The Video Input Formatter at conclusion delivers

4:2:2 YUV outputs into separate chroma and luma

data paths. The chroma path will be discussed here.

The chroma output of the Video Input Formatter is

directed to a chroma low pass 19-tap FIR filter. The

filter bandwidth is selected or the filter may be by-

passed via the CONTROL_1 register. The pass-

band of the filter is either 650 KHz or 1.3 MHz and

the passband ripple is less than or equal to 0.05 dB.

The stopband for the 1.3 MHz selection begins at

3 MHz with an attenuation of greater that 35 dB.

The stopband for the 650 KHz selection begins

around 1.1 MHz with an attenuation of greater than

20 dB.

The output of the chroma low pass filter is connect-

ed to the chroma interpolation filter where upsam-

pling from 4:2:2 to 4:4:4 is accomplished. The

chroma digital data is fed to a quadrature modulator

where they are combined with the output from the

subcarrier synthesizer to produce the proper modu-

lated chrominance signal.

Following chroma modulation the chroma data

passes through a variable gain amplifier where the

chroma amplitude may be varied via the C_AMP

8-bit host addressable register. The chroma then is

interpolated by a factor of 2 in order to operate the

output DACs at 2 times the pixel rate. The interpo-

lated filters help reduce the sinx/x roll-off for high-

er frequencies and reduce the complexity of the

external analog low pass filters.

Luma Path

Along with the chroma output path, the CS4952/3

Video Input Formatter initiates a parallel luma data

path by directing the luma data to a digital delay

line. The delay line is built as a digital FIFO where

the depth of the FIFO replicates the clock period

delay associated with the more complex chroma

path.

Following the luma delay, the data is passed

through a variable gain amplifier where the luma

DC values are modifiable via the Y_AMP register.

The output of the luma amplifier connects to the

sync insertion block. Sync insertion is accom-

plished by multiplexing into the luma data path the

different sync DC values at the appropriate times.

The digital sync generator takes horizontal sync

and vertical sync timing signals and generates the

appropriate composite sync timing (including ver-

tical equalization and serration pulses), blanking

information, and burst flag. The sync edge rates

conform to RS-170A or CCIR specifications.

The luma only path is concluded via output interpo-

lation by a factor of two in order to operate the out-

put DACs at two times the pixel rate.

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