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walid

Hum in the incoming signal

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In Palestinian satellite channel when a signal come to the continuity studio from outside, there is a hum on it (hum in video and audio), and this is clearly obvious when we take the main electric power from our generator, but the internal signals from the studio (cameras and VTRs ....) are very good and have not any hum. Sometimes the hum is only on audio especially when we take the sig. from Microwave. Please help me to solve this problem.
Thanks.

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Guest SM2GXN

Hi walid!

It sounds to me that your hum is caused by a groundloop, when you say hum do you mean 50 or 60hz?
Size of the antenna wont cause any hum, a bad aimed or to small antenna would cause drop outs in digital transmissions and sparks for other on the picture.
Your equipment is connected so it produces a groundloop and thus causing hum, that's my contribution  ;D

Bjorn

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Hi, thank you all for your responds, first of all I want to tell you that the hum in audio is 50Hz and in video is two bars scanning the monitor screen from top to bottom or from bottom to top, the scanning is very slowly.

Mr. Bjorn said that: “Your equipment is connected so it produces a ground loop and thus causing hum, that's my contribution”
I think this ok but please tell me how an equipment may be connected so I produces a ground loop? And how we overcome this?
Thanks, 
Walid.
:o

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Guest SM2GXN

Hi walid!

Usually interconnection between several different equippments cause this so called groundloop. I don't now how your setup looks like so it's hard to tell but asume that your satellite receiver has got an audio out which is fed to an amplifier that has its chassie gronded to safety ground and same satellite receiver has an video out which is fed to an VCR which has its chassie grounded to safety ground, both audio and video has screened cables. These cables will make a loop from ground at the audio amplifier to satellite receiver and finally back to the VCR through the screen. That's a loop, hum can be induced by power cables running in parallel with signal cables, power cables should cross signal cables in 90 degrees.
Hopefully this will help.

Bjorn

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please I need more info.....
why hum increases when the main AC power is taken from our generator.
It is true, there is power lines in parallel to signal lines in the same place, is this one of the causes producing hum.
I notice that when toutching the receiver  outside body you feel some electricity, is this related to hum.
thanks

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running power along signal cables can cause line noise this is most well known from car audio and also in some experiments i have done with alternatepower sources, I have found that generators and other power supplies will give you either interference or ground loops or both

epspecially without conditioning the signal from the power source.

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Guest SM2GXN

Hi!

I agree with tnk2k and mrohler but when it comes to shielding I must say that it is pretty hard get a good shielding for strong mains fields.
Remember a case where a strong hum was induced in an on stage pa system, that was caused by the power cables beneath the stage floor, when they switched on the spots the hum became real strong  ;D
It sounds that the hum is caused by that your power cable is running in parallel with signal cables (as I wrote before), an increasing current in the power cable will build up bigger 50Hz field and thus giving more hum.  Even tough there isn't a ground loop there can be hum, power cables are leaky and will induce hum.
One thing more, don't ever leave your cables coiled this will bring even more hum.
Isolations transformers as tnk2k can be very helpul in man cases.
You have to check the grounding as tnk2k suggested.

Bjorn

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I guess you've already checked this, since it is one of the first things i learned about sound engineering:

Signal cables and electrical cables should be atleast 20cm apart from eachother, and if they absolutely have to cross eachother, do it at 90 degrees. Checking the ground connection would be a good idea as well.

However, if you cant find a solution, you could EQ most of it away

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Guest SM2GXN

Hi jarmund!

I don't think you can eq away in this case, the only way is to get rid of ground loops and induced hum from power cables.
If you mean by placing an isolation transformer I do agree.
It's not an easy task to solve hum even tough some people state it is.
Are you working with sound systems or are you a musician jarmund?

Bjorn

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Hi, I see that we limit the problem in two things:
(1) (a) Grounding problem and I must check this. Can anybody tell me the fastest and best method to check this, what instrument may I use and what acceptable reading should I looking for, how to decide that ground is OK or need to strengthening it.
(b) Tnk2k said: “If it is a big problem and you can not identify the source in a reasonable time, use isolation transformers to break the ground loops.” What type of transformers can I use and where to connect it?
(2) Existence of power cables very close to signal cables, this is difficult to make them far from each other, can I use a low pass filter to pass hum (50Hz sig.) to ground, or what you comment.
Thank you all very much, Walid Farra.
???

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Guest SM2GXN

Hi walid!

For the audio a isolation transformer is a good choise but when it comes to video signals it's getting more difficult. I'm not sure what you meant when you say that you take the power from you generator.
Are you running two power systems at the same time?
Would it be much work to make a drawing on your setup, cable type, balanced, unbalanced power cable and so on.

Bjorn

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Your audio cable carries a low level signal and hopefully is electrostatically shielded.
The mains cable carries a high voltage and current and its magnetic field goes right through the electrostatic shield of the audio cable. They must be separated.

You could filter out all the bass of the audio and completely eliminate the hum. But do you want tinny sound?
You could notch-filter out just the fundamental mains frequency, and be left with its harmonics.
I don't remember what happens to video without low frequencies but I think it would look really messed up.

