Understanding Video and Other Unbalanced Interfaces.
Apr 1, 1998 12:00 PM, Bill Whitlock
Ground noise is a rich mixture of power line harmonics and any otherhigh-frequency noises that exist on the power line-think about listening tothe power line with the treble turned way up (don't try this at home!).Ground noise should not be confused with plain old noise, which is more orless uniformly spread across the signal spectrum, heard in a audio systemas the sound of a waterfall or seen in a video system as grainy movement orsnow. A predictable amount of this (gaussian) electronic noise is inherentin all electronic devices and must be expected. Ground noise producesartifacts such as hum, buzz, clicks or pops in audio systems and slowlyrolling dark bars, bands of specks or herringbone patterns in video systems.
Last column we explained that, in real world systems, significant groundnoise currents will flow in any wire connecting two devices and thatsignificant ground noise voltages will exist between the local grounds ofthese two devices. This column will explain why unbalanced interfaces lackany inherent ability to suppress the effects of these ground noise currentsand voltages, making them particularly vulnerable to hum and otherso-called ground-loop problems. Unfortunately, both standard 75 W coaxialvideo and standard consumer audio are such unbalanced interfaces.
Just how much ground noise is tolerable depends on whether the signal isaudio or video and the expected level of performance from the system.Obviously, an audio monitoring system in a recording studio needs to bemuch more immune to ground noise and interference than a paging system at aconstruction site. In general, video systems can tolerate more interferencethan audio systems. For broadcast video, a signal-to-noise ratio of 40 dBis considered excellent, and even expert viewers find it difficult todetect interference at -50 dB. On the other hand, a 1995 AES paper by LouisFielder of Dolby Labs suggests that, for audio reproduction in a homelistening situation, the threshold may be about -120 dB.
Common-impedance couplingUnbalanced interfaces use two-conductor connectors and cable having twocoaxial conductors, often called "single conductor shielded" cable. Themost widely-used connectors are the RCA (properly IHF) or two-conductor1/4-inch phone (or TS for tip/sleeve) for audio signals, the BNC or RCA for75 W video signals, the F for 75 W RF signals, and various RS-232connectors for data interfaces.
As shown in Figure 1, ground noise current will flow in any wire connectedbetween points A and B. Because the wire has impedance, a small voltagedrop will appear across it. In an unbalanced interface, this wire alsocarries the signal. The signal actually delivered to device B is the sum ofall the voltages in the loop from point A to C. Because the wire'simpedance is "common" to both signal and ground noise current paths, thiscoupling mechanism is called common impedance coupling. Common impedancecoupling happens when two currents flow in the same conductor.
The common impedance includes the shield impedance of the cable and theshield contact resistance at each connector. The impedance of the cable'sinner conductor has negligible effect on the coupling because it's inseries with the much higher impedances of device A's output and device B'sinput.
At power line (hum) frequencies, the impedance of a wire (or cable shield)is effectively equal to its DC resistance. According to Ohm's Law, E = I xR. Therefore, the low-frequency noise voltage, E, depends upon the groundnoise current, I, and the resistance, R, of the cable shield. Consider a 25foot (7.6 m) interconnect cable with foil shield and a #26 AWG drain wire.>From standard wire tables, its shield resistance is calculated to be 0.41W. If the ground noise current is 300 mA, the ground noise voltage will be300 mV. Because the normal reference signal level for consumer audio is 300mV, the noise 20 x log (300 mV/300 mV) or only 60 dB below a referencesignal.
Ground noise current for floating (two-prong AC plug) equipment is usuallyrelated to the equipment's power consumption, which dictates the size andtherefore the primary to secondary capacitance of its power transformer. Itis this power transformer capacitance that causes the ground noise currentsto circulate in the first place. Ground noise current can range from a fewmicroamps for a turntable or CD player to nearly a milliamp for an audiopower amp or video monitor (see Jensen AN004 for more details).
Ground noise current for grounded (three-prong AC plug) equipment can bevery high because the ground noise in the building's wiring is effectivelyforced across the unbalanced cable's shield. Currents may reach 100 mA and,in some situations, noise voltage may actually be larger than the referencesignal.
Reducing the couplingGround noise coupling is an inherent weakness in unbalanced interfaces, andthe coupling becomes worse as cables get longer and equipment power supplyconnections become physically farther apart. In spite of this, unbalancedinterfaces seem to be well entrenched even in very large audio and videosystems where trouble is virtually guaranteed. There are only two ways toreduce ground noise coupling.
First, reduce the impedance of the shield. Keep cables as short aspossible. Longer cables increase the coupling impedance. Also, use cableswith heavy gauge shields. Cables with shields of foil and thin gauge drainwires increase coupling impedance. Use cables with braided copper shields.The only property of cable that has any significant effect on noisecoupling is shield resistance. Maintain good connections. Connectors leftundisturbed for long periods can develop high contact resistance. Hum orother interference that changes when the connector is wiggled indicates apoor contact. Use a good commercial contact fluid and/or gold platedconnectors.
Second, reduce the circulating ground current. Don't add unnecessarygrounds. Additional grounding of equipment generally increases circulatingground noise currents. Never, never disconnect or lift a safety ground orlightning protection ground to solve a problem; the practice is bothillegal and very dangerous. Use ground isolators at problem interfaces.These isolators pass the signal while electrically breaking the shieldpath. This stops the ground noise current flow and the resulting coupling.A number of commercial isolators are available for audio, video, and CATVsignal paths.
Ground isolatorsThere are two basic types of ground isolator. Active devices usedifferential amplifiers. Most diff-amp circuits make poor receivers forbalanced audio lines and are nearly useless for unbalanced lines. Theycan't deal with ground noise voltages over about 10 V, even for spikes, andthey usually use integrated circuits prone to degradation or failure causedby such transients. The Sonance AGI-1 is an example of such a device foraudio.
Passive devices use transformers. Their tolerance of source impedance,which makes them such excellent receivers for balanced lines, makes themoutstanding isolators for unbalanced lines. They require no power and areessentially immune to voltage transients and RFI. The ISO-MAX CI-2RR is anaudiophile-grade ground isolator for audio.
Figure 2 compares 60 Hz hum rejection of the example audio isolators. Overthe 200 W to 1 kW range of typical consumer source (output) impedances, theactive isolator can achieve only 15 dB to 30 dB of hum rejection. Under thesame operating conditions, the passive isolator achieves 90 dB to 110 dB.
Cable shielding and RFIUnbalanced interconnect cables are also subject to noise pick-up viamagnetic and/or electrostatic induction effects. Unlike balanced cables,this pick-up cannot be nullified by the receiving input. The cable's outershield, if it completely surrounds the inner conductor, preventselectrostatic noise pickup. Foil shields usually have this 100% coverage.Braided shields, because they are woven and have small openings, generallyvary from 80% to 95%, but they are adequate for all but extreme cases.
Likewise, strong AC magnetic fields radiate from any conductor operating ata high AC current. Building wiring, power transformers, electric motors andCRT displays are a few sources of very strong AC magnetic fields. Becauseof the impedance imbalance, unbalanced interfaces cannot take fulladvantage of twisted or coaxial cable construction to nullify magneticnoise pick-up. Strong AC fields very near cables will induce significantnoise. Increasing the distance between cables, and the offending magneticfield will always reduce pickup. Cable shielding, whether copper braid oraluminum foil, has no significant effect on magnetic fields.
The ability of equipment to reject high-frequency ground noise or RFIdepends upon how well it is designed. Sadly, the performance of mostcommercial equipment degrades when such interference is coupled to itsinput. Symptoms can range from actual detection of radio, CB or televisionsignals, heard as music, voices or buzz in the case of television signals,to much more subtle distortions, often described as a veiled or grainyquality in the reproduced audio. Video, RF and data systems can exhibit awide range of symptoms.
Designer cablesBeware of marketing hype. In my opinion, much of the unexplainable audibledifferences among audio cables is due to unrecognized variations inultrasonic and RF common impedance coupling. Even low levels of such highfrequencies coupled to (mixed with) an audio signal are known to causespectral contamination in any downstream active device (amp). It seemsobvious to me that the real solution is to prevent the coupling in thefirst place with a ground isolator, not agonize over which designer cablemakes the most pleasing small improvement.
Some designer cables have very high capacitance and will seriously degradehigh frequency response, especially if used on a long cable run driven by ahigh impedance consumer device. For such applications, consider a lowcapacitance, low shield resistance cable such as Belden's #8241F. Forexample, its 17 pF per foot (305 mm) capacitance allows driving a 200 foot(61 m) run from a 1 kW output while maintaining a -3 dB bandwidth of 50kHz. Its 2.6 mW per foot shield resistance is equivalent to #14 gauge wire,minimizing common-impedance coupling. It's also quite flexible andavailable in many colors.
Finally, marketers of some designer cables imply that audio cables behaveas transmission lines. Real science tells us that audio cables are nottransmission lines in the engineering sense and don't require low impedancetermination networks until they reach thousands of feet in physical length.
In an upcoming column, we'll discuss the limitations of power linetreatments such as power isolation transformers and so-called balancedpower in dealing with ground noise.