DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

( )
PULSE T
The risetime, from 10% to 90%,
of a T step is about 1.18 times the
risetime of a T pulse:
-1
4sin 0. 9-0. 1
tr= t
π HAD
≈ 118 . tHAD
HAD
125 ns
2
PULSE T
HAD
250 ns
STEP T
tR 125 ns
Figure 45.9 T pulse, 2T pulse, and T step waveforms are shown for 480i systems, where T has
the value 125 ns. In 576i systems, bandwidths are generally higher, and T has the value 100 ns.
Figure 45.10
T pulse spectra
Amplitude (relative)
1.0
0.5
0
Although the term raised cosine or sine squared properly
applies to a complete pulse, from baseline to
maximum and returning to baseline, it is loosely applied
to a transition from one level to another. For example,
the leading edge of analog sync ideally has a raised
cosine shape. When applied to a step, the parameter T
denotes the usual risetime from 10% to 90% as in electrical
engineering. Figure 45.9 above shows the waveforms
of a T pulse, a 2T pulse, and a T step.
Figure 45.10 below shows the frequency spectra of
a T pulse and a 2T pulse, along with the spectrum of
a triangle-shaped pulse having the same risetime.
A good approximation of a raised-cosine step waveform
between zero and one, like that of the step at the
right in Figure 45.9, can be obtained without trigonometry
by evaluating this cubic polynomial for values of
time (t) between zero and one:
3 2
3t −
2t
2T pulse
T pulse
Triangle with
125 ns risetime
0 2 4
Frequency, MHz
CHAPTER 45 SDTV TEST SIGNALS 543
10%
50%
90%
Eq 45.2
6 8