AD7183 datasheet(13/40 Pages) AD

13 / 40 page

REV. 0

ADV7183

–13–

CHROMINANCE PROCESSING

Figure 17 shows the chrominance data path. The 10-bit data

from the Y ADC (CVBS mode) or the C ADC (S-video) is first

demodulated. The demodulation is achieved by multiplying by

the locally generated quadrature subcarrier, where the sign of

the cos subcarrier is inverted from line to line according to the

PAL switch, and then low pass filtering is applied to removed

components at twice the subcarrier frequency. For NTSC, the

phase of the locally generated subcarrier during color burst is

the same as the phase of the color burst. For PAL, the phase of

the color burst changes from line to line, relative to the phase

during active video, and the phase of the locally generated

subcarrier is the average of these two values.

The chrominance data is then passed through an antialiasing

filter which is a band-pass filter to remove the unwanted lumi-

nance data. This antialaising filter dramatically reduces the

external antialaising filter requirements as it has only to filter

components above 25 MHz. In component mode the demodu-

lation block is bypassed.

The next stage of processing is a shaping filter that can be used

to limit the chrominance bandwidth to between 0.5 MHz

and 3 MHz; the CSFM[2:0] can be used to select these

responses. It should be noted that in CVBS mode a filter of no

greater than 1.5 MHz should be selected, as CVBS video is

typically band-limited to below 1.5 MHz. In S-video mode a

filter of up to 2 MHz can be used. In component mode a filter

of up to 3 MHz can be used as component video has higher

bandwidth than CVBS or S-video.

The chrominance data is then passed through a resampler to

correct for line length variations in the input video. This

resampler is designed to always output 720 pixels per line for

standard PAL or NTSC. The resampler used on the ADV7183

is of very high quality as it uses 64 phases to resample the video,

giving 1/64 pixel resolution. The resampler is controlled by a

sync detection block that calculates line length variations on the

input video.

The final stage in the chrominance path, before it is applied to

an output formatter block, is chroma comb filter.

ANTI-

ALIASING

LPF

SHAPING

LPF

RESAMPLE

CHROMA

COMB

FILTERS

SYNC

DETECTION

U/V

CV/C

27MHz

SINE

ANTI-

ALIASING

LPF

COSINE

13.5MHz

6.75MHz

SUBCARRIER

RECOVERY

INTERLEAVE

Figure 17. Chrominance Processing Path

FREQUENCY – MHz

–60

4.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

–10

–20

–30

–40

–50

SH2 SH3 SH4 SH5

SH6

SH1

Figure 18. Chrominance Shaping Filter Responses

FREQUENCY – MHz

1.0

–1.0

4.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.8

0.4

–0.2

–0.6

–0.8

0.6

0.2

–0.4

SH2 SH3 SH4 SH5

SH1

SH6

Figure 19. Chrominance Shaping Filter Responses

(Close-Up)

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