Info wanted for an FAQ on laptop noise

Mike Daniell · Post by **Mike Daniell** » Wed Nov 14, 2007 5:35 pm

I sometimes get questions on what to do about laptop noise, ie the electrical interference from (mostly) laptops that gets thru to the sound system. I'd like to put together an FAQ on the subject and welcome contributions. If you can provide any advice for this FAQ please post your advice here or email me directly.

Thankyou.

SimnaWeb · Post by **SimnaWeb** » Thu Nov 15, 2007 7:49 am

A DI with a ground lift often helps me. If I am strapped and without a pair of DIs, I hate to admit it but I have a ground lift on the plug.

I own a Dell. People have mentioned over and over that the Dell power supplies are very noisy. Ground loops a'plenty.

I have also noticed occasionally that I get a random pulsing that is stopped by turning off the wireless adapter for the laptop. Often noise comes from the CD drive as well. Mute the CD drive in the windows mixer to clear that one up.

I may be out of the norm. A lot of the venues I work are horrible for noise. The college I work in has RF bleeding into the AC, ground loops in the building, it's crazy. Other places are just as bad. One has an elevator that is powered off the same legs as the rest of the building without being isolated.

Hope some this helps.
Eric

Nick · Post by **Nick** » Thu Nov 15, 2007 9:06 am

I find that the main source of noise experienced with internal soundcards in Laptops (and even worse in many desktops) is some sort of data buss noise relating to the operation of the Hard Disk, sounding a bit like very quiet Modem signal. It starts and stops with the drive activity.

I've not found a way around this other than using an external interface, and even then we sometimes experience the same effect if that interface is buss-powered (on some computers - HP desktops for instance, but not my NEC or Lenovo laptops). Often on quickie shows I'll use a simple stereo USB interface for the feed to the main FOH speakers, and use the internal sound card to feed upstage effects speakers, which are further away from the listening audience, making the faint buss noise less an issue. On bigger shows we use MOTU interfaces that are externally powered and have no issues.

This is of course not specific to SCS - it affects any sound playback.

cheers,
nick

danbaird · Post by **danbaird** » Thu Nov 15, 2007 6:09 pm

I have found a couple issues with laptop audio...

The most frustrating was a laptop (I think an HP) which output analog audio but had a very distinct "digital" signal noise underneath. If you have ever heard a raw digital bitstream you know what I am talking about! We tried everything, DIs w/ ground lift, dedicated isolation splitter