John Huntington’s Entertainment Technology Blog has an interesting posting Toshiba’s Very Cool “Space Chair”Ad. It consists mainly of two videos. The first video shows the cool Toshiba TV ad. The second video is the “Making of” video. It tells how the ad was made using several high definition cameras rigged to a big weather balloon that lifts the whole system to the sky.
An EE’s Guide to Survival Between Microphone and Voice Coil is an interesting slide set amde by Netherlands´ well known audio engineer Bruno Putzeys and put to web by Hypex Electronics and Grimm Audio. It was presented in AES convention at 2007. It is a very good presentation of the whole audio electronics path from microphone to speaker. If you are into audio electronics, read this slide set.
Prosessori magazine reports that traditional CRT display technology is dying much sooner than expected earlier. The sales of CRT based displays has practically ended in Western Europe, Japan and North America. And the sales of traditional CRT based TVs is also dropping very quickly on developing markets as well. According to Displaysearch research center people in China do not want anymore to buy old fashioned TVs with picture tubes. The change to flat screen displays is so quick, that the manufacturers of LCD TVs have hard time to produce as markets need. It is expected that 130 million LCD displays will be sold this year. And the prices will continue dropping.
Isolator for digital audio is needed in cases where coaxial S/PDIF digital audio link forms a ground loop in your audio system. Typically in coaxial S/PDIF connections the coaxial cable shield is connected to equipment cases. This arrangement easily creates ground loops which can cause noise problems in various places in audio system. A suitable transformer can be used to isolate the coaxial S/PDIF signal and avoid ground loop problems (other option is to use optical connection instead of coax). Here is the circuit of the S/PDIF signal isolator I have used:
ELFA catalogue product 56-550-55 “PE65612 Trafo dig.siirt.” is a tranformer suitable for isolating S/PDIF digital audio signal. The manufacturer for this product is Pulse Engineering and their product code is PE-65612.
Ratio: 1:1
Bandwidth: 100 kHz-55 MHz ±3 dB
Here is a picture of the S/PDIF signal isoltor I built (box open):
More information on S/PDIF and related circuits can be found on my SPDIF document.
According to Reuters news article the European Commission has issued new volume standards for MP3 players on Monday this week. The new standards will require that MP3 players will play at a safe volume by default. There will also be a health warning so consumers who choose to override the default settings know the risks. Those new rules will require small technical changes to I-Pods and other MP3 devices. According to Yahoo news article “EU to set volume limits on MP3 players” new standards would see new players include a maximum sound level default at 80 decibels.
According to EU’s executive commission said the maximum sound levels of players now being sold range between 80 and 115 decibels with the supplied earphones. Using different earphones could add to those levels by up to 9 decibels. Above 120 decibels is equivalent to the level of noise generated by an airplane taking off.
A normal way to troubleshoot ground loops problems in audio systems is to listen to the audio signal at the mixer with headphones. If the buzz exists at the mixer then do the following: One by one, disconnect the inputs and outputs to and from the mixer and note if the buzz decreases. This same connecting and disconnecting wires method can be used on other components of audio and video systems.
Disconnecting and then reconnecting the cables take a lot of work. This caused that ground loops are frustrating to troubleshoot in large systems where there are lots of cables.
I have found that a clamp type multimeter can help to troubleshoot ground loop problems. The ground loop noise is normally caused by the extra noise current flowing on the shields and ground wires of the cables. That noise current is normally mains voltage frequency (50 Hz / 60Hz) or it’s harmonics. Normally the signal cables should not carry any (or very little) mains frequency current in them, so by measuring this kind of current flowing on the cable it is possible to determine where the noise current flows. A clamp type multimeter is a very good tool for doing the measurements, because you can easily measure the noise current flowing on the cable with it without need to disconnect the cable or disturb the signal inside the cable. This means that you can troubleshoot a live system with clamp multimeter in AC range.
The wires which have considerable current on them are part of ground loop. The wires with most current on them are pushing most noise current to the whole system. So first locate the wires that have highest or otherwise very high current flowing in them. Then you can try to disconnect them and test if that stops the noise. Usually there is one or few cables that cause all or most ground loop current on the system. That noise current gets typically flowing around in different cables on the system, causing more or less noise problem here and there around the system. Then the real noise source or sources are disconnected, suddenly the whole system becomes noise free. When you have found out the problem source then just add suitable cure to that connection (typically signal isolation transformer or similar device).
