Some of these
problems directly impact the audio and/or picture quality.
Let's examine each type of powerline disturbance below. Please
understand that a full discussion of each of these issues
would require a large chapter in a book -- these definitions
merely scratch the surface.
In order to
fix a problem, it helps to define the problem. Some of the
problems are electrical only, but manifest themselves in the
audio or video circuitry. Sometimes a problem actually originates
in the audio circuitry itself.
Some percentage, perhaps 5%
to 25% above of the "stated", "nominal",
or "specified voltage".
the opposite of the overvoltage condition, above. The original
issue of brownouts was when in large cities, heavy current
demand for air conditioners in the summer put such a strain
on the power grid that the voltage sagged, or went down. Sometimes
this happens TO the power companies, and sometimes this is
caused BY the power companies. (i.e. controlled or rolling
brownouts) The end result is the same: less current is available
and lower voltage is available.
condition caused by the powerline source being too high an
impedance to supply the necessary current demand; could be
milliseconds (such as from a motor start capacitor drawing
inrush current ) or many seconds (any continual appliance,
such as an electric iron, clothes dryer, heater, etc. This
is more often than not caused by too small a wire used in
an installation or by Aluminum wire used in place of copper.
The sine wave
simply stops for a few milliseconds, or perhaps as long as
a few cycles. This might be caused by a lightning protection
changeover relay device somewhere upstream in the power grid,
or, it could even be caused by an intermittent connection,
and although that would be a rather rare condition, it does
occurences which ride on top of the sinewave; perhaps up to
750V or so. A spike might be considered the shortest duration,
perhaps a few microseconds to a few milliseconds. A surge
might be from a few milliseconds to a few seconds. And a peak
condition might last from a few seconds to a few minutes.
to 5000V lasting from hundreds of nanoseconds to hundreds
of microseconds. It is these higher voltage spikes which burn
out equipment, often in mysterious ways. Typically these high
voltage spikes are caused by lightning somewhere on the grid,
from a few feet to a few miles away
The opposite of peaks: V-shaped
cutouts in the sine wave as seen on an oscilloscope
is the phase relationship between the VOLTAGE condition of
the line and the CURRENT condition of the line. A totally
resistive load (i.e. a lightbulb) has "unity" power
factor. A load that has a CAPACITIVE or INDUCTIVE component
REACTS with the sinewave; therefore this is said to be a "reactive"
load. This causes a harmonic imbalance of the sinewave, resulting
in Harmonic Distortion, explained below. One goal of load
balancing for example on a 240 Volt circuit is to even out
the different power factors so as to NOT introduce either
a voltage imbalance or a harmonic imbalance on the power line.
even order distortion of the 60 Hz sinewave causes mechanical
humming in transformers. The waveform, instead of being a
'pure' 60 Hz sinewave, contains some combination of even (120,
240, 360, 480 Hz) or odd (180, 300, 420, 540 Hz) harmonics.
Noise (in a transformer)
'humming' sound emanating from a transformer: the transformer
windings are magnetically saturated, often because the voltage
is too high. This saturation starts turning the sine waves
into square waves. Square waves, by nature, are composed of
a train of odd harmonics. These harmonics mechanically vibrate
the transformer and its mounting apparatus, and that is what
you hear 'mechanically'. In the instance of a subwoofer, you
might think the subwoofer [speaker] is making the noise, but
actually it's the transformer inside that is making the noise
and you are hearing this transformer noise through
the paper or cone material of the speaker. This noise is often
a mix of both 60 Hz and 120 Hz, and some of both of their
(radio frequencies, i.e. from an AM radio station, TV station,
CB radio, Ham/ Taxi / Police / industrial walkie-talkie radio,
etc etc) amplitude modulating the sine wave envelope. This
is commonly referred to as "picking up RF". See
also rectification, below.
Noise common to BOTH sides
of the line (sometimes a problem when operating from 240 Volts)
up on one side of the line measured to ground. Therefore in
the case of a 240 V line the noise would be only or mostly
on ONE side or leg, i.e one of the 120 V "legs".
or currents caused by bad, loose, or high impedance ground
connections; can also be caused by rectification in the grounding
by dissimilar metals such as aluminum / copper junctions which
then rectify the RF in the air and add the resulting modulation
voltage to the power line.
digital "audio" or "power supply" circuit
feeds its switching noise back into the power line and the
noise then shows up in another piece of equipment.
Sonic or ultrasonic
"whine" from a poorly designed switching power supply
leaking back into the power line. Expect a whole new generation
of problems with so-called "switching" audio amplifiers
- especially as they get old.
Caused by loose
or dirty mechanical connection; even caused by such seemingly
innocent devices as pushing a wire into a captive wire slot
in a switch instead of properly screwing the wire down securely
under the connection screw. May also come from relays or motor
brushes. If a switch was opened or closed at EXACTLY
the zero crossing point of a sine wave, since there is no
voltage present there is no current flowing. Since this is
not a very likely occurrence, most of the time when either
a relay opens or a person opens a switch the waveform is at
some other spot than zero, and therefore there IS current
flowing. The current attempts to continue to flow across the
airgap, causing arcing. Arcing then oxidizes, pits, and eventually
wears out the mating surfaces. Some [better] circuits use
relays filled with nitrogen which will not allow an oxidizing
arc to form... since there's no oxygen inside the relay.
noise from the previous circuit; this often implies improper
gain staging or matching, but may be a defective component
as well, acting as a noise 'generator'.
Noise: Hum at 60 Hz
A true ground loop! A true
ground loop is ONLY a 60 Hz component !
Noise: Hum at 120 Hz
a power supply filter problem; however most people "think"
that 120 Hz hum is a ground loop.
Perhaps a bad
capacitor, noisy transistor junction, or IC / OpAmp going
bad or latching up.
typically 120 hz that is clipped, (or where the filtering
circuitry in the power supply has faulted) generating a series
of ODD harmonics, i.e. a mix of 120, 360, 600, 840, 1080 Hz,
distinctly measureable by specific test equipment, such as
a spectrum analyzer. Since it is clipped, it has turned into
square waves; square waves by definition are odd-order harmonics.
So a true buzz has a very recognizable sound. Please note
that Hum and Buzz are completely different phenomena.
demodulated audio from AM, FM, TV, ham, shortwave, Marine,
Taxi, CB, etc etc. This is NOT the same as leakage, where
one signal leaks into another undesired area usually because
solid state "switches" are used rather than mechanical
Noise: Common Mode Noise
A sound that
seems ike a combination of a buzz and noise, typically caused
by "office" fluorescent lights and their starter
Noise: Transverse Mode Noise
This is the
noise you hear from "dimmers" where the buzzing
changes pitch as the dimmer knob is rotated; you are hearing
the duty cycle of the dimmer change.
Noise: Ticks and Pops
condition such as a spike will usually produce a tick in an
audio circuit. Only the very best power supplies will really
filter this out, and even they have no control of this if
the tick is riding on an audio signal which is properly going
through an amplifier. This could also be caused by a refrigerator
motor or washing machine motor (on the same circuit) starting
up. The higher impedance the entire chain of wiring is, the
more likely this will happen and be noticed.
Noise: Video Interval Beating
59.94 hz beating
against 60 Hz; often caused by ground differentials between
a feed from a "TV cable company" and the local house
or studio ground; perhaps as high as 65 volts differential.
This manifests itself as a buzzing sound, cyclical in nature,
sort of repeating every 14 seconds. Once this sound is heard
and memorized it is VERY recognizable.
A series of articles by Bill
Whitlock, here: www.jensen-transformers.com/apps_wp.html