audio interference in Vegas

[Renton Maclachlan]

I posted this over on the Audio forum but as it involves Vegas in part, I'll shoot it up here as well...

I took the plunge and got some Wharfedale 8.1 Studio Monitors so that I could get some better sound when editing a video project than I have been getting from computer speakers...

They turned up this morning and a friend lent me cabling for them so I could get them going. The cable has a mini jack to go into the computer (actually I have a mini-jack cable coming from the computer, into which I plug either speakers - past or present ones, or my headphones for when the house needs to be quiet), which switches to two XLR plugs to go into the balanced inputs in the new speakers.

However there is a teething issue in getting them going correctly...there's interference of some sort occurring and I wonder if anyone here can tell me what it is...

When nothing is playing on the computer (though the computer is on) there is a light/soft hum and I supposed what you could call a sort of soft crackle - for want of a better word, coming from each speaker. This increases as you turn the volume up at each speaker.

When editing in Sony Vegas and using the phones I don't hear any of the interference at all and nor did I when using the old speakers. But as soon as I plug the new speakers in I do. It happens when I'm playing the timeline in Vegas and seems to occur at the transition between frames. I have a Shuttle pro which allows me to wind down the timeline a frame at a time and I hear it when doing this...also when playing at normal speed. Once again it is more pronounced when the volume is up at the speakers.

Not sure if it has anything to do with it but rather than plugging the speakers minijack into the lead from the computer, I touch my watch with it, I get a crackle thorugh the speaker (depending on which par of the plug hits my watch). The speakers are both powered of course and when I touch my watch the power is on and the two XLR's are plugged into the speakers.

Could it be something to do with the cable or its proximity to something else?

[Juris Lielpeteris]

You should use a higher quality sound card.
I'm using the following one http://www.creative.com/emu/products...aspx?pid=15185 (discontinued).

[Dave Baker]

Hey Renton, I can't answer your question, but I must give you a bit of advice.

Never, ever connect or disconnect loudspeakers with the power on, it can result in blowing the amplifier output stage (or sound card) and/or the speaker drivers.

Dave

[Renton Maclachlan]

Thanks for the caution but this was happening before I may have done that. There is no stern warning re doing such in the manual, just not fiddling around inside the case with the power on.

I notice they talk of poor A/C distribution possibly resulting in hum through poor grounding...

[Renton Maclachlan]

The noise is not there when the computer is turned off...though there is a very faint hum you can here when your ear is right against the speaker...

[Dave Baker]

Originally Posted by Renton Maclachlan

I notice they talk of poor A/C distribution possibly resulting in hum through poor grounding...

Yes they would, earthing problems are almost a degree course in their own right.:-)

Years ago when I was a Hi-Fi nut, one of the biggest problems was getting rid of hum, it was usually caused by an "earth loop", where the system, or part of it, was earthed by more than one route, but that could be anywhere in the system. Either that or an earthing conductor was disconnected.

Two things come to mind. First, your active (powered) speakers are taking the output from your sound card and amplifying it along with any noise that is there, noise you would not have heard before. Second, I'm not trying to rubbish your new speakers as I don't know anything about them, but I notice that these days many manufacturers quote signal to noise ratios and distortion levels that would have given us audiophiles apoplexy years ago. What makes it worse is it's so much cheaper and easier to make clean amps than it was back then!

What I would do is disconnect all input cables to the speakers, then switch them on and listen for the hum and yes, I'm going to be pedantic about switching off! If the hum is gone, then connect back up bit by bit starting (obviously!) at the speaker input, until it returns. If the hum hasn't gone, it's the speaker amp.

Dave

[Mike Kujbida]

What sound card do you use?
You mentioned using XLR cables. Do you use an adapter to go from unbalanced from your computer to balanced on your new speakers?

[Renton Maclachlan]

I used to use an Audigy but my son in law took it out saying the onboard sound was as good...so at present I don't have one. My board is an Asus P8Z68 Delux/GEN3...

Re XLR's...no adapter...just a cable that has a mini jack leading to two XLR's...

[Adam Stanislav]

Originally Posted by Renton Maclachlan

Re XLR's...no adapter...just a cable that has a mini jack leading to two XLR's...

That may explain the interference. A mini jack is electronically incompatible with an XLR (let alone two).

Adam

[Renton Maclachlan]

I thought Adam you might be onto something so rang a friend who is an audio wiz and he confirmed your analysis.

So...seeing the speakers have unbalanced RCA inputs as well, I tried linking it all up with RCA cables...
but behold, the interference is still there.

I have attached a file recorded and maximized illustrating the sound as it occurs in Sony Vegas Pro. First is normal speed down the time line, then me winding along the timeline slowly using the Shuttle Pro. The video consists of me speaking, with various graphics replacing me and being shown...some are photos, some are png files. You distinctly hear the sound change when the cursor comes to a graphic or photo...

The audio volume has been wound right back except on one small section I where wind it up a bit then down again.

Friend suggests it could be interference from the power supply, as the current varies to deal with the graphics etc and the frames down the time line...

Next step is to put the sound card back in and see if that changes anything...

[Mike Kujbida]

Renton, to confirm that the speakers are ok, can you hook them up to your stereo system thereby taking the computer out of the equation completely.

[Renton Maclachlan]

I tried what you suggested Mike and it's as I thought, not an issue with the speakers...

I tried putting the video card back in last night and the computer did not recognise it...

[Juris Lielpeteris]

I understand Your desire to circumvent this proposal, but it seems that this is not possible:

Originally Posted by Juris Lielpeteris

You should use a higher quality sound card.

[Paul R Johnson]

Now we can hear it, the cause is bus noise. Pretty common to be honest, and usually a feature of either the cheap Realtek internal sound chips or plug in cards that are not well designed. Video cards and on motherboard graphics adapters produce this noise which is picture dependent, and it leaks from the video bus into the sensitive section of the audio section. An external d/a is the usual cure, the one audio people use. They have this problem in audio recording all the time. Inside the. Is, close to the motherboard is the worst place for audio. Nobody notices on the cheap low if speakers many people use.

[Renton Maclachlan]

Why would the interference not be heard through the headphones and cheap computer speakers?

What do you think is the best method of overcoming it? A friend has said he has a DAC he could lend me to see if makes a difference...which he reckons it would...