Clamp on multimeter allows you to easily measure the current on cables. Just clamp the meter over the audio cable and get the AC current reading. If you want you can clamp several audio/video cables inside the clamp and get the reading of the sum of their noise currents (remember that there is possibility that if there are two cables with exactly same noise signal but different direction you get zero reading). Clamping the meter around a number of signal cables speeds up the troubleshooting process where there are lots of cables, for example near audio mixer. If the group of cables you measured with clamp meter shows a considerable noise current, then measure the cables individually to see which one has the most current flowing. If there was no considerable current on the cable group, continue measuring next cable group. Besides audio cables you can do the measurement with video cables, mains power cables and other signal cables.
There are few things to consider the selection of the clamp type multimeter. First the multimeter needs to measure the AC currents with the clamp. You don’t need the DC current measuring capability, although getting a clamp meter with also DC capability can make the meter more useful for other applications (usually the DC capable clamp meters are more expensive than AC only). The second thing to consider is the resolution of the meter. The ground loop currents you normally want to measure are in few mA to 1A range (in some severe case the current can be considerably more). It is preferred to have a clamp meter that can measure currents down to few mA. Unfortunately many meters with this good resolution are usually quite expensive.
Usually the cheap clamp type multimeters have 10 mA or 100 mA resolution, meaning that they can’t detect anything lower than 10 mA or 100 mA. A multimeter with 100 mA resolution is practically useless in ground loop problem solving, because over 100 mA ground loop current are not seen often. A multimeter with 10 mA resolution is already useful to troubleshoot ground loop problems, but it will not reveal you all the details in most cases. Usually 100 mA-1A current on cable means very serious noise problem on audio and video systems, currents in 10-100 mA range cause some noise problems. Usually when the current is well below 10 mA there are no considerable noise problems.
I have used a clamp meter with 10 mA successfully for troubleshooting ground loop problems, but when used that I wished I had a meter that can show even lower currents down to 1 mA or less. So if you are buying a clamp meter, consider trying to get as good resolution as possible with the money you are willing to spend it. When looking for multimeter for this application the actual measuring accuracy (measurement error percents promised) is not important, we are merely making checks if there is current flowing or not and approximately how much (just some approximation around how much current is enough).
This picture shows the cheapest I know well working clamp multimeter that can measure currents AC down to few mA currents. The meter has 1 mA resolution at 2A measurement range (the display started showing current higher than 2 mA). You can get this 1.3″ LCD Clamp Style Digital Multimeter with Pouch from Dealextreme for around 20 US dollars.
Remember that a real life return path for current is not an ocean of zero impedance. Some engineers draw every ground as a wire because even copper planes have impendance. This approach might be one reason that makes some audio engineers more think of using single-point grounding. By discarding ground planes in favor of thin traces that wind back to a single-point ground, some audio engineers get slightly better distortion measurements, but at the expense of poor immunity to RF. The problem is that every audio circuit must work at 2 GHz — not to pass any signal but to reject noise from cell-phone radiation. Remember that every one of those long, spindly “ground” wires is an antenna. For RF noise immunity performance ground planes and connectors tightly connected to equipment metal case are good things.
Video isolation transformers are primarily used in CCTV application in fields of security, manufacturing, avionics and display. The video isolation transformer is an extremely broad bandpass 1:1 isolation transformer. Its hum isolation is very good and it can sustain very high noise voltages without degradation. Isolation decreases with increasing noise frequency.
Video signals can transformer isolated in many applications. Most video signal transport paths are AC coupled, so this kind of signal can pass through a suitable transformer. Video isolation transformers are used CCTV applications where they solve ground loop problems or protect system against ground potential differences between different locations.
The design of a high bandwidth transformer which can go to very low and very frequencies is very hard. Video signal can have significant signal components from 50 Hz up to 6 MHz. Unfortunately in practical transformers you have to always make some compromises on low and high frequency responses (highest components of composite video can be attenuated even few dB). Signal amplitude at low frequencies is limited by core saturation and coil inductance. High frequency response is limited by leakage inductance and winding capacitance. Many video isolation transformers are only designed for CCTV other not so demanding applications application, where more signal distortion is accepted than in broadcast industry.
Lundahl LL1575 is a high bandwidth video isolation transformer designed for CCTV (closed circuit television) applications. I have used that transformer successfull to build video video isolators. That transformer has 20 Hz – 11 MHz +0 /- 3 dB frequency response (possible with special bifilar winding technique and special core construction). That transformer gives 2 kV rms isolation between primary and secondary windings.
The LL1575 datasheet recommends that if DC current is present, the transformer must be decoupled (with large capacitor). Because in many video circuits there can some DC present especially in video outputs, I thought that it would be a good idea to include suitable DC blocking capacitor to the circuit input side (on the left, connected to video signal source output). A suitable capacitor value should be 1000 uF or higher (practically demands using electrolytic capacitor) and have good high frequency characteristics (electrolytic capacitors are not good at this, ceramic etc.. would be much better here). The capacitor I used was built from 4700 uF electrolytic capacitor (gives capacitance) wired in parallel with 100 nF ceramic capacitor (handles the high frequencies where electrolytic capacitor is not good at). 25V or higher voltage rating is suitable for the application (you normally see 1Vpp video signal plus maybe few volts DC).
The circuit is best constructed to a small plastic box with suitable connectors. For professional applications I recommend to use BNC connectors. If all your equipment use RCA for video signal, then use those connectors if they are easier for you. Plastic box is easiest for the construction because in this way it is easy to guarantee good electrical isolation between input and output side connectors (in metal box needed isolation could be hard to do reliably).
Here ia picture of the video isolator I have built:
Ground loop elimination in video systems does not always ask for a complete isolation of the grounds. There are passive hum suppressor transformers that will very effectively remove the hum from the video signal, but do not affect the video signal otherwise. Those special transformers act like a common mode coils, which stop the annoying ground loop currents on the shield of the coaxial cable, but provide a straight path for the signal inside the cable. This type of device is capable of passing the signals from DC to tens of MHz without problems. The hum suppressor transformer both reduces the current flowing on the cable shield and compensated the voltage differences that would otherwise be between cable ends and eventually get to the signal.
The humbugging transformer/coil consists just of many (tens to hundreds) turns of 75-ohm miniature video coaxial cable wind on a suitable transformer core. The aim is to have a coil that has high enough inductance to keep the cable shield current low and still withstand the voltages/currents it gets exposed to in typical applications without saturation. This kind of hum reduction coil works by mutual inductance. The coax cable is wound around a transformer core so that both the inner and shield of the cable become inductors. The tight coupling ensures that any voltages/currents flowing on the cable shield, caused by variations in earth potential differences, are transformed into the inner conductor. This type of transformer is typically constructed to a specially selected toroid transformer core.
This type of hum suppression transformer has found it’s way to the professional video application (rental companies) and computer video applications (computer to video projector connections). The transformers of this type are usually called “hum bug transformers”, “hum-bucking transformers”, “anti-hum video transformers”, “hum suppressor transformers” or “video isolation chokes”. T The humbugging transformer presents a simple transmission line to the signal, so that its signal amplitude capability is essentially unlimited, and its bandwidth extends from DC to that frequency where line losses become excessive. Isolation increases with increasing noise frequency. The choke provides protection against moderate levels of 60 Hz hum, and protection against very high levels of RF noise. ypical performance figures are 30-40 dB hum level reduction up to 2-10V volt voltage differences at mains voltage frequency (50 Hz or 60 Hz). The maximum voltage the humbugging transformer can handle at low frequencies depends on the transformer specifications, usually limited by transformer core saturation. The humbugging transformer choke is primarily used in Broadcast TV because it passes the DC component of the signal. It is used in studio, and in remote ENG.
This picture shows the inside a commercially sold “video hum bug transformer” sold by Finnish company RGB. The price range of this kind of commercial device is usually 100-200 US dollars range. I had to open this unit to change the BNC connector that was broken on a field use. This device consists of a toroid shape transformer core and around 50 turns of very thin 75-ohm video coaxial cable wrapped around it. You can make your own working similar device by taking he core from a 100-400W toroidal mains power transformer and winding 30-50 turns of thin 75-ohm coaxial cable around it. This will work well but is somewhat larger and heavier than the commercial devices like this. It is possible to optimize the size with careful selection of suitable core material and number of turns.
Here is an inside views of some humbugging video transformer products that I have made. They have performed very well on many video systems to solve ground loop problems.
RC filter or amplifier’s lowpass filter at the input of a delta-sigma ADC is normally put there to reduce noise. But sometimes adding a filter can produce a noisier digital output than without that filter. Using an analog filter to inject noise article tells that it is as easy to eliminate higher-frequency noise with an analog lowpass filter as it is to inject noise into the frequency band below the corner frequency of your filter. If you change your circuit design by reducing your filter’s resistor values, you will increase the noiseless bits in the circuit.
