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Cables and Interconnects
July 2002
A
cable, or interconnect, should be “transparent” to the signal traveling over
it. In other words, it should pass the signal along in its pure original form
and add nothing. Poor quality cabling will introduce distortions such
as
Frequency Distortions.
If you analyze the physical construction of any
cable, you find a textbook example of a passive filter network. The diagram
above shows this. The effect of having a filter network in line with an
electrical signal is to attenuate (reduce) the different signal frequencies in
differing amounts. For example: with a video cable, the video image would lose
sharpness because the higher frequencies are reduced. With an audio cable, you
would lose the higher frequency sounds (Note: it is often said that cables act
as “tone controls” in an audio system. Frequency distortion is the reason
for this.) Unfortunately, there is no way to eliminate these effects entirely,
but through proper engineering the frequency attenuation effects can be reduced
to imperceptible levels.
Phase Distortions.
Because cables act like passive filters, they also
impart phase (time) distortions. Because signals of different frequencies travel
through wires at different speeds, the effect is that higher frequencies
actually arrive at the end before lower frequencies do! The diagram below
illustrates this.
There are two fronts on which to attack this
problem. First, engineers can design the cable so that the values of R, L and C
are as low as possible. Second, they can design the cable so that the higher
frequencies are slowed down to match the lower frequencies. This is what is done
with most high end cables.
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R.F.Interference
Lines - Copyright Columbia Tristar 2001 |
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Local Interference Distortions.
The cables and wires used in a home theater system are, in essence,
long pieces of wire, and long pieces of wire act like antennas to various noise
sources. The two most common noise sources in the average home are local
broadcast stations and AC power wiring. Local broadcast stations are of
different types. The most common are local television stations, AM and FM radio
stations, Ham radio and other RF (Radio Frequency) sources. These sources of
interference can be very powerful and can easily radiate into your A/V system to
cause wavy lines throughout the video picture (see the diagram on the previous
page.) AC power wiring, on the other hand, can induce “60Hz” hum throughout
a A/V system and can be heard in the audio, as well as seen in the video.
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Termination
Distortions.
All cables have a certain impedance (resistance to current flow.) For example,
the impedance of RG-6 coax cable is supposed to be exactly 75 Ohms. The
manufacturers of Audio/Video equipment design their units to match the impedance
of the cables connected. The problem is that some cables are “not to spec”
and do not match the equipment impedance. The result is a signal reflection that
appears as ghosts and other nasty distortions. Another way that termination
distortions can occur is when interconnect plugs oxidize. That is why the best
cables have gold plated connectors.
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The Following illustrates
the typical cables and interconnects in a home entertainment system.
Suggested cable internal construction details for high performance and
immunity to interference are shown.
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3) Audio
Interconnects
Carries:
Audio baseband signals (line level audio)
Frequency: 1 Hz to 100 kHz, Impedance: 300 Ohms
Maximum run length: 50t*
Features to
look for:
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24k Gold plated machined connectors.
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Internal Balanced Twisted Pair construction for noise
Immunity.
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Conductor Wingdings of different gauges to eliminate phase
-distortion.
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4) S-VHS Video Interconnects
Carries: Video baseband
signals.
Frequency: DC to 100 Mhz, Impedance: 75 Ohms
Maximum run length: 10 feet*
Features to look for:
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24k gold plated connectors for resistance to
oxidation.
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Dual Shielding (minimum) for RF interference Elimination.
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OFC center conductor and low loss dielectrics for -precision
75 Ohm performance.
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