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 other high-frequency noises that exist on the power line-think about listening to the 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 or less uniformly spread across the signal spectrum, heard in a audio system as the sound of a waterfall or seen in a video system as grainy movement or snow. A predictable amount of this (gaussian) electronic noise is inherent in all electronic devices and must be expected. Ground noise produces artifacts such as hum, buzz, clicks or pops in audio systems and slowly rolling dark bars, bands of specks or herringbone patterns in video systems.

Last column we explained that, in real world systems, significant ground noise currents will flow in any wire connecting two devices and that significant ground noise voltages will exist between the local grounds of these two devices. This column will explain why unbalanced interfaces lack any inherent ability to suppress the effects of these ground noise currents and voltages, making them particularly vulnerable to hum and other so-called ground-loop problems. Unfortunately, both standard 75 W coaxial video and standard consumer audio are such unbalanced interfaces.

Just how much ground noise is tolerable depends on whether the signal is audio or video and the expected level of performance from the system. Obviously, an audio monitoring system in a recording studio needs to be much more immune to ground noise and interference than a paging system at a construction site. In general, video systems can tolerate more interference than audio systems. For broadcast video, a signal-to-noise ratio of 40 dB is considered excellent, and even expert viewers find it difficult to detect interference at -50 dB. On the other hand, a 1995 AES paper by Louis Fielder of Dolby Labs suggests that, for audio reproduction in a home listening situation, the threshold may be about -120 dB.

Common-impedance coupling Unbalanced interfaces use two-conductor connectors and cable having two coaxial conductors, often called "single conductor shielded" cable. The most widely-used connectors are the RCA (properly IHF) or two-conductor 1/4-inch phone (or TS for tip/sleeve) for audio signals, the BNC or RCA for 75 W video signals, the F for 75 W RF signals, and various RS-232 connectors for data interfaces.

As shown in Figure 1, ground noise current will flow in any wire connected between points A and B. Because the wire has impedance, a small voltage drop will appear across it. In an unbalanced interface, this wire also carries the signal. The signal actually delivered to device B is the sum of all the voltages in the loop from point A to C. Because the wire's impedance is "common" to both signal and ground noise current paths, this coupling mechanism is called common impedance coupling. Common impedance coupling happens when two currents flow in the same conductor.

The common impedance includes the shield impedance of the cable and the shield contact resistance at each connector. The impedance of the cable's inner conductor has negligible effect on the coupling because it's in series with the much higher impedances of device A's output and device B's input.

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 x R. Therefore, the low-frequency noise voltage, E, depends upon the ground noise current, I, and the resistance, R, of the cable shield. Consider a 25 foot (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 0.41 W. If the ground noise current is 300 mA, the ground noise voltage will be 300 mV. Because the normal reference signal level for consumer audio is 300 mV, the noise 20 x log (300 mV/300 mV) or only 60 dB below a reference signal.

Ground noise current for floating (two-prong AC plug) equipment is usually related to the equipment's power consumption, which dictates the size and therefore the primary to secondary capacitance of its power transformer. It is this power transformer capacitance that causes the ground noise currents to circulate in the first place. Ground noise current can range from a few microamps for a turntable or CD player to nearly a milliamp for an audio power amp or video monitor (see Jensen AN004 for more details).

Ground noise current for grounded (three-prong AC plug) equipment can be very high because the ground noise in the building's wiring is effectively forced across the unbalanced cable's shield. Currents may reach 100 mA and, in some situations, noise voltage may actually be larger than the reference signal.

Reducing the coupling Ground noise coupling is an inherent weakness in unbalanced interfaces, and the coupling becomes worse as cables get longer and equipment power supply connections become physically farther apart. In spite of this, unbalanced interfaces seem to be well entrenched even in very large audio and video systems where trouble is virtually guaranteed. There are only two ways to reduce ground noise coupling.

First, reduce the impedance of the shield. Keep cables as short as possible. Longer cables increase the coupling impedance. Also, use cables with heavy gauge shields. Cables with shields of foil and thin gauge drain wires increase coupling impedance. Use cables with braided copper shields. The only property of cable that has any significant effect on noise coupling is shield resistance. Maintain good connections. Connectors left undisturbed for long periods can develop high contact resistance. Hum or other interference that changes when the connector is wiggled indicates a poor contact. Use a good commercial contact fluid and/or gold plated connectors.

