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Banana Skins – April 2019 (#172-179)

172.  An example of the ‘Pin 1 Problem’ causing interference in professional audio systems

The Sound Dept. was asked to design sound effects for a fairly standard play. To deliver the sound effects, sixteen amplifier/loudspeaker positions were required, six of them on stage. All went well during the rehearsals and during the first day of production. The sound designer seemed to have done a good job, so I went back to my office to plan a touring season. But on the second day of production, I was summoned to the stage by the director of the play. His artists could not concentrate on what they were doing because of loud clicks coming from the on-stage loudspeakers.

Sure enough, every time someone used the lift to and from the fly floor, a loud noise could be heard in every loudspeaker on the stage. The problem was traced to the input cables for the amplifiers. The input cables were neatly taped to the floor, following the line of the set. Unfortunately, the cables ran parallel with the lift power wiring, which was under the stage and out of sight. On inspection it was found that the input cables to the amplifiers ran parallel with the lift wiring for at least seven meters. Every time the lift was used, the magnetic field generated by the switching current, coupled with the amplifier cable shields and the switching noise was injected into the input circuit of each amplifier. When the input cables were re-routed, the noise was reduced, but did not totally disappear.

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Some years later I found out why. In each case, the cable shield was only connected to ground at one end. This is still the most common method used for trying to control low frequency noise, such as hum or buzz, for equipment that connects the cable shield to the internal circuit 0V conductor (this is now known as “the pin 1 problem” because the three-pin XLR style connectors used in pro-audio connect the cable screen to pin 1).

Each cable shield was thus an excellent receiving antenna for frequencies whose wavelengths were a small fraction of the cable length. The switching noise was duly induced in the cable shields and the noise currents delivered to the output connectors of the mixing console (the cable shields being disconnected at the amplifier ends), where the noise was injected into the mixing console ground conductors. Thus the switching noise was added to the signals delivered from the mixer to all sixteen power amplifiers.

(From Tony Waldron, Technical Manager of Cadac Electronics Ltd, http://www.cadac-sound.com, recalling an incident when he was Head of Sound at the Royal National Theatre in 1986.)


17
3. DECT phone interferes with computer

Today I heard about a case where an ISDN terminal was susceptible to a DECT phone next to it while a 900 MHz GSM did not cause anything. This was in the field, not in test lab. In test lab my experience is that if it passes below 1GHz it also passes above it.

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(Posting from Ari Honkala to emc-pstc@ieee.org, 10th Jan 02)

Try putting a mobile phone next to your computer mouse! Even more fun if the computer has speakers!

(One of the replies to the above, from Peter Flowerdew, 10th Jan 02. We recommend making sure all data is backed up on removable media before trying this or other interference experiments on PCs, MACs, or other computers.)


174. 
Electronic Article Surveillance (EAS) system resets pacemaker

The Japanese Ministry of Health, Labour and Welfare announced that there was a case where the operation of a heart pacemaker was influenced by the electromagnetic field from an anti-pilferage device when the patient walked through the exit of a library. Fortunately, the effect was to reset the personalised parameters in the pacemaker to the initial (non-personalised) settings, and the patient didn’t feel discomfort with that.

The ministry believes that risk of hazard caused by electromagnetic interference between pacemakers and anti-pilferage devices is low, and a recommendation to the patients was not to stay long time near anti-pilferage devices. The information source was: articles on newspapers (Asahi, Yomiuri, Mainichi, etc.) at 17 January 2002, and Pharmaceuticals and Medical Devices Safety Information No.173 (January 2002) from the ministry.

(From: Tom Sato, 19th Jan 02.)


175. 
Digital cell phones interfere more with pacemakers than older analogue types

Scientific studies have reported increased interference effects in pacemakers caused by digital phones that did not occur with the older analog technology. Cell phones have decreased in size so that they are often carried in a shirt pocket directly adjacent to an implanted medical device. There are a number of wireles technologies in use today which involve different combinations of power levels and modulation schemes.

(From “Immunity testing for Active Implantable Cardiovascular Devices” by Daniel D. Hoolihan of Hoolihan EMC Consulting, ITEM 2001, p 45.)


176. 
Financial risks of poor power quality (1)

The inability to trade can result in large losses that far exceed the cost of the operation. In a recent example a claim for £10m compensation was made as a result of a 20 minute power interruption.

(From “Introduction to Power Quality” by David Chapman of the Copper Development Association, in  “The Power Quality and Application Guide”, published by The Power Quality Partnership at http://www.cda.org.uk.)


177. 
Financial risks of poor power quality (2)

It is estimated that power quality problems cost industry and commerce in the EU about €10 billion per annum while expenditure on preventative measures is less than 5% of this.

(From “The Cost of Poor Power Quality” by David Chapman of the Copper Development Association, in  “The Power Quality and Application Guide”, published by The Power Quality Partnership at http://www.cda.org.uk.)


178. 
Financial risks of poor power quality (3)

There are no official statistics on the severity and distribution of voltage dips but some medium scale measurements are now in progress and can be expected to yield valuable information in due course. One study, carried out by a major generator, measured voltage disturbances at 12 sites with demand between 5 and 30MVA.

In a ten-month period 858 disturbances were logged, 42 of which resulted in disruption and manufacturing loss. Although all 12 sites were low technology manufacturing operations making low added value products the financial loss totalled €600,000 (average €14,300 per event or €50,000 per site), with the highest individual loss of €165,000. The table below gives some typical values:

Industry Typical financial loss per event
Semiconductor production €3,800,000
Financial trading €6,000,000 per hour
Computer centre €750,000
Telecommunication centre €30,000 per minute
Steel works €350,000
Glass industry €250,000

(From “The Cost of Poor Power Quality” by David Chapman of the Copper Development Association, in  “The Power Quality and Application Guide”, published by The Power Quality Partnership at http://www.cda.org.uk.)


179. 
Adjacent channel interference problems with U.S. emergency services’ radiocom system

In the USA, many of the emergency services (fire, police, ambulance, etc.) use a radio system which operates at 800MHz. The base-stations for these systems are often quite widely spread, to reduce the cost to the public purse. These systems are known to suffer from ‘adjacent-channel’ interference, which seems to be on the increase due to crowding of the spectrum. The interference has resulted in documented cases where officers or other have been put at risk. The main problem appears to be intermodulation in the RF front-ends of the handsets, caused by out-of-band signals from other licensed transmitters.

It is often forgotten that most radio receivers achieved their narrow channel bandwidths in the intermediate frequency processing stages, and that the bandwidth of the earlier stages is much wider, making them prone to interference from powerful signals at nearby frequencies.

(Taken from: “Interference to Public Safety 800MHz Radio Systems, Interim Report to the FCC, Dec 24, 2001” which can be downloaded from http://www.apco911.org.)


The regular “Banana Skins” column was published in the EMC Journal, starting in January 1998. Alan E. Hutley, a prominent member of the electronics community, distinguished publisher of the EMC Journal, founder of the EMCIA EMC Industry Association and the EMCUK Exhibition & Conference, has graciously given his permission for In Compliance to republish this reader-favorite column.

The Banana Skin columns were compiled by Keith Armstrong, of Cherry Clough Consultants Ltd, from items he found in various publications, and anecdotes and links sent in by the many fans of the column. All of the EMC Journal columns are available at: https://www.emcstandards.co.uk/emi-stories, indexed both by application and type of EM disturbance, and new ones have recently begun being added. Keith has also given his permission for these stories to be shared through In Compliance as a service to the worldwide EMC community.

We are proud to carry on the tradition of sharing Banana Skins for the purpose of promoting education for EMI/EMC engineers.

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