Balanced or un-balanced

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Permalink Reply by RANDY HAMILTON on May 10, 2010 at 1:32pm

balanced is the way to go XLR or quarter inch tip ring sleeve if you are running in stereo . the way to fix your feeback problem is with a FDS eq and a compressor with a gate. the FDS feedback detection system eq will light up on the frequency that is causing the feedback . . the compressor with the gate squeezes the wave and compresses it to a tight signal instead of up and down waves that can cause distortion or a distorted signal . the gate can be set so that you have to yell into the mic to open it
or set it so that it is closed all the time until some one speaks into the mic and it will open thus releasing the signal flow .
the main thing is called unity gain. that means you have as much signal coming in as you have going out . the best is to set it so your incoming is equal to your outgoing signal . the way you do that if pfl check every instrument and mic to see how hot the signal is .!!! and set it so it is not clipping !!! .that may solve a lot of your feedback issues . the easy way to explain it is say you have a 10 inch pipe that has 100lbs of water pressure at the pump end and it drops down to 3/4 inch pipe all that water can"t fit through the small pipe thus causing it to back up and cause friction . that is the same with your input and output signal on your sound board if your gain structure is not set correctly you will have all kind of problems . every time i change out a band with new instruments the first thing i do before anything is set the gain structure because some guitar pedals are hotter than others. and watch out for pedalboard tweekers who will reach down in the middle of the set and turn the gain output on their pedaboard up thus causing a hotter signal and you will be like what just happened . now i am dealing with feedback and all other knd of growling screeching noises hope that helps .
GOD BLESS YOU

Permalink Reply by John Earnshaw on May 10, 2010 at 11:59pm

Thanks for the reply our budget is a £0.00 so guess EQ is out of the question but regards a noise gate / compressor (I'm a guitarist) is all familiar gound I can make one and curtail the frequencies with pass band filters.You have certainally given me some thoughtful ideas.I thought that balanced is the right way as other wise you have a resistive shunt that would mean you need the sensetivity up and the level on the deck to compensate , also the phantom power won't be as efficient.
Again thanks for the help I should hopefully be trying to re-create the feedback over this week and find a cure.Might just be a proximaty thing with the microphones as there is not a great deal of space.
I will post the findings on here .
Thanks
John

Permalink Reply by Wayne Pau on May 10, 2010 at 6:05pm

I think here may be some term confusion here.

The reason balanced lines were invented is not to get rid of feedback, but to lower the amount of noise (hum) in the line. Was happens is that the using the +/-/GND lines, we can filter out a lot of noise that is generated via cross-talk phenomenon. This happens in really long lines of unbalanced cables, which picks up small variations due to fields induced by other higher powered lines and other field emitting equipment. We basically can subtract out the small fluctuation in the reference line, but using the + and - lines. (If you look at this on a scope, it's not flat-lined, if you get what I mean.)

This is from the Yamaha Sound Re-enforcement Handbook (I believe):

http://www.yamaha.com/yamahavgn/CDA/ContentDetail/WrappedTextDetail...

A lot of noise can be generated *if* you're wiring is incorrect as well. The web-page has a good picture of how balanced to unbalanced connections should be wired. However, ideally a good DI box should be used to bridge to either.


This site has a good simple explanation of balanced vs. unbalanced.

http://www.atlascables.com/index.php?id=75:


Lastly, it's generally a good idea to keep balanced line for microphones. Every so often someone will bring a condenser microphone that needs to be powered by the mixer via 'Phantom Power'. This is not possible via a two-conductor unbalanced line. (It needs all 3 conductors to pass power to the microphone.)

So my answer would be, where possible go with balanced line for microphone, etc. They can go for much longer lengths due to their design. It also allows for the use of powered condensers microphones that need 'Phantom Power'.

As for the feedback, that may be a whole separate issue. (vs. Hum, interference or cross-talk...)

Hope that helps.

BTW crosstalk can also be avoided to judicious design of cable runs. Don't run higher powered cables in parallel to lower-power cables, and if they need to cross, try to do it at 90 degrees.

Permalink Reply by John Earnshaw on May 11, 2010 at 12:07am

Thanks for that I think that might explain some of the other problems they are having at present the unbalance has earth and pin 2 shorted and pin 3 to run paraell so that is the unbalanced line which I shall be changing.The cross talk will explain some stuff but I do think that if ALL mike leads are the same length it will stop the latancy. I.E.imagine 2 cars traveling at the same speed and one has futher to go than the other .The one with the greater distance will arrive later hence an echo effect in the monitors.
Thanks for the comments and web sites I certainally will give it a look.
Thanks
John

Permalink Reply by Wayne Pau on May 11, 2010 at 12:45am

er. Hm...Latency is not usually typical in a church set-up. How big is your church. I have spooled cable for SkyDome (sports stadium) in Toronto on spools that was over 500+ feet. With a lot of hand-waving, the electrical representation of a audio signal travels at the speed of light. (See: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html). We did have echo/pre-echo issues in the SkyDome, but that was more the speed of sound vs. the speed of light (sound moving slower in air vs. the electrical signal in cables, which necessitated digital delays to make it sound more real). How big is your sanctuary? How long is your cable run? I can't even imagine this being an issue unless your church is the size of the Crystal Cathedral or Notre Dame.

When we electrically talk about cross-talk, it's rarely audibly distored. With two cables, one may bleed into the other, but not so intelligible you could determine the culprit. Usually it's a distortion of the original signal, not an addition of a new signal. It's not an audio finger-print. We're talking phasing and fields here.

Any echo effect your are hearing in your monitors is most likely do to "reflections", where sound emitted from speakers reflect back off walls and other solid objects, only to be redirected back into a microphone at a slightly delayed time (distance there and back X speed of sound, accounting for room conditions). A small amount of this gives some realism to the sound, often associated with "reverb", but if the delay between the primary source and the secondary reflection will cause a noticeable 'echo' effect.

Technical speaking, when this secondary reflection reaches a breaking point, it becomes feedback, as it gets amplified again and again until the system saturates and squeals. That is feedback. This is why you get some distorted, echo like sounds just priority to an outbreak of ear peircing feedback.

Feedback destroyers work by monitoring very tiny notches or slivers of frequency with an algorithm that looks for an incredible steep gain, and then using a notch filters, eliminates the source, cutting the endless cycle of infinite amplification. (If you really want to know, it's like signals class all over again. They take F-T Transform of the signal to get the amplitude of particular frequencies and then take the derivative to see the how quickly it's changing. If the derivative is beyond a threshold, the system assumes it's feedback and works to negate it, bringing down the amplitude of the offending frequency/frequencies until it's no longer beyond the range.)

I hope you're not getting confusion with similar phenomenons. Sometimes a little knowledge is dangerous. Eliminate the amplified signals "bleeding" back into your amplification system and you should be fine.

If you truly want to get your head around the physics, might I suggest what used to be the stand book on Sound Enforcement:

http://www.amazon.com/Sound-Reinforcement-Handbook-Yamaha-Products/...

I have both the first and second edition. Great book. Very technical, but 100% accurate.

Permalink Reply by John Earnshaw on May 11, 2010 at 12:24pm

Thanks fo the advice .No you didn't loose me just put it into terms and a way that works for me.The latancy I have since come to the conclusion is caused by the cable box wiring and this should be cured once I get the soldering iron on the cables.
The signal that is causing the feedback I hope to detect using some ham software that I use in diagnostics of audio / mike radio gear.It ives a specograph of frequencies present and lets you monitor and see visually what is happening.
Thanks to everyone as you can see I'm in at the deep end with a system that needs improving.
God Bless
John M0JFE

Permalink Reply by Wayne Pau on May 11, 2010 at 1:56pm

John,

Actually if you have the equipment, this may work for you. Sometimes poor EQ causes what people perceive as clarity issues. To the human ear things don't sound nature and when the brain thinks they should sound one way, and they are reproduced sound differently, this can be awkward for some vocalist.

There could be a issue with the EQ at your church. We often 'ringing-out' a system to track down and turn down frequencies which are prone to feedback. However, the second part of 'ringing-out' a system is to try to make the system seem more natural and pleasant to the ear. So the process is more an 'art' than a science in some case. (A system that never feedback but sounds like a dying cat defeats the purpose.)

If you have good frequency spectrum analyzer, you can combine that with a Pink Noise generator and see how 'accurate' your system is current at reproducing sounds. We usually a combination of Real-Time Analyzers and Spectra-Graph. If you have that software, you're pretty good to go.

http://en.wikipedia.org/wiki/Pink_noise

While not as accurate as the very expensive pro-level equipment, you can actually download pink noise samples.

Someone has posted a clip from a AV video from Hal Leonard here. While it's not the whole video, it basically gets you going in the right direction. My personal opinion is you need to balance what is done in the video, which is to reduce the probability of feedback with having a system that is EQ'd "true". (Note: Perfectly flat on the spectra-graph may not still sound pleasant. Sometimes a human touch is necessary. Nothing beats a good, well trained ear.)

Mic & Speaker placement will get you only so far. Sometimes you're up against it because the whole sound system is out. Ringing out the system may help a lot, but depends on your set-up. Not sure if you have a multi-band equalizer. A very very basic two channel can be had used for very very little (Kijiji or Craigstlist). We're talking <$40. Some amps already have multi-band EQ control

Hopefully it works out for you.

Permalink Reply by John Earnshaw on May 11, 2010 at 8:04pm

Great minds think alike I was refering to the spectroscope software I use for setting up radio equipment and the pink noise generator is most definatly better idea than using a side tone .I was also thinking of plugging in a scope to see just what noise was on what channel.Our only EQ is on the desk I.E. high , notch ,sweep for mids and a low .The thing that I do feel is the bug bear is down to the un balanced connections which I shall change and re wire.There is too much to change at once but if I do it in steps the rest of the team can keep up with me.
Thanks for the help and I believe that through God all things ARE posible.
God Bless
John

Permalink Reply by Wayne Pau on May 11, 2010 at 8:10pm

oops.. the link didn't show up. There link for the short video clip is here:

http://takelessons.com/videos/view/how-ring-out-audio-system

=) Sounds like you got a plan...

Permalink Reply by John Earnshaw on May 11, 2010 at 8:43pm

That is so cool gadgets we love em.My idea to test the system once we have done the alterations is to pass a designated frequency through it first at desk then on stage and measure the deviation that occurs as gain is increased and faders are employed at various ports ie headphone ,monitor output ,front of house output.This way you can eliminate if it is the amps or cables that are failing .
We use the deviation meter on radio gear to make sure we don't push our bandwidth and thus tansmit out of band ...a big no no .It also shows up clipping and any audio processors that are close to there limit.
John

Permalink Reply by Dave Hartwell on May 10, 2010 at 6:22pm

Agreed, (un)balanced lines don't cause feedback.

Regarding balanced / unbalanced lines:
* We use only balanced mics (e.g. all mics are XLR)
* I make a rule that we use DI boxes for musicians, and their 1/4" jacks cables should be quality cables and relatively short (15-25 feet usually is fine).

Assuming the feedback problem is a mic / monitor mix issue (most common problem):
A really cool trick to find feedback is to boost the EQ high gain as far as possible, with the monitors and house zeroed (quiet). Then slowly bring up the monitors one by one. You will discover which is the offending mike/monitor mix. Then (a) figure out why, and (b) work with the EQ to isolate out that frequency.

Have fun!
-- Dave. <>/body>

Permalink Reply by John Earnshaw on May 11, 2010 at 12:13am

Thanks for the advice I will test that out later on in the week.It certainally sounds more feasable to hopefully eliminate it tottally.Re musicians well the guitar will be altered to take a feed off his line out from the amp direct to the desk and the keyboard will be direct to the desk then out to it's monitor.As I say money is tight.
God Bless everyone and thank you
John

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