Audio interferance
 

Hi,

Anybody got any bright ideas on how to solve audio interferance / noise from a PC?

Got my PC going through a mixer (nothing wrong with the mixer) and the interferance noise coming from the PC is quite bad - thing is - it actually goes alway when the hard drive is being accessed! Wierd....

Any ideas?

Re: Audio interferance
 

Problem with PC's is that it's a whole lot of electrical frequencies in one box which means any electrical connection to it also carries a load of Sproggies, I used to get the same so started using a SPDIF cable (even better is optical) for the audio as digital is a lot less susceptible to harmonic interference. Doesn't though help you much with the mixer unless you have a digital in.

Re: Audio interferance
 

Is this a separate sound card or an onboard one?

If onboard it is likely to cause noise as and when you access the PC. The amplifier within the mixer will also be picking up on this and thats why it will be more apparent.

Also check you do not have your audio cables touching power cables. This can also lead to more noise.

I used to have this problem with noise and got a seperate PCI soundcard which solved it.
I use this one:

http://www.dolphinmusic.co.uk/produc...hz-24-bit.html

Re: Audio interferance
 

It's the onboard sound chip. My Yamaha SW1000XG had to be pulled and put into my other computer as there are not any 64 bit drivers for it.

Thanks for the SPDIF tip Kymmy - unfortunately it does not have any digital inputs.

I have an old SB Audigy - but I think the audio out connection is duff on it...

Re: Audio interferance
 

It's likey to be the motherboard noise finding it's way on to the output of the onboard soundcard, either that or a ground loop.

Re: Audio interferance
 

For sound recording, we use these at work. Bit expensive, but they've been out a couple of years so I would be surprised if you couldn't find them a lot cheaper.

USB powered. Two channel inputs (6 1/4 inch or XLR). Both with gain controls. XLR is phantom powered. SP/DIF in/out/passthrough and a separate headphone output. In short a basic mixer as well.

Re: Audio interferance
 

Are there any audio cables inside the case that could be re-routed away from noisy areas, things like the cable connecting the motherboard to front panel headphones/microphone.
It might be worth putting ferrite beads on such cables. https://secure.wikimedia.org/wikiped...i/Ferrite_bead

Re: Audio interferance
 

Ferrite beads rule, especially in a mixed audio/RF environment.

Re: Audio interferance
 

SPDIF in its modern form covers both coaxial and optical connections

http://www.topbits.com/spdif.html

I thought ( but I am not sure on this as its not my field) that as long as enough information is passed digitally even if slightly corrupted gets corrected ?

Re: Audio interferance
 

Always used balanced connections between your audio interface and the mixer. Also, try to use balanced connectors to your monitors. Balanced lines reduce the likelihood of interference as the cables have an additional level of shielding over a 2 conductor unbalanced link.

Common balanced connectors are: 1/4" or 1/8" TRS (tip, ring and sleeve) jacks or XLR

The problem is that your Audigy sound card possibly doesn't support balanced connections

ps: digital solves a lot of problem as above but I doubt you have a digital mixer :)

Re: Audio interferance
 

Nope, there's no parity or error correction on SPDIF unlike newer systems (like AES) it does have a few extra bits in it's 20bit format but they're used for equipement type and copy protection codes. Remember also that SPDIF isn't synched as it's one way only so does have a sync fallback but still can't correct errors (instead it just misses out the corrupt section)

In this case though I suggested it purely due to it's supression of analogue harmonics whether you use coaxial (some motherboards do have SPDIF co-axial) or Optical is dependant on what's available and what you're plugging it into

Re: Audio interferance
 

mine has both as does my amp :)

Re: Audio interferance
 

I have SPDIF on the motherboard (old asus) but regulary use optical or co-axial via my cambridge audio DTS decoder with the multimedia players and SAT

Lee, check your onboard sound chip, if it's only a few years old you might find that you have an SPDIF header onboard, an old SPDIF decoder on ebay will be dirt cheap and will enable you to plug directly into your mixer/amp

Re: Audio interferance
 

Nope - all cables are external - plugs into the 3.5mm jack.

My cables go into a cable tray in my desk - worth moving the audio cables out of there?

Re: Audio interferance
 

Does the Audigy card have a drivebay mounted break-out box connected via ribbon cable?

Re: Audio interferance
 

Yes it does. I've got the platinium version. The Audigy and drive bay is not installed at the moment and as I said - I think it's broken...

Re: Audio interferance
 

Okidoki no problem, I was going to say if the drivebay box was installed it could be providing another path to ground which is not desirable. Ground loop issues aren't always characterised by hum, they can also be the source of digital noise problems like you're experiencing.

Switched mode power supplies can be bad guys too and if you're running through a PA on stage with these issues unresolved then you can forget it. On the occasions I've used my laptop and firewire audio interface live it's been with a fully balanced chain with all gear running off a power conditioner. Even then you can still detect a bit of digital hash in the noise floor when nobody is playing.

Re: Audio interferance
 

For audio stuff you are best taking it away from you PC internals. Most audio cards and onboard sound systems are not for those actually wanting to use them for anything worthwhile.

My old ASUS M/B with onboard sound had a noise floor 40dB above the theoretical value. A cheap and cheerful USB one (circa £30) I borrowed from work was about 10dB above.

Moving cables around inside the PC will have a limited effect as much of the noise is induced in the ground/shielded side anyway. Just because two points are electrically ground does not mean they are at the frequencies flying around inside a PC. At 3.4GHz a tiny unshielded wire is a very effective aerial.

Yamaha do some very good pro equipment for PCs. A bit expensive though so it depends how serious you want to be.

As for it going away when the HD is being accessed...most switching power supplies require a minimum load for stability. Few peripherials in a PC use 5V apart from HDs and CDs/DVDs. If for effiency the design of the power supply is a "burst mode" design (I have no idea one way or t'other), they are notorious for radiating from DC to XRAYs. :)

As previously mentioned, a balanced input with twisted pair wires will gain you alot.