Specifications

minimum, the technician can determine, with
some degree of success, the likelihood of
proper data recovery by measuring the eye
pattern level, comparing the estimated jitter at
transition crossovers to the allowance in the
table, and verifying that the eye is open to
some degree. When signal level is down near
the limits of a Class A receiver’s capability, only
the intended receiver or a serial digital monitor
will suffice. The best course of action is to
invest in a component monitor that can make
definitive signal margin measurements, which
will guarantee that you know your proximity
to the operational cliff.
Slowing to a Stop
Hopefully this article proves helpful and
removes some of those SDI implementation
roadblocks in your path. I intended to present
enough information to clarify the real issues
for and against SDI routing with analog
equipment. Our position at Extron is to
recommend routing serial digital with
equipment specifically designed for the
purpose of minimizing design time and risk.
Our stance is often in opposition with some
equipment manufacturers that visibly push the
application of their analog products but do
not always provide enough information to
guide a complete, reliable design. We feel that
if you have the education, test equipment,
and the confidence to proceed with
application of analog systems for SDI
distribution, then by all means, take
advantage of it, as it can be highly successful
and lower in cost. However, if you do
get caught in an analog speed trap, give
us a call…we’ll find a way to fix
your ticket.
1. “Routing Switcher Technology,” by
Barry W. Albright, Broadcast Engineering,
July 2000.
2. “A Guide to Digital Television
Systems and Measurements,” by David K.
Fibush, pp. 21-36, Copyright 1997,
Tektronix, Inc.
January – March 2003 ExtroNews 14.1 9
TECHNICALLY SPEAKING...
AES/EBU Audio
Serial NTSC
Serial PAL
Serial Component
Parallel NTSC
Parallel PAL
Parallel Component
Parallel HD
Period
(ns)
Standard
Jitter
Spec
% of
Clock
163
7
6
4
70
56
37
7
40.0
0.5
0.5
0.5
10.0
10.0
6.0
1.0
25%
7%
9%
14%
15%
20%
16%
14%
you can see that the more open the eye
pattern, the easier it is to discern the signal. As
jitter and amplitude loss increase, the eye
pattern closes down until the receiver is no
longer able to lock onto the data transitions
and recognize the clock signal within. When
the receiver loses sync with the recovered
clock, you have encountered the cliff effect.
As long as the jitter does not exceed a
certain threshold (which is solely dependent
on the receiver design), digital signal recovery
is possible and the signal is synthesized back
to its original quality. In practical systems, the
clock signal is extracted from the data stream
and will still contain some jitter, depending on
the qualities of its PLL system. Having some
jitter is not necessarily detrimental if the clock
jitter is following along with the data jitter and
positioning the decoder in the middle of the
eye pattern.
2
Table 3 lists some typical jitter
specifications.
DC Component
Analog television signals are usually AC-
coupled from one system to another. For basic
signal amplification, this works well, since the
signal’s positive and negative excursions will
center on the average DC value of the signal.
This ensures that the signal stays within the
linear range of the amplifier. AC-coupling is
desirable also because terrestrial transmission
of the signal, recording to tape, or signal
processing hampers the maintenance of this
DC reference point. Cascaded signal handling
systems either have their own DC operating
reference or cannot process a signal with a DC
component. The video display, ultimately,
must restore the signal’s DC component.
Why? During creation, the black reference
for the signal was tied to a specific DC
voltage, which could be used to calibrate the
light output threshold of the display system.
Reestablishment of this threshold is essential
for proper control of the point where the
display just produces light output.
Even though SDI is digital, the
establishment of a DC operating point is
critical to its proper decoding. The AC coupled
data stream, without a DC component, will be
centered about the central operating point of
the input signal equalizer/amplifier. A next
step in the process involves lowpass filtering of
the data signal, which re-establishes the
correct DC operating point for the incoming
data. Restoring the DC component in the
receiver provides the correct switching
threshold for the downstream processing
logic.
The Mechanic’s Tools
Evaluating SDI transmission quality is
straightforward with the right test equipment.
The best tool to have in the trunk alongside
your jumper cables is a serial component
monitor. Specialized serial digital test gear will
evaluate the signal electronically via
computerized measurement algorithms to
certify its performance level at any point in a
distribution system. While an actual waveform
display is not necessary for characterization of
signal performance, it provides the technician
with visual confirmation of the testing activity.
Serial digital signals can be roughly
evaluated with a standard oscilloscope,
provided it has enough bandwidth. At a
Table 3. Typical Jitter Specifications.
2
REFERENCES: