Ground loops effect in video output
As the source and destination of a video signal can be at
differing ac or dc earth potentials, earth loop currents flow and cause
longitudinal hum to be introduced into the video signal. Video hum is low
frequency (50 or 60 Hz mains frequency or it's harmonics) noise from the ground
lines which has influenced the video signal, causing degradation of the
displayed signal. Video hum is usually observed as bars rolling vertically
through the video image, video hum may also cause video distortion or even tearing
of the picture in severe cases. Video hum maybe a problem in any system where
video sources and display devices are connected to different A/C power sources
with varying grounding potentials.
Typically the humming can be seen as slowly vertically
moving horizonal bars in normal TV video signals. The same kind of bars can be
also seen in computer screen, but typically they are not as visible because
bars are moving so fast that you see them as some strange flashing in screen.
The picture below is a real world example of the effects
of a ground loop and what it causes a a video picture received from the cable
TV network:
Ad you can see that video signal has strong hummign bars
and other interference in it. Those have entered the cable TV signal because of
ground loops in the system. Ground loops in the video systems can have
following effects:
•Hum Bars: The mains frequency (50 Hz or 60 Hz) can cause
stationaly or moving horizonal humming bar to appear on the video signal (as
shown on the picture above). If you have light dimmers nearby those humming
bars can easily become quite severe and easily visible.
•RF Interference: Herring bone interference on video line
is caused by a ground loop (that includes your coax shield) acting as an AM
radio antenna. Any large loop of wire makes a good AM antenna. These antennas
are especially adept at picking up AM broadcasts if most of the loop is
vertical.
•Cross-Talk: Ground loops can cause one signal to
interfere with another, because every cable should ideally return through the
corresponding shield conductor, but there's an alternative path through the
other shield conductor which causes undesirable voltage differences to nearby
cables.
Isolating video signal is more complicated than isolating
audio or antenna signals, because the DC level of the video signal is important
and video signals have very high frequency spectrum (normal composite video can
have bandwidth from 50 Hz to 6 Mhz).
Isolating video signal needs typically active technology
which involves electro-optical isolation or differential amplifier with a
floating ground on the input connector. Those both technologies are usable in
real world situations. Differential input with floating ground works nicely for
small ground potential differences and this approach is used in some
professiona video equipments (some video projectors I have seen have had
differential inputs and option to disconnect input ground connection). Differential
inputs are also used in applications where a video signal is transmitted
through twisted pair wiring (some CCTV applications which use twisted pair
interfacing equipments).
Electro-optical isolation works well in applications
where complete electrical isolation is necessary. There are some this type of
isolation devices on the market and some special video distribution amplifiers
have this kind of option built in.
Ground loop elemination does not always ask for a
complete isolation of the grounds. There are passive hum suppressor
transformers which will very effectively remove the hum from the video signal
(typically around 40 dB hum level reduction), but do not effect the video
signal otherwise. Those special transformers act like a common mode coils,
which stop the annoying ground loop currents on the shield of the coaxial
calbe, but provide a straight path for the signal inside the cable. This kind
of devices are capable of passing the signals from DC to tens of MHz without
problems. This type of hum suppression transformers have found their way to the
professional video application (rental companies) and comouter video
applivations (computer to video projector connections). The transformers of
this type are usually called "hum bug transformers", "humbucking
transformers", "anti-hum video transformers" or "hum
suppressor transformers". Generally term hum-bugger refers to any circuit
(often a special coil) that introduces a small amount of voltage at power-line
frequency into the video path to cancel unwanted ac hum.
There are also special wideband isolation transformer
which can isolate video signals. A transformer which can nicely transfer the
whole video frequency spectrum without much distortion is very hard to produce
so there are not many of them on the market. Some of the isolation transformers
are only designed for CCTV application, where more signal distortion is
accepted than in broadcast industry.
The choke (humbugging transformer) is primarily used in
Broadcast TV because it passes the DC component of the signal. It is used in
studio, and in remote ENG. The isolation transformer is primarily used in CCTV:
security, manufacturing, avionics, display, etc.
Differential video amplifiers
Differential amplifier approach uses an operational
Amplifier. Operational Amplifiers only amplify the difference between the two
input lines. This method eliminates common mode noise between the incoming
signals by making A-B=C, as only the difference between A & B are amplified.
Operational amplifiers is maintain wide bandwidth signals throughout your
system while eliminating ground loop problems that are caused by power and
video. Diffeerential video amplifier inputs are used in some video equipments
(typically some video projectors) and video distribution amplifiers to fight
against ground loop problems.
Differential video amplifiers have a limitation on their
input voltage range which gives some limitations how much common mode signal
those circuits can tolerate. If the ground potential difference is more than
few volts, then operational amplifier based isolators don't work effectively.
Too high voltage difference can cause problems from very distorted video signal
to damaged differential video amplifier. If the voltage difference is a
substantial proportion of the DC supply voltage of the amplifier, you will
probably have trouble using an amplifier alone.
It is a good idea to measure the voltage difference
before using differential video amplifiers to be sure not to damagze them.
Measuring can be done using a multimeter (check using both AC and DC ranges) or
better using a scope earthed to the mains supply, and put the probe on the
earth connection of the incoming video cable. If you many potential difference
which are many volts, then you have quite propably something wrong in the
grounding of the building and you should consult a qualified electrician to
check and correct this potentially dangerous problem.
Good back porch black level clamp
If the video signal input has well designed fast black
level clamp circuitry that can also solve small common mode noice problems
caused by ground loop. Back porch ground level clamp circuit adjusts the black
level of the video circuits according the incoming video signal. If black level
clamp circuit is active circuitry which samples the black level saparately for
every can line the ground loop bars are quite effectively eliminated because
the the low frequency noise (50 Hz power or harmonics) is sampled at start of
every scan line and suppressed then from the rest of the line. This works quite
nicely with those low frequency humming bars, especially if combined with
differential video inputs. back porch black level clamp system does not help in
fighting against higher frequency noise which might be injected to the video
system through the ground loop.
Active video isolators
Video Isolator passes a video signal from its input to
its output with no electrical connection and is able to provide complete
isolation, for the video signal, passing through it. Having the Video Isolator
in the video signal path makes it possible to have Standard Safety earthing of
all equipment with no associated earth loop problems. In the studio, feeds
between different buildings are no longer a problem and it is no longer
necessary to run technical earth to non critical locations such as viewing
rooms.
Electro-optical isolators convert video signal voltage to
blinking LED and other part of the circuit receives that light and convert it
to back video signal voltage. This method guarantees very good isolation (complete
galvanic isolation), but has typically bandwidth and linearity problems. Poor
bandwidth will result in fuzzy images and poor linearity will result in an
inability to produce the same gain for all signal levels (most noticeable in
gray-scale patters).
Anti-hum video transformers
Anti-hum video transformers are not real transformers,
they are common mode chokes! Anti-hum transformers work as a series inductor
offering a series impedance to the circulating earth currents thus effectively
reducing the current flowing in the loop which will reduce the voltage dops on
the cable shields and equipment (that reduces hum). Those coils can reduce the
currents on cable shield very effectively because they have very high impedance
at 60 Hz and above and there presents a high impedance to common mode signal
differentials between the input and output. Earth loops typically have low
resistance a quite the inductance will not have to be very huge to start to
help. The coil itself will then have quite much voltage difference on the input
and output grounds (the potential difference is now over the transformer insted
of distributed to whole cable), but the common mode coil construction
guarantees that this difference is not supped to the differential signal inside
the cable. Since the signal and ground lead are coincident, the differential
signal is unaffected.
Hum reduction transformers or common mode coils are
constructed with either 75 Ohm twisted pair (made of fine wire) or coaxial cable
wrapped around a very high permeability core. Most basic hum isolation
transformers are basically just coax cable wound on a toroid-type core. They
work by mutual inductance. The coax cable is wound around a transformer core so
that both the inner and shield of the cable become inductors. The tight
coupling ensures that any voltage in the shield caused by variations in earth
potential are transformed into the inner conductor.
The method is an ancient idea and can cope with very
large ground loop signals, and has very large bandwidth with very little loss.
This type of anti-humming transformer also provides DC continuity between the
input and output leads which is a good thing. transformer does not stop ground
loop current flowing (the amout of current is lower bause the added indictance)
but the transformer reduces the current cancel the effect of ground loop
current. Good one can reduce the ground loop effect up to 40-50 dB.
Hum isolation transformers are effective solutions for
hummign problems. The downside of them is that they are somehow bulky devices
because of the large core needed to do the job. The boxes I have seen have been
packed in metal case have weighted at least one kilogram. Hum isolation
transformers are typically stand-alone passive boxes which are added to video
system when problems are encountered.
Video isolation transformers
There are special wideband isolation transformer which
can isolate video signals, but not without problems. The design of a high
bandwidth transformer which can go to very low frequencies is very hard. You
have to always make some compromises on low and high frequency responses
(highest components of composite video cna be attenuated even few dB). All real
isolation transformers have one serious drawback which can't be avoided: they
can't pass the DC level through. So any system that relies on the video having
any particular DC reference will not function properly. There are many video
systems around which need particular DC reference level, but there are many
which are AC coupled.
Some of the isolation transformers are only designed for
CCTV other not so demanding applications application, where more signal
distortion is accepted than in broadcast industry. So a video isolation
transformer might be OK for a security camera installation if a complete
isolation is needed, but I would not put it on any professional video studio
system.