ML6421

10

Figure 12. ML6421 Reconstruction Performance in the Frequency Domain

due to discrete sampling. The fall-off follows a sinx/x

response. The ML6421 filters have a complementary boost

to provide a flatter overall response. The boost is designed

for 13.5MHz Y/C and CV sampling and 6.75MHz U/V

sampling. Note: The ML6421 has the same pin-out as the

ML6420.

In a typical application the ML6421 is used as the final

output device in a video processing chain. In this case,

inputs to the ML6421 are supplied by DAC outputs with

their associated load resistors (typically 75

Ω or 150Ω).

the input of the ML6421.

The ML6421 will drive 75

Ω source termination resistors

(making the total load 150

Ω) so that no external drivers or

amplifiers are required.

FILTER PERFORMANCE

The reconstruction performance of a filter is based on its

ability to remove the high band spectral artifacts (that

result from the sampling process) without distorting the

valid signal spectral contents within the passband. For

video signals, the effect of these artifacts is a variation of

the amplitude of small detail elements in the picture

(such as highlights or fine pattern details) as the elements

move relative to the sampling clock. The result is similar

to the aliasing problem and causes a “winking” of details

as they move in the picture.

Figure 12 shows the problem in the frequency domain.

Curve A shows the amplitude response of the ML6421

filter, while Curve B shows the signal spectrum as it is

distorted by the sampling process. Curve C shows the

composite of the two curves which is the result of passing

the sampled waveform through the ML6421 filter. It is

clear that the distortion artifacts are reduced significantly.

Ultimately it is the time domain signal that is viewed on

a TV monitor, so the effect of the reconstruction filter on

the time domain signal is important. Figure 13 shows the

sampling artifacts in the time domain. Curve A is the

original signal, Curve B. is the result of CCIR601

sampling, and Curve C. is the same signal filtered through

the ML6421. Again the distortions in the signal are

essentially removed by the filter.

In an effort to measure the time domain effectiveness of a

reconstruction filter, Figure 14 was generated from a

swept frequency waveform. Curves A, B, and C are

generated as in Figure 13, but additional curves D and E

help quantify the effect of filtering in the time domain.

Curve D and Curve E represent the envelopes

(instantaneous amplitudes) of Curves B and C. Again it is

evident in Curve D that the envelope varies significantly

due to the sampling process. In Curve E, filtering with the

ML6421 removes these artifacts and generates an analog

output signal that rivals the oversampled (and more ideal)

signal waveforms. The ML6421 reduces the amplitude

variation from over 6% to less than 1%.