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Reply to Bjorn: You ask: “I'm not sure what you meant when you say that you take the power from you generator. Are you running two power systems at the same time?”
No most of the time we take the power from the local electric company, but when the company cut off feeding electricity then we use a standby generator about 500KVA. At the period before the generator handle the load, there is a huge UPS carry out the load for say 15 to 30 seconds.
I said earlier that the hum increases especially in audio sig. When we operate on our generator not from the electric company.

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Reply to audioguru:
First of all welcome, I wait for your reply you are my great teacher…
(1) You said: “Your audio cable carries a low level signal and hopefully is electrostatically shielded.”
What you mean by shield, is this shield which we find inside any antenna cable or you mean that to put signal cables far enough from power cables, if you mean this we then fall down to the first square……
(2) From the rest of your reply I conclude that making a LPF is not the good choice, so, what to do?
Yours, Walid.
>:(

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Hi Walid,
Antenna coax cable has an electrostatic shield and so does audio cable with RCA or DIN connectors that connect stereo components together. All audio signals (not speaker outputs) must use shielded cable to avoid picking up interference. Audio cables that are installed in a building and won't be moved very much have a shield made of metalized plastic film and shields very well.

Your hum problem is caused by the power cable and the signal cables being beside each other much too close together. About the only way to fix it is to re-install either cable with a big space from the other.

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If walid can feel electricity by touching the case (as he stated several posts above) then he has a grounding problem to the case. The case is also the main shielding device. Unplug the unit, and check the ohms from the grounding prong of the power cable to the case. This will most likely give you a clue. Check for loose connections and secure the ground to the case. This might be all you have to do to get rid of the problem. Even if it does not cure your problem, this is where you should start. It is also a safety hazard.

MP

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hi boys, thank you for your interst ,
in the last few days we have an expert who seperate between the earth of the main power and the generator earthing , but the problem is not solved
, so i want to summarize the problem as follow :
when our equipments operate on local authority main power there is no hum on audio or video , but when our equipments operate on our generator ( 500 kva )  there is a strong hum on video signal.
thank you for the help
walid

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If the generator is grounded to exactly the same point as the mains, and used the same wires to your equipment, then the problem of hum shouldn't exist.

exactly it is grounded to exactly the same point as the mains, and used the same wires to the equipments.
we notice the following:
if we disconnect the main circuit breaker of the digital studio either it is oprating or not, the hum is reduced to 90% or more
note also we have two other analog studios and hese not contribute to the broblem.
this problem appears only the last 3 monthes and at that time we had the same conditions we have now, only we as i told you enhance the grounding abnd the problem remain.
thanks very much.

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Hi walid,

The hum; can you establish if it is the mains frequency 50 or 60 Hz whichever you got?
What kind of alternator do you have, I am especially interested in the exciter circuit! ::)

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Guest SM2GXN

Hi walid and all the others!

All hum you got in your studio is caused by groundloops or induced mains field into signal carrying cables, to fix this in a big studio could be a real big pain.
The only cure is to break the groundloops wherever they are and be sure that power cables (especially those carrying high current) not are running in parallel with signal cables audio/video and rf.
I don't think a scope is of any help, the generator probably produce a fine sine wave  and proper voltages.
audioguru it doesn't matter if grounds are connected to same spot the thing is that signal cables between all gears usually create a loop which causes hum on audio and video.
Starground in a amplifier will save lots of trouble but the difference here is that grounds of all the stages (stages=equippments) is also connected through the cable shield, right?
If a volume pot is connected through a Shielded cable usually only one end is grounded because otherwise it will create a loop created by a current flowing in the metal chassie itself to starground and finally back to the other ground connection of the cable shield, hopefully you see my point  ;D
I wouldn't recommend breaking the safety ground as some people suggest you could in no time be prepared to buy a one way ticket to the graveyard  :'(
Walid As I understand your studio is a big one and therefore it might be to much to ask you to make a  simple drawing on all connections  ::)
Hm...not much but lots of text  ;D

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Hi 4-1000A,
I thought if the generator used exactly the same wires as the mains, then there wouldn't be a problem since the mains works well with no coupling and no ground loop.

Maybe the generator is grounded at its location, far from the mains ground.

Inside a grounded metel cabinet, I always ground a shield only at the receiving device to avoid a ground loop. ;D

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Guest SM2GXN
Maybe the generator is grounded at its location, far from the mains ground.
Could be like that, I don't know what would happen if the ground at the generator isn't as good as the one comming from the power company or vice verse.
One question walid, from your generator is there 4 or 5 conductors. Please make a drawing how you run these cables from the generator, breakers etc and how the system is grounded.

I think we covered these earlier on.  Walid got it narrowed down to generator induced as it does not show up when running on regular power.  That would indicate posssible miswiring of the generator or improper ground connections or incompatible output configuration.  Worse case he should get a power monitor unit to capture the generator power configuration: phase voltages, phase currents, ground current and have it evaluated for proper impedance loading and grounding.


tnk2k you are right that's a good way to check that everything is ok from the generator.
Sorry for repeating myself haven't read the posts for a while, I should have updated my memory which become shorter and shorter, got to replace it with a new one with higher speed and size  ;D

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