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Earth Loop problems with Touchscreens

3 Simple Rules to reduce Noise Caused by Earth Loop faults in Audio Visual systems

A familiar problem for audio visual installers is the level of noise induced into the audio path created by earth or ground loops especially when using touchscreens but also projectors.

The cable screen/shield should be connected to the chassis earth only at one point (one end of the cable). This should be at the host connection for the best noise immunity.

The cable screen/shield is often connected to the chassis earth at both ends of the cable connection. Since this makes an earth to earth connection between the computer and touchscreen, it can create an “earth loop” or “ground loop” problem. Since the earths are probably at different voltages, a large current (often several amps in magnitude) can flow between the earth points if the shield has a lower impedance to the current flow than the normal earth path. For correct shielding, all cable shields should be connected together, and then earthed at a single point (at the host’s chassis earth - normally a computer in AV systems)

The signal earth must not be connected to the chassis earth

A chassis earth is intended as a safety ground for power supplies, EMI filters, voltage spike protection circuits, AC neutral returns, and all manner of AC and DC driven devices. As a result, the chassis earth can carry relatively large voltages and currents. The signal earth is not the same earth as the cable screen/shield earth which connects to the chassis earth  - it's the earth used for the audio signal

Power cable and signal cables must be segregated

The most common error when installing AV cabling is running power cables (110v or 240v  AC) in the same trunking or conduit as the audio visual cables (or running them alongside in ceiling voids) which is guaranteed to cause interference. The practice of running them together is often done to reduce the cost of cable installation but ignores the interference risks.  And if the AC cable is powering relays, motors, or fluorescent lighting with high voltage spikes then additional noise above the mains "hum" will be induced into the adjacent audio and video cables.

Systems using VGA cables are the main culprit as the audio is in analogue format and easily affected by induced hum and other noise, HDMI cables are less likely to be affected by low frequency noise - simple inline isolators can be used with VGA cables to overcome or reduce the problem

D J Edis-Bates C.Eng MIET

Meet at Hong Kong Electronics Fair 13-16 October

HK Autumn 2014

We don't have a stand but I'll be at the Hong Kong Electronics Exhibition between 13th - 16th October - it would really good to meet up if you are planning to be there too.

email or call me to arrange a time and place to meet - we have many new products including Android Digital Signage Player, Classroom Tablets with e-Learning and Touchscreen Sound bars with HDMI that we are launching

For more information about these and other Edis products please e-mail me, or call me by mobile : 0086 13922082500

Classroom audio from Edis

Stereo Headsets for the Classroom

Two new Headsets with or without boom microphone for the classroom 


Touchscreen Soundbars

TouchscreenSoundbarTouchscreens are often supplied with internal speakers that are inadequate for classroom use - the speakers are usually too small, the amplifier power is only 7 or 10 watts and the small eliptical speakers don't face the front so much of the sound is lost in the touchscreen casing and since speakers are directional it's important that they point towards the audience.

An answer to this is to use forward facing speakers at the right height - the height of your listeners ears. A soundbar fixed under the touchscreen is the best solution as the speakers can be large enough, the speakers face forward at the correct level and the sound output can be designed to suit young peoples hearing for voice at the correct sound level. Additional speakers can be wired to the rear of the classroom to ensure uniform sound levels ideally at 75dBA (15dB above normal speech)

Digital Signage lags the East

SignageboxThe enormous difference I notice in the UK compared with China is the scarcity of digital signage.

In China digital display marketing is not only huge in terms of the physical size of screens but also there are a huge number of digital displays everywhere - in hotels, offices, shops entertainment areas, bars – you name it - digital signage is booming. 

Why is this ? Well the cost of displays and signage players are sold at much lower cost in China due to competition so the displays, devices and software do not bear the high price tags that I've seen in the UK and Europe where fewer sources of supply, higher prices and monthly charges must be a factor in putting off end users who seem to stick with old style paper billboards.

Also the biggest advertising agency for digital signage here, in Zhanjiang, is the local newspaper, they've retained and built their teams of copy writers and graphic artists, previously used for newspaper advertising, which is nosediving into oblivion, they create stunning displays with sound.

Our local newspaper has 8,000 digital signage players spread throughout the city, hundreds in the foyers of apartments and office buildings where people queue for lifts to get to the office every morning. The newspaper also has billboard or digital displays inside the lifts. Typical costs are a few hundred £'s per week including the graphic design costs with adverts running on the 8,000 screens with content repeated 10-12 times an hour.

Most of the screens, like those for lifts, are small with a signage player linked back via the internet to the newspapers servers. But they do have kiosk screens and huge multi-screen displays as well. Hotels have dozens and a big variety of displays to welcome guests, advertise events, services and display menus and special deals in the foyer, restaurants, lifts and corridors. Public transport including buses and taxis here often have WiFi connection and digital signage displays.

In China the access to digital advertising is easy and low cost, just call the local newspaper ! The cost of advertising is low, equipment costs are low, the coverage is widespread and the displays well positioned for maximum return.

To stimulate the digital signage market outside China we've developed an Android solution that reduces equipment and hosting costs, simplifies the purchasing process, and includes a signage player with client app pre-loaded plus three years free CRM hosting. A real out-of-the-box digital signage system. 

for more information ...

Edis Soundbar with HDMI for Touchscreen

EA54 Soundbar with HDMIEdis launch a Soundbar with HDMI for Touchscreens for a professional classroom and meeting room sound system. HDMI and RCA inputs with HDMI video output for touchscreen connection.30 Watt or 80 Watt amplifier, with remote control, and options with USB and SD card player.

Superb wall mounted Touchscreen audio system

Android Digital Signage Solution from Edis


Check out the Edis fully featured Android Signage Solution with client-server CRM system, an Android Signage player and a whole host of accessories to support your Digital Signage needs. More ...