3G HD-SDI and the Trinix Advantage
whitepaper
3G HD-SDI and the Trinix Advantage
Marc Walker, Senior Engineer, Grass Valley
March 2011
How the emphasis on design excellence inside
the routing frame of the Grass Valley™ Trinix™
NXT router can reap dividends by creating
much-needed extra headroom outside of the
frame, especially when dealing with legacy
equipment and cabling systems.
www.grassvalley.com
Contents
The Challenge of 3 Gb/s HD-SDI
1
A Review of Digital Advantages
1
Cable and Circuit Board Frequency Response Characteristics
2
Cable Frequency Response Comparison
3
Cable Distance Considerations
4
Interconnection Problems
5
Crosstalk
8
The Trinix NXT Advantage
9
3G HD-SDI and the Trinix Advantage
The Challenge of 3 Gb/s HD-SDI
The transition to 1080p60 digital video has created more challenges in transmission of SDI signals. This new signal at 2.97
Gb/s is often called 3G HD-SDI, or simply 3G. This is twice the
data rate of the 1080i60 HD-SDI signal that has been in use for
about ten years.
Doubling the data rate has increased the difficulty of building
equipment and systems. The bits are now about 3 inches or 7.5
cm long in a cable, and about 2 inches or 5 cm long on a circuit
board trace. Almost all circuit interconnections must be treated
as transmission lines. Cable losses increase by 40%, connector
discontinuities become twice as significant, the signal band-
width doubles, the crosstalk potential increases, and amplifier
gain is harder to achieve at the higher bandwidth. All of these
factors make building equipment more than twice as hard at
3G.
The good news is: it can be done successfully, with careful
design and attention to details. We will focus this whitepaper on a review of digital advantages, cable and circuit board
frequency response, cable frequency response comparison,
cable distance considerations, interconnection problems, and
crosstalk.
A Review of Digital Advantages
Analog storage and transmission systems have existed for a
long time. Analog is generally simpler to implement. Analog
signals can be damaged by things such as noise, crosstalk, bad
frequency response, reflections, etc. Some of this damage can
be fixed or partially repaired in an analog signal, but it is seldom
the same as the original. The quality of analog recordings,
photographs, or office copies deteriorates with each generation
of copies.
A digital video signal is just a series of numbers representing
the image. Digital copies can be flawless, if the numbers are
recovered correctly from the source media, and transferred correctly to the destination media. Digital audio and video signals
usually originate as analog signals that are converted to digital
signals. Some of the fine details of the analog signal are lost
in this conversion, but the infinite number of possibilities in an
analog signal are converted into a finite set of numbers that can
be stored and transmitted. How well the digital signal represents the image depends upon the precision, accuracy, and
quantity of the numbers. This requires decisions to be made
concerning sampling rates and the number of bits per sample.
The digital signal numbers can be stored and transported in
www.grassvalley.com
many ways without additional quality losses. Possible storage
media includes hard disks, data DVDs, magnetic tape, paper
tape, or even Roman numbers written on paper towels in old
English script.
The numbers can be transmitted in many different methods,
but the generally accepted format in professional television is
SDI, as standardized in SMPTE standards. This is an 800 mV
binary serial digital signal, transmitted at various data rates on
coax cable. The binary representation has two possible values
at the center of the bit time, 1 or 0. This digital signal is actually an analog representation of the numbers that represent
the image, and becomes subject to the problems of an analog
system. The challenge is to tell the difference between the two
binary values, at the destination, with sufficient accuracy to
recover all of the numbers correctly.
The advantage of digital transmission is much better immunity to noise and distortions. The price to pay is a significant
increase in bandwidth. If the digital numbers are correctly
received, the image is successfully transported with NO additional impairments.
1
3G HD-SDI and the Trinix Advantage
Cable and Circuit Board Frequency Response Characteristics
Coax cable has signal losses that increase with frequency,
much like a low pass filter. Some of the losses are due to
resistance of the wire and skin effect, while other losses are
caused by dielectric absorption in the insulation. The loss curve
is approximated by the formula where L is the loss in dB per
unit of cable length:
L = A + Cf + B

f
A, B, and C are constants that are dependent upon the type
of cable and unit length. “f” is frequency. In many cases, the
A and C components are ignored, resulting in the common
approximation of cable losses being proportional to the square
root of the frequency. This means, if the frequency is multiplied
by 4, the attenuation of the cable, expressed in dB, doubles.
This can be a good rule of thumb, but the other terms are still a
part of the losses and may be important.
2
To properly recover the serial bits, it is desirable that the system, from the transmitter to the bit detector, has a frequency
response that is nearly flat to at least ½ the clock rate. It is also
desirable that the roll off be reasonably gentle out to 3 times
the clock rate. Belden 1694A cable is specified to have a frequency dependent insertion loss of 26 dB per 100 meters at 1.5
GHz. This amount of loss requires equalization in order to work
successfully for serial digital video. Since the length of a cable
is not always predictable, adaptive equalizers have become the
expected solution. They automatically adjust the equalization to
match the apparent cable losses, up to a specified equalization
limit.
Circuit boards also have losses that increase as the frequency
increases. These losses are usually much greater per foot, than
coax cable, but the trace lengths are usually less than a few
feet. As traces become longer on circuit boards, the losses
increase, and equalization may become necessary on the internal connections. Some ICs have built in equalization capability
with fixed settings, but they may not be optimum for a given
path. As a general rule, backplane signal interconnect lengths
should be kept to the minimum necessary.
www.grassvalley.com
3G HD-SDI and the Trinix Advantage
Cable Frequency Response Comparison
Not all video cables are created equal.
Be careful that your choice of cable for
long cable runs is acceptable. Many early
equalizers for 270 Mb/s systems were
optimized for Belden 8281 cable, since
there was a lot if it already installed in
analog systems. Belden 8281 is an excellent cable for analog video. Other cables
are now available with better performance for digital video systems.
This whitepaper discusses various
Belden cables, as an example, but the
concepts also apply to cables from other
manufacturers. If you examine the shape
of the loss curve for Belden 8281 cable,
the losses appear similar to Belden
1505A, but shape of the loss curve is
somewhat different. 8281 has lower
losses at low frequencies, such as the
analog video it was designed for. 1505A
has lower losses at the high frequencies encountered in Serial Digital Video
systems. This is partially due to the use
of foam versus solid dielectrics.
Figure 1 – Comparison of 86 meters of Belden 1505A and 100 meters of Belden 1694A to 86
meters and 68 meters of 8281. The response of 86 meters of 1505A and 100 meters of 1694A are
almost identical to each other.
It is desirable to match the cable loss
curve to the equalizer correction curve
for best system performance. New
equalizers are usually optimized for
Belden 1694A and similar cables, but
8281 will probably work in most applications. Many of the new cables, for digital
video, have similar shapes to their loss
curves, but the loss per 100 meters is
different for the individual cables. For
example, 100 meters of 1694A has
about the same losses across the signal
spectrum as 86 meters of Belden 1505A.
This gives a family of cables to choose
from when deciding between cable size
and cost.
Figure 2 – Cable length comparison for equal loss at 1.5 GHz.
www.grassvalley.com
3
3G HD-SDI and the Trinix Advantage
Cable Distance Considerations
How long can your cables go? How far do you
need to go? 100 meters has been the common
length for 1.5G cable lengths using a cable with
losses similar to Belden 1694A. By just looking at
cable loss equations, this would equal about 70
meters of the same cable for an equivalent 3G
system. Is 70 meters enough for your system?
SMPTE 292M and SMPTE 424M state that the
input cable distance should be specified by the
manufacturer of the receiver. These two standards
also suggest this will be common when the cable
losses are in the range of 20 dB at one half of the
clock rate of the signal. SMPTE 259M, for 270
Mb/s, 360 Mb/s, and 540 Mb/s systems, suggests
a cable loss range of 20 dB to 30 dB.
Belden, and other cable manufacturers, may give
a suggested cable length for a given cable. This
is based upon an assumed value for the equalization capability of the receiver. The maximum cable
lengths listed by Belden are for a 20 dB equalizer
in the receiver. In real systems, this distance will
increase or decrease depending upon the receiver
characteristics. If a better receiver is available, the
maximum cable lengths will be extended.
Choose your cable types wisely. In general, a
physically smaller coax cable will have higher
losses, resulting in a shorter maximum distance.
You must also be careful about the characteristics
of the cables within a given physical size.
Figure 3 – Comparison of frequency response for 100 meters of various cables.
Belden 1505F, the more flexible version of 1505A, has about
43% more losses than regular 1505A. This gives 1505F about
the same losses as the physically smaller 1855A up to 1.5
GHz, and greater losses above 1.5 GHz. The plenum rated
cable, Belden 1506A, has losses slightly higher than 1505F, and
significantly higher than 1505A. The plenum rated cable, Belden
1695A, has losses 26% greater than 1694A. Belden 7731A has
lower losses than most of the other digital cables, but it is also
4
larger and heavier. 7731A may help you on long cable runs to
the far ends of your facility.
You must consider what happens when you mix cable types.
If your receiver is specified for 100 meters of 1694A, it may
not work correctly if you use 70 meters of 1694A connected to
30 meters of 1855A. This will be the equivalent of about 120
meters of 1694, exceeding the receiver specifications, even
though the physical length is only 100 meters. Losses or problems in the interconnecting barrels or patch panel and connectors may further reduce the working distance.
www.grassvalley.com
3G HD-SDI and the Trinix Advantage
Interconnection Problems
The interconnection between digital video devices is subject
to many issues. The best option is a single length of coax cable
that has less loss than the capability of the receiving device
to equalize. Patch panels, transitions to other cable types,
damaged cables and connectors, and even coax barrel connections can cause additional signal losses, impedance bumps,
and signal reflections or standing waves. A single weak barrel
or connector can cost 1 meter of 1694A cable length, while a
good barrel will have much smaller losses.
It can be demonstrated that one impedance bump or discontinuity between a signal source and destination is not too
serious. It will have some effect upon the signal transmission. The reflected signals will be absorbed at the ends of the
transmission line. Figures 4 and 5 demonstrate that a good 75Ω
barrel and connector cause very little disturbance in frequency
response. Figure 6 shows that a single 50Ω barrel increases
the losses at 1.5 GHz by 0.5 dB, and starts to put ripples in the
frequency response. Figure 7 shows significantly worse return
loss than achieved with the 75Ω barrel.
Figure 4 – The blue curve is the frequency response of two ½ meter
cables connected with a 75Ω barrel. The horizontal red line is a 0 dB
reference line and the red curve is a 1 meter cable. The response is
down about 0.4 dB at 1.5 GHz and almost matches the 1 meter cable.
Figure 5 – Return loss of two ½ meter cables connected with a good
75Ω barrel. The return loss is better than 20 dB to 3 GHz.
Figure 6 – The blue curve is the frequency response of two ½ meter
cables connected with a 50Ω barrel. The horizontal red line is a 0 dB
reference line and the red curve is a 1 meter cable. The response is
down about 0.9 dB at 1.5 GHz. Some evidence of reflections is present.
Figure 7 – Return loss of two ½ meter cables connected with a 50Ω
barrel. The return loss is worse than 15 dB at frequencies above 800
MHz.
www.grassvalley.com
5
3G HD-SDI and the Trinix Advantage
Interconnection Problems (cont.)
Two impedance bumps that are separated by a few inches,
such as an old patch panel, a couple of bad barrels, or even bad
connectors, can cause reflections to occur between the two
bumps, setting up standing waves and significantly distorting
the frequency response of the connection system. These multiple impedance discontinuities will increase the interconnection
losses and reduce the distance the signal can travel.
A TDR, or time domain reflectometer, can be used to look at
impedance discontinuities in a system and observe the reflections that result. Most TDR units are built for 50Ω systems, but
they can also function for 75Ω systems. A reading of 200m corresponds to 75Ωs. A reading of 0 corresponds to 50Ωs.
Figure 8 is the actual TDR for two 50Ω barrels separated by
22 cm of cable. The actual impedance of the barrels measured
about 53.3Ω. The second barrel is identical, but appears different
because some of the original TDR pulse was reflected back by
the first barrel. There is a small reflection visible 2 ns after the
second barrel. This is a result of a reflection from the second
barrel bouncing off the first barrel and then off the second barrel
again.
Figures 9 and 10 show the frequency response and return loss
for this combination. Note the large ripples in the frequency
response. The return loss is also much worse than the single
barrel.
Figure 9 – Frequency response of two 50Ω barrels separated by a 22 cm
cable, with a 50 cm cable on each end.
6
Figure 8 – TDR of two 50Ω barrels separated by a 22 cm cable, with a 50
cm cable on each end. Note the reflection two divisions before the end
of the trace. This is due to reflections between the two barrels.
Figure 10 – Return loss of two 50Ω barrels separated by a 22 cm cable,
with a 50 cm cable on each end.
www.grassvalley.com
3G HD-SDI and the Trinix Advantage
Interconnection Problems (cont.)
Figure 11 is a set of simulated TDRs for
the two 50Ω barrel conditions. 200 mV
corresponds to 200m and is equivalent
to 75Ω. 0 mV corresponds to 50Ω. The
purpose of this simulation is to compare a simulation of the actual TDR to
simulated TDR plots with slower rise and
fall times. The slower rise and fall times
correspond to various SDI data rates. It
demonstrates that with lower data rates,
having slower rise and fall times, the
discontinuities caused by the barrels are
less disturbing. Thus, as the data rate is
increased, we must be more careful with
cabling and connectors. The discontinuities become more significant as they
become a larger fraction of the bit time.
Figure 11 – TDR simulation of two 50Ω barrels separated by a 22 cm cable. The top trace TDR
pulse shape is set to match the actual TDR. The next three traces show simulations matching the
pulse shapes of 3 Gb/s, 1.5 Gb/s, and 270 Mb/s pulses.
www.grassvalley.com
7
3G HD-SDI and the Trinix Advantage
Crosstalk
Crosstalk is another issue of concern in signal-dense situations. Crosstalk is similar to adding noise into the signal, but it
is not random. It occurs when one or more interfering signals
are coupled into the desired signal. Crosstalk may have many
sources. Crosstalk between cables in a cable tray is theoretically possible, but has not been observed to be a problem in
television systems. The signals are generally well contained
inside coax cables and connectors that are in good condition.
Crosstalk is usually more of a problem where many signals
come together in a piece of equipment, such as a multichannel
distribution amplifier.
In general, crosstalk will be at least proportional to frequency.
This means an increase of at least 6 dB in crosstalk with each
doubling of the data rate. In digital systems, crosstalk will usually show up as increased jitter. A typical crosstalk specification
for regular analog video switchers is –60 dB, or 0.1%. This is
usually to a frequency of 5 MHz or less. If that amount of coupling, is translated to 1.5 GHz, using an increase of 6 dB each
time the frequency doubles, the crosstalk will be about -10 dB.
This is too much crosstalk for the serial digital system to work
well. Thus careful mechanical and electrical design is necessary
to get adequate isolation.
Cable equalization at the switcher inputs compounds this
problem. If the interfering signals are coming in on short
cables, from a nearby source, and the desired signal is coming
on a cable near the maximum length, you have what is called
hostile crosstalk. Crosstalk at the switcher inputs reduces the
capability of the equalizer circuit to properly recover the signal.
When the desired signal is boosted 26 dB by the equalizer, the
interfering crosstalk is also boosted 26 dB. Therefore, if you
assume a necessary budget of 20 dB signal to crosstalk ratio,
and 26 dB of equalization in hostile systems, you need at least
46 dB of isolation between the sum of the hostile signals and
the input being used. This isolation is needed at frequencies up
to at least one half of the clock rate, or 1.5 GHz for 3G video.
For 36 dB of equalization capability, at least 56 dB margin is
required between the sum of the crosstalk sources and the
selected input. This is roughly equivalent to 106 dB of isolation
at 5 MHz
There must be sufficient margin between the interfering signals
and the desired signal to get the full benefit of a long distance
equalizer circuit. If the isolation is not adequate, it will reduce
the usable reach of the equalizer system, when hostile crosstalk is present.
Crosstalk can also occur within the switcher circuitry, following the cable equalizer circuits. At this point, the signals inside
the switcher have been equalized, and are all at the same
amplitude, but crosstalk there can still damage or destroy the
signal. Careful attention to the internal design and construction
is required, due to the high number of high-speed signals in a
very small space.
Effect of Crosstalk Occuring Before Cable Equalizer
Frequency Response in dB
Frequency in MHz
Figure 12 – Crosstalk occurring before the equalizer is amplified by the equalizer
along with the incoming signal.
8
www.grassvalley.com
3G HD-SDI and the Trinix Advantage
The Trinix NXT Advantage
The Grass Valley Trinix NXT system design allows us to provide
exceptional performance within the issues discussed above.
We studied all of the components we could find and tested
several of each type. We chose the combination of parts that
gives us the best performance. When new parts come along
that make a better system, we will use them. Trinix NXT has
the extra performance capability that allows you to use the
same 100 meters of cable you installed for your 1.5G system,
and make it work at 3G. It also gives you an extra margin to go
longer distances, or headroom for some problems in cabling
systems.
The Trinix NXT system has input modules, output modules, and
matrix modules like many other distribution switchers. It also
has a backplane board to interconnect the signal modules. The
mechanical and electrical design of the system was conceived
to minimize the length of the traces on the high-speed backplane. The longest signal traces on the backplane are about 35
cm or 14 inches in length. Most of the traces are much shorter.
This minimizes the high-frequency signal losses in the backplane, and improves the signal integrity within the Trinix NXT
system. The module-to-backplane connectors were selected to
have excellent performance for 1.5 Gb/s signals, and provide
isolation between the signals. Although the original design
target for the Trinix NXT backplane was 1.5 Gb/s, we didn’t stop
with good enough. We made it the best we could. That same
backplane also performs extremely well at 3G. We even had it
tested by an independent lab to assure the backplane and module interconnections would be adequate for 3G. They are more
than adequate, and they do work well.
With Trinix NXT, it is less than 2.5 cm or 1 inch from the end of
your cable to the circuitry on the board. The I/O circuit boards
plug directly into the back of the BNC connectors, using a
patented design. Every input and output on the board is almost
identically spaced and placed near the BNC connectors. This
gives excellent matching between channels, resulting in consistent performance. Since the input equalizers are located very
close to the input connectors, it minimizes the opportunity for
weak signals to be corrupted by crosstalk from strong signals
nearby.
Figure 13 – Trinix I/O connector panel system gets the circuitry very
close to the BNC cables.
www.grassvalley.com
9
3G HD-SDI and the Trinix Advantage
The Trinix NXT Advantage (cont.)
Other systems usually have at least one circuit board or set of
cables to connect the BNC connectors to the I/O boards. These
are often called “mid plane” designs. This can cause several
problems:
• First, no connector is absolutely perfect. These systems
need an additional connector and the interconnecting circuit
board or cables. The second connector adds additional
imperfections to the impedance matching of the system.
This can cause standing waves to occur between the two
connectors, and attenuate the high-frequency components
of the signals. The extra connector and the distance between
the connectors make it more difficult to meet return loss
specifications. Many times, the interconnection length will
vary from path to path through the interconnecting board or
cables.
• Second, the interconnecting media can enable more
crosstalk to occur, due to the close physical proximity of the
signals, especially signals received through short cables are
close to signals received through long cables.
• Third, this interconnecting media will also have high
frequency losses, whether it is miniature coax, circuit board
strip line, or microstrip transmission lines.
The Trinix NXT design allows us to reach 140 meters of 1694A
cable at 3G with hostile crosstalk on the surrounding inputs.
This means the surrounding inputs are all being driven through
short cables from other 3G signals. At NAB 2009, a Trinix NXT
system was demonstrated at 3G, with no errors, running 170
meters of 1694A into the Trinix input. This illustrates the longer
distances possible when hostile crosstalk is eliminated.
Miniature I/O connectors have captured a lot of attention. They
allow the rear panel to be smaller for a given number of inputs
and outputs. This can be an advantage in tight places, but it
also has risks:
• First, the connectors may not be well matched to 75Ω, and
thus cause impedance bumps.
• Second, the signals are closer together, enhancing the
possibilities for crosstalk to occur between inputs.
• Third, the small connectors require small cable to be
installed in them. This may be fine for tight spaces and small
facilities, but if you need to go a long distance, you need
better cables. This requires a transition panel from small
cables to regular sized cables. The higher losses of small
cable limits the signal reach of your inputs and outputs. You
must plan for these losses in your cable budget.
Trinix NXT is not as I/O-dense on the rear panel as some other
systems, but that is not necessarily a bad thing. How many
cables can you manage per square foot on the rear of your
switcher? How many cables can fit within, between, or under
your racks? Do you really want to use small cables and transition to other cables? What is more important, cable reach or
rack units? Does it matter that another switcher is smaller, if it
does not work well?
Don’t just take our word for it that the Trinix NXT has the best
3G system performance. The Trinix NXT system, along with
systems from several other router manufacturers, was tested
by a respected independent laboratory in Germany. They ran a
3G signal through five passes on the system. Trinix NXT was
the only system that passed with no CRC errors. In fact, we
have run a 3G signal 32 passes through the Trinix NXT with no
errors! The lab was amazed by the length of cable that Trinix
inputs can accept. They were also surprised by the good and
consistent return loss at the system inputs and outputs.
We encourage you to explore the Trinix NXT advantage, and
find out for yourself.
SALES
SUPPORT
Local and regional sales contacts can be found by visiting
www.grassvalley.com/sales
Local and regional support contacts can be found by visiting
www.grassvalley.com/support
© Copyright 2011 Grass Valley USA, LLC. All rights reserved. Grass Valley and Trinix are trademarks of GVBB Holdings S.a.r.l. All other tradenames
referenced are service marks, trademarks, or registered trademarks of their respective companies. Specifications subject to change without notice.
RMC-4025M
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement