CS7654 datasheet(14/62 Pages) CIRRUS

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CS7654

Internal Filters

The CS7654 has an internal low-pass chroma filter

to reduce the effects of color aliasing. This filter is

enabled by writing a value of 0 to bit 4 of register

01h at SA 0x00h. The CS7654 also contains a luma

peaking filter to enhance the edges of blurred imag-

es. This filter is enabled by setting register 05h bit

3 to a value of 0 at SA 0x34h. By default the low-

pass chrome filter is off and the peaking filter is on.

Analog Video Timing Generator

All timing generation is accomplished via a

27 MHz input applied to the CLKIN2X pin.

The Video Timing Generator is responsible for or-

chestrating most of the other modules in the device.

It automatically disables color burst on appropriate

scan lines and automatically generates serration

and equalization pulses on appropriate scan lines.

Color Subcarrier Synthesizer

The subcarrier synthesizer is a digital frequency

synthesizer that produces the appropriate subcarri-

er frequency for NTSC or PAL. The CS7654 gen-

erates the color burst frequency based on the CLK

input (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.

Controls are provided for phase adjustment of the

burst to permit color adjustment and phase com-

pensation. Chroma hue control is provided by the

CS7654 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). Horizontal sync to

color burst phase adjust is possible by program-

ming the SCH register (register 17h, SA 00h).

Chroma Path

The Video Input Formatter 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 can be

bypassed) via the CONTROL_1 Register. The

passband 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 than

35 dB. The stopband for the 650 KHz selection be-

gins 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 in which up-

sampling from 4:2:2 to 4:4:4 is accomplished.

Following the interpolation filter, the U and V

chroma signals pass through two independent vari-

able gain amplifiers in which the chroma amplitude

can be varied via the U_AMP and V_AMP 8-bit

host addressable registers.

The U and V chroma signals are fed to a quadrature

modulator in which they are combined with the

output from the subcarrier synthesizer to produce

the proper modulated chrominance signal.

The chroma then is interpolated by a factor of two

in order to operate the output DACs at twice the

pixel rate. The interpolated filters enable running

the DACs at twice the pixel rate and this helps re-

duce the sinx/x roll-off for higher frequencies and

reduces the complexity of the external analog low

pass filters.

Luma Path

Along with the chroma output path, the CS7654

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 in

which the depth of the FIFO replicates the clock

period delay associated with the more complex

chroma path. Brightness adjustment is also provid-

ed via the 8-bit BRIGHTNESS_OFFSET Register.

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