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EuroWire – November 2009
69
technical article
A major improvement in chip design in
the last two decades has been the ability
to withstand, and continue operating
with, large amounts of reflected signals.
The ideal passive device would be exactly
75 ohms with no reflection at all, but this
is not possible. This is where the return loss
guarantee, previouslymentioned, becomes
a serious requirement to maximise the
HD performance of cable, connectors and
similar components.
One should also note that the SMPTE
return loss requirement (–15 dB or
3.16% reflection) is extremely generous.
Installers are cautioned that component
manufacturers who claim to meet the
SMPTE standard are suggesting only that
their components are no better than the
minimum requirement, not a very positive
start to an installation.
Translate to 1080p/60
When converting to 1080p/60, the clock is
doubled to 1.5 GHz, and the third harmonic
raised to 4.5 GHz. Memory space, tape,
disc, hard-drive size is essentially halved to
store these images, audio, and metadata.
The standards for this are contained in
SMPTE 424M. Return loss minimum, under
this extended specification is –15 dB to 1.5
GHz and –10 dB to 3 GHz
[note2]
. Components
should be tested to 4.5 GHz and some
return loss guarantee should be assigned.
One cable manufacturer now routinely
tests many of its HD cables to this new HD
standard with a guarantee of –23 dB from
5 MHz to 1.6 GHz and –21 dB from 1.6 GHz
to 4.5 GHz. A similar guarantee should be
sought for all passive devices.
Cable distance
Digital signals have a significant problem
with distance. Receiving chips can perform
basic error correction until the error rate
becomes greater than the chip can handle.
This means that the digital image is perfect
until that critical distance where the data,
the image, can no longer be resolved.
The chip rapidly goes from perfect picture
to no picture in only a few feet of cable.
This is commonly called the digital cliff.
The real concern is that an installer or user
could be inches away from the cliff, and
not know it. By simply inserting a patch
cable, even a patch cable specifically made
for digital signals, the user might push that
signal over the cliff.
In the SMPTE 292M standard, there is
a formula to determine the maximum
distance on any given cable. It simply states
than when the signal drops 20 dB at half
the clock frequency, that is the distance
limit.
Table 3
shows some common cable
sizes with this calculated distance for HD.
Also shown is the maximum distance
for SMPTE 424M, running 1080p/60, again
20 dB loss at half the clock.
However, the distances in
Table 3
are
based on a formula, not on real-world
applications. The real-world distances are
obviously very chip-dependent and really
good chip sets would perform over longer
distances than those shown
[note4]
.
The distances in
Table 3
are approximately
halfway to the digital cliff with an average
chip set. Thus, a user could probably double
these distances before reaching the cliff.
The numbers shown are, therefore, ‘safe’
numbers, designed to keep an installation
operational even when there are flaws, a
poor connector or two, a bent cable or an
older device with yesterday’s chips.
Testing distance
If it is decided not to rely on
Table 3
, or
similar distance charts, then cables must
be tested. Given that a high-quality
network analyser can cost $60,000 or
more, most installers are content to use
a chart. There are, however, ways to test
HD and 1080p/60 for little or no cost and
determine, in an approximate way, where
the cliff is. To be most effective, an installer
should use a single cable type, produced
by a single manufacturer. Mixing cables
and manufacturers makes the situation
even more unpredictable.
Testing requires an HD source. It doesn’t
matter what the source is as long as it
produces the appropriate HD or 1080p/60
output. Also required is an HD or 1080p
monitor. A professional broadcast-quality
display has one valuable feature: the image
can be shifted so that the centre of the
monitor is the black ‘retrace’ area between
images. When looking for bit errors, these
will show as flashes of pixels, the most
recognisable being a black pixel turning
white (where 0 is misread as 1).
A normal video image can often hide these
bit errors, especially if the image is ‘busy’,
so the black inter-frame area is the best
choice. Then it must be decided how far
to be from the digital video cliff. Whether
ten, twenty or fifty feet, once a number is
chosen the tester will take a piece of cable
of that length, matching the coax cable
used in the installation. Assuming for this
example that a distance of 50 feet has
been chosen, the tester will take a piece
of cable that is 50 feet long. Connectors
should be attached at each end and, at
one end, a female-to-female adaptor. This
adaptor must have low return loss at the
highest frequency required to pass. It is a
male-to-female ‘extension cable’.
The installer simply adds this extension
cable to any existing cable under test
between the HD source and the monitor
– it can be added at the source or desti-
nation end, it is simply extending the
original installed cable. If, after a few
seconds, small white flashes cannot be
seen in the black inter-frame area, then
the cable length is at least 50 feet short
of the cliff. If flashes are discernible, it
could mean that the cable under test is
damaged, the connectors poorly attached,
or the wrong connectors or cable used.
It could mean that the cable under test is
simply too long for that signal.
There is a choice of solutions: check the
cable and connectors, replace it temporarily
to test if it is creating the problem.
Move equipment in the rack to change the
length of cable, or change to a larger cable
with lower loss.
Return loss
Match Reflected
-10 dB
90%
10%
-15 dB
96.84% 3.16%
-20 dB
99%
1%
-21 dB
99.21% 0.79%
-23 dB
99.5% 0.5%
-25 dB
99.68% 0.32%
-30 dB
99.9% 0.1%
-35 dB
99.97% 0.03%
-40 dB
99.99% 0.01%
Table 2
▲
▲
:
Return loss versus match
Cable
Diameter
HD distance 1080p/60 distance
7731A RG-11
0.405"
550
ft
360
ft
1694A RG-6
0.275"
400
ft
270
ft
1505A RG-59
0.235"
310
ft
220
ft
1855A 'Mini'
0.159"
260
ft
150
ft
179DT 'Micro'
0.100"
110
ft
80
ft
Table 3
▼
▼
:
Cable distance by cable type and signal