RF interference

[ulot]

i want to ask why mics and sometimes keyboards pick radio stations and how can it be avoided?

 

[David Bridgen]

This happens because of non-linearity, either in an active circuit or sometimes in a poor connection (which can behave like a rectifier).

A.m. signals entering such equipment due to inadequate shielding/screening will be demodulated (detected) in them.

 

[Lsharptec1]

During the 7 years that I was was a broadcast engineer, I ran into this problem many times.

The simple answer is "SHEILDING". Microphone cables must be properly sheiled, and preferably "balanced", to avoid picking up RF interference.

Larry

 

[ulot]

Thanks alot but you all are talking too "big"
i think you're trying to say that loose and bare cables contribute the this right? so how best to shield a cable? will strong taping of joints help and using cables with thicker insulation?

 

[Nigel Goodwin]

Thanks alot but you all are talking too "big"
i think you're trying to say that loose and bare cables contribute the this right? so how best to shield a cable? will strong taping of joints help and using cables with thicker insulation?

Taping will do nothing! - you need METAL shielding, use high quality screened cable on microphone leads, and preferably balanced connections. The units the microphones connect into should be well screened as well, using metal casings - NOT plastic, as cheaper units often do. Also inputs should all contain low pass filters, to reduce any RF interference entering that way.

Basically it's mostly down to good practices, bad practices (plastic casings, poor screening of the leads etc.) will leave you open to potential problems.

Do you have a specific problem?, or is this just a general enquiry?.

 

[ulot]

thanks it's just a general question. We have that problem alot around here and it gets embarassing at times. So what exatly is screening?

 

[ThermalRunaway]

screening is the same concept as metal shielding, except that it's applied to connecting leads. If you've ever cut away the plastic of a piece of coax cable you'll know exactly what we're talking about - underneath the plastic the wire is completely wrapped in a metalic shielding, and this is what is meant by "screened cables".

The metalic shielding helps to avoid the problems caused by RF interference and makes the connecting leads much less suseptical to "pick-up" from other signals. Have you ever used a poor quality SCART lead? We use them all the time at work. The SCART cables are screened, but the quality of the screening is very poor and a significant part of the lead is left open to interference at each end of the lead itself. Using these leads we quite often find that we can see interference in the background of the pictures, and this is caused by the bad quality of the screening used on the cable. If I remove the cheap SCART lead and try my own good quality type which I always keep in my toolbox, the signal pickup dissapears completely.

This is why screening is important and it's also why you get what you pay for when you buy connecting leads.

Brian

 

[Nigel Goodwin]

This is why screening is important and it's also why you get what you pay for when you buy connecting leads.

Yes, but don't get conned into buying stupidly expensive leads!, or gold plated ones - generally a massive waste of time!. And don't EVER buy anything called a 'monster cable'!.

 

[ThermalRunaway]

Yeah I know what you mean. Every time you purchase something from Comet they try and get you to buy one of their monster cables - £50 for a SCART lead. Luckily I'm quite immune to their bullsh*t, but I wonder how many people they actually manage to conn?

Brian

 

[Nigel Goodwin]

Yeah I know what you mean. Every time you purchase something from Comet they try and get you to buy one of their monster cables - £50 for a SCART lead. Luckily I'm quite immune to their bullsh*t, but I wonder how many people they actually manage to conn?

I work in the Derbyshire Dales, where we have almost zero terrestrial digital coverage - as such we don't sell the boxes (although we keep a couple in stock), and if anyone enquires we ask where they live, and explain why we won't sell them one.

We've had a number of customers who've refused to accept this, and gone to Comet to buy a Freeview box - upon getting it home it doesn't work - it's not got a prayer of doing so, no digital transmitter remotely with in range. The customer then goes back to Comet (who won't take the box back, as they are sold as non-returnable), who ask what SCART lead they are using - Comet then sell them a £50, telling them that will make it work!.

The also do the same on deliveries, you have to pay for delivery at Comet, the van eventually turns up, and if you don't have a SCART lead they sell you one for £50. If you refuse to pay the £50, they say you'll have to pay the delivery charge again for them to come back and set it up another day!.

BTW, where I work we do free deliveries, and usually provide free SCART leads as well - not great leads, but reasonable quality ones - if there's any problems with the lead, we change it for a more expensive one. Obviously this only applies to reasonably priced items - we don't do free deliveries on portable TV's, DVD players, or VCR's anymore - they are too cheap.

 

[ulot]

thanks alot for you help thats answers my questions.

i think i'll just ask this other question here
we had a heavy rain fall and a keyboard was soaked bad with rain, every single part of it. we opened it up and let it dry in the sun, it works now but the sound is almost inaudible. what could be wrong? speakers or something else?

 

[Nigel Goodwin]

thanks alot for you help thats answers my questions.

i think i'll just ask this other question here
we had a heavy rain fall and a keyboard was soaked bad with rain, every single part of it. we opened it up and let it dry in the sun, it works now but the sound is almost inaudible. what could be wrong? speakers or something else?

It could be anything, water is quite corrosive, but if the paper speaker cones were soaked it could well have ruined them - try plugging headphones in and see how it sounds then?.

 

[ulot]

thanks i'll try that and give you fit back

 

[ulot]

the sound is okay when using headphones or connecting to a jack so is the problem with the speakers?

 

[Nigel Goodwin]

the sound is okay when using headphones or connecting to a jack so is the problem with the speakers?

Assuming the headphones are fed off the same amplifier as the speakers (which is usually the case), it can't really be anything else.