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Intercom, and Pink Noise...

Discussion in 'Sound, Music, and Intercom' started by Schniapereli, May 2, 2007.

  1. Schniapereli

    Schniapereli Active Member

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    Location:
    Provo, Utah, United States
    Our intercom has been having some problems when we take it down to the front of house station. We use these belt packs here and this power supply here.

    The problem seems to be that whenever we have a really long cable, the signal goes sqwueakhle. It is only like a 25 foot run of XLR cable. But, then how do the intercom stations backstage still work, since they have an even longer cable run (without and signal amplifiers..that I know of...)?
    Is there some little dooby we need to buy to strengthen the signal at some end of the cbale path?
    Is there something else we are doing wrong? (our long cables work on the mics, but not for the com...?)



    Also, I am now confused about Pink noise. While an interview on ProSoundWeb with Mick Hughes says:

    Big Mick: Pink noise, being completely flat, means that there is no variable as there is with a CD EQ; hence you can look at each band of the P.A. frequency response, its peaks and troughs.

    (I have also heard of other techs refering to pink noise as being flat.)

    but, wikipedia says that Pink noise is not flat, and that it decreases 3 db per rising octave. (link here)

    Which is it? (and what about all the other colors of noise?)
     
  2. mbenonis

    mbenonis Wireless Guy Administrator Premium Member

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    Are your beltpacks configured for balanced or unbalanced operation? I see that these beltpacks can be used in either mode, so that they're compatable with ClearCom systems. If one beltpack were misconfigured, I could see this sort of problem occur.

    As far as pink noise goes, the reason we use it is because it contains an equal amount of energy per octave. Since our equipment works in this fashion (i.e., is also logarithmic), we use pink noise to test whether our frequency response is flat. See the Wikipedia article for more.
     
  3. icewolf08

    icewolf08 CBMod CB Mods

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    Occupation:
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    Not sure about your headset issue. I think you need to provide more info. What part do you move to the FOH position. The manual for your power supply should say how many beltpacks/outstations it can support and the maximum amount of cable it can handle.

    In terms of your white/pink noise question, I think both definitions are technically correct. Pink noise which is equal power per octave, produces a flat response on a logarithmic scale. White noise, which is equal power per frequency, produces a flat response on a linear scale. This is the way I understand it/was taught, could be wrong...
     
  4. CURLS

    CURLS Member

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    Well man im not real sure what to tell ya on your com issue, maybe you could try a ground lift or something.
    However, your Pink noise question sparked my attention. Wikipedia does a poor job of explaining it.
    Ok first, Mick's comment "Pink noise is flat" </u>compared</u> a cd. Well yea what he is saying is a cd on a RTA and to our ear jumps all over the place the frequency's are at different magnitudes at different times consistently.
    But, he says pink noise is flat and Wikipedia says it decreases 3db per octave, so your going ok well if it decreases 3 db per octave, how is it equal? Well, for one it is still equal per octave hence why 1/3rd(aka. "logarithimic") ocatave eq's are so prvalent in audio engineering. Also, many engineers state that it is flat because it is more than one frequencey at a relatively same amplitude at any one time. So you say White noise is too, yea your right White noise is still a good test measurement generator for measurement systems like TEF a program unlike smaart who takes out many variables such as reflections and as with most microphones they have a flat frequency response, UNLIKE our ear see "Fletcher-Munson" curve in Google. That should explain why when you see pink noise in flat response microphones on Smaart the response usually looks relatively flat. Whereas when you pink a system the lows usually seem a bit higher in amplitude, well, it's because they are.
    I really hope that did'nt confuse ya, it did me when i first didn't get it!
     
    Last edited: May 7, 2007
  5. SHARYNF

    SHARYNF Well-Known Member

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    Another way to look at the white noise vs pink noise, Pink noise more closely matches a sound that we would hear as being flat regarding loudness.

    I'd also look for cable connection that take the ground and connect it to the shell of the connector. The other thing is easy to miss is that if you have a weak connection (wire is starting to break, only a few strands are connected) it will typically meter correct, but will not carry the current you need. (ie voltage vs amps) SO even though you have an electrical connection, it is like using a awg of wire that is way to fine for the level you need, So the power that the system needs to power the units, is not sufficient

    Sharyn
     
  6. Andy_Leviss

    Andy_Leviss Active Member Premium Member

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    In com systems, this wouldn't generally cause a squeal; it would, however, cause a hum.

    Are you extending the headset cable, or the beltpack? Headset cables are very length-sensitive, and the nulling system can get very unstable if you extend it, causing a low feedback threshold.

    The other common cause of intercom squeals is improper termination. I'm not as familiar with that base station, but check whether it has a termination switch, and if that's on or not. Are you per chance using multiple power supplies/base stations? If so, check to be sure only one is terminated. Both unterminated systems and double-terminated systems can be prone to either feedback or circuit oscillation that sounds like feedback.

    Hope this helps,
    Andy
     
  7. SHARYNF

    SHARYNF Well-Known Member

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    when I mentioned a problem with the ground (pin 1) remember that this carries power, so it is possible if there is a problem on this connection that you may get problems other than hum.

    Sharyn
     
  8. wolf825

    wolf825 Senior Team Emeritus Premium Member

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    Location:
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    Can you narrow your Com issue down to a specific cable, beltpack or junction?

    You said it always happens when you go to the FOH station--how do you get there "normally" when you do this? Is there a hardwire run? Do you loop thru a beltpack to get there? Is this the same beltpack/cable/headset every time?
    Do you have DC wallpacks or just beltpacks

    Also--you don't "Y" cable any of your com feeds, do you? Your PS for your com can combine or isolate channel feeds--how many packs do you place per channel? do you run diefferent channels or are you single thruout? Are you balanced or unbalanced--and are you sure your cables are ok? Is the cable you use for the FOH runs different from the others you use (different impedance, guage, Canare quad or single etc etc)--IOW, does it cause squealing if you place it elsewhere in the chain that you already know works? Does your powersupply show a SHORT when this happens? Are you intermixing with Clearcom or other intercom systems? Mixing multi-channel beltpacks with a single channel system etc etc?

    Just tryin to narrow down the issue....if you could descrbe the situation and methods and any lights or beeps or whatever that happens on the packs or th ebase station etc it could help narrow down the problem. I mean with Com it could be something as simple as the mic gain/side-chain on a pack is set too high if that is a feature...to a shorting cable or connection (a braided sheild to pin 1 on a odd cheap mic cable could be a super big issue for COM vs a direct wire (or dual wire as reccomended by telex) for connection on pin 1).... a poor termination somewhere....a number of possibilities... sorry for the 20 questions--but narrowing down th eissue is the best way to find and fix the problem.

    -w
     

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