TMC2490AR2C datasheet(21/36 Pages) CADEKA

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PRODUCT SPECIFICATION

TMC2490A

REV. 1.0.2 2/27/02

Filtering Within the TMC2490A

The TMC2490A incorporates internal digital ﬁlters to estab-

lish appropriate bandwidths and simplify external analog

reconstruction ﬁlter designs.

The chroma portion of the incoming digital video is band-

limited to reduce edge effect and other distortions of the

image compression process. Chrominance bandwidth is

selected by CHRBW. When LOW, the chrominance pass-

band attenuation is <3 dB within

±650 kHz from fSC. The

stopband rejection is >26 dB outside fSC

±2 MHz. When

HIGH, the chrominance passband attenuation is <3 dB

within

±1.3 MHz from fSC. The stopband rejection is

>33 dB outside fSC

±4 MHz.

Figure 9. Color-Difference Low-Pass Filter Response

The Chroma Modulator output and the luminance data are

digitally ﬁltered with sharp-cutoff low-pass interpolation ﬁl-

ters. These ﬁlters ensure that aliased subcarrier, chromi-

nance, and luminance frequencies are sufﬁciently suppressed

above the video base-band.

Figure 10. Chrominance and Luminance Interpolation

Filter – Full Spectrum Response

-70

-60

-50

-40

-30

-20

-10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

Frequency (MHz)

24490A

Wideband

Narrowband

-70

-60

-50

-40

-30

-20

-10

10 11 12 13

Frequency (MHz)

24487A

Virtually all digital-to-analog converters have a response

with high frequency roll-off as a result of the zero-order hold

characteristic of classic D/A converters. This response is

commonly referred to as a sin(x)/x response. The sin(x)/x vs.

sampling frequency is shown in Figure 12.

Figure 11. Chrominance and Luminance Interpolation

Filter – Passband Detail

The TMC2490A’s digital interpolation ﬁlters convert the

data stream to a sample rate of twice the pixel rate. This

results in much less high frequency sin(x)/x rolloff and the

output spectrum between fS/4 and 3 x fS/4 contains very lit-

tle energy. Since there is so little signal energy in this fre-

quency band, the demands placed on the output

reconstruction ﬁlter are greatly reduced. The output ﬁlter

needs to be ﬂat to fS/4 and have good rejection at 3 x fS/4.

The relaxed requirements greatly simplify the design of a ﬁl-

ter with good phase response and low group delay distortion.

A small amount of peaking may be added to compensate

residual sin(x)/x rolloff.

Figure 12. Sin(x)/x Response

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

0123456

Frequency (MHz)

24488A

-6

-5

-4

-3

-2

-1

456

Fs=13.5Msps

Fs=27.0Msps

(Oversampled)

Frequency (MHz)

24489A

TMC2490AR2C Datasheet(PDF) 21 Page - Cadeka Microcircuits LLC.

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