Second, reduce the circulating ground current. Don't add unnecessary grounds. Additional grounding of equipment generally increases circulating ground noise currents. Never, never disconnect or lift a safety ground or lightning protection ground to solve a problem; the practice is both illegal and very dangerous. Use ground isolators at problem interfaces. These isolators pass the signal while electrically breaking the shield path. This stops the ground noise current flow and the resulting coupling. A number of commercial isolators are available for audio, video, and CATV signal paths.

Ground isolators There are two basic types of ground isolator. Active devices use differential amplifiers. Most diff-amp circuits make poor receivers for balanced audio lines and are nearly useless for unbalanced lines. They can't deal with ground noise voltages over about 10 V, even for spikes, and they usually use integrated circuits prone to degradation or failure caused by such transients. The Sonance AGI-1 is an example of such a device for audio.

Passive devices use transformers. Their tolerance of source impedance, which makes them such excellent receivers for balanced lines, makes them outstanding isolators for unbalanced lines. They require no power and are essentially immune to voltage transients and RFI. The ISO-MAX CI-2RR is an audiophile-grade ground isolator for audio.

Figure 2 compares 60 Hz hum rejection of the example audio isolators. Over the 200 W to 1 kW range of typical consumer source (output) impedances, the active isolator can achieve only 15 dB to 30 dB of hum rejection. Under the same operating conditions, the passive isolator achieves 90 dB to 110 dB.

Cable shielding and RFI Unbalanced interconnect cables are also subject to noise pick-up via magnetic and/or electrostatic induction effects. Unlike balanced cables, this pick-up cannot be nullified by the receiving input. The cable's outer shield, if it completely surrounds the inner conductor, prevents electrostatic noise pickup. Foil shields usually have this 100% coverage. Braided shields, because they are woven and have small openings, generally vary from 80% to 95%, but they are adequate for all but extreme cases.

Likewise, strong AC magnetic fields radiate from any conductor operating at a high AC current. Building wiring, power transformers, electric motors and CRT displays are a few sources of very strong AC magnetic fields. Because of the impedance imbalance, unbalanced interfaces cannot take full advantage of twisted or coaxial cable construction to nullify magnetic noise pick-up. Strong AC fields very near cables will induce significant noise. Increasing the distance between cables, and the offending magnetic field will always reduce pickup. Cable shielding, whether copper braid or aluminum foil, has no significant effect on magnetic fields.

The ability of equipment to reject high-frequency ground noise or RFI depends upon how well it is designed. Sadly, the performance of most commercial equipment degrades when such interference is coupled to its input. Symptoms can range from actual detection of radio, CB or television signals, heard as music, voices or buzz in the case of television signals, to much more subtle distortions, often described as a veiled or grainy quality in the reproduced audio. Video, RF and data systems can exhibit a wide range of symptoms.

Designer cables Beware of marketing hype. In my opinion, much of the unexplainable audible differences among audio cables is due to unrecognized variations in ultrasonic and RF common impedance coupling. Even low levels of such high frequencies coupled to (mixed with) an audio signal are known to cause spectral contamination in any downstream active device (amp). It seems obvious to me that the real solution is to prevent the coupling in the first place with a ground isolator, not agonize over which designer cable makes the most pleasing small improvement.

Some designer cables have very high capacitance and will seriously degrade high frequency response, especially if used on a long cable run driven by a high impedance consumer device. For such applications, consider a low capacitance, low shield resistance cable such as Belden's #8241F. For example, 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 50 kHz. Its 2.6 mW per foot shield resistance is equivalent to #14 gauge wire, minimizing common-impedance coupling. It's also quite flexible and available in many colors.

Finally, marketers of some designer cables imply that audio cables behave as transmission lines. Real science tells us that audio cables are not transmission lines in the engineering sense and don't require low impedance termination networks until they reach thousands of feet in physical length.

In an upcoming column, we'll discuss the limitations of power line treatments such as power isolation transformers and so-called balanced power in dealing with ground noise.