Specifications

8 ExtroNews 14.1 January – March 2003
TECHNICALLY SPEAKING...
which can affect frequency components of
the SDI unevenly, like speed limits changing
from one city block to another. Pre-emphasis,
or peaking, which may be beneficial to
analog computer graphics transmission, can
cause distortion or group delay effect,
further limiting reliable serial digital data
recovery. Group delay is the rate of change of
signal phase shift with respect to frequency.
Linear phase shift is a requirement for SDI
routing devices. Moments ago, we learned
of the effect of non-standard bandwidth roll-
off on receiver equalization.
The physics of analog losses affect
components of digital signals just like analog
signals. Digital signals are simply the
conveyance of a number or numbers
represented by subsequent changes in
electrical value or level. These level changes
and the rate at which they change (the rise
and fall time) are analog features that are
affected just like any other analog signal.
Digital receivers reach the “cliff” and stop
decoding the proper message when levels
and transitions can no longer be recognized.
Other loss factors are cable loss (the largest
one), system noise leading to signal jitter,
and any imparted DC component.
Cable Loss
Applying reclocked devices and following
coaxial cable loss guidelines for SDI will
provide a significant measure of confidence
in system reliability. Well-designed receivers,
called Class A type, can recover serial digital
data as low as –30db at one-half the clock
rate from a pristine source, or about
25 millivolts. The one-half clock rate
frequency is used to calculate SDI cable loss.
Follow the cable’s specified insertion loss
figure for 100 feet at the one-half clock rate
value. For 270 Mbps component SDI, the
rate would be 135 MHz. Divide that dB loss
value into 30 and the result will be the
number of multiples of 100-foot lengths that
can be used. Cable manufacturers often
provide a table of distances that shows the
SDI or HD SDI rates that can be run with each
type of appropriate cable in the product line.
Usually, the lengths indicated include a
10% length reduction allowance to establish
some margin against the cliff effect. See
Table 2 for some examples.
Timing Jitter
Signal jitter, or random timing uncertainty,
occurs as serial digital signals pass through
the various devices in any distribution
system, digital or analog. Small amounts of
internal phase noise, power supply noise
modulation, and processing stage threshold
variations are all factors contributing to
increased signal jitter. Figure 3 shows a
typical, clean SDI signal. Compare that to
Figure 4, where the jitter level is substantial.
The opening in the pattern is called the
“eye” and the pattern is routinely called an
“eye pattern”. Comparing the two figures,
Table 2. Recommended Serial Digital (SDI) transmission
distances through coaxial cable.
Figure 3. Standard reference level SDI signal conforming to
SMPTE 259M.
Figure 4. Eye pattern shows normal cable losses, yet jitter is
still well within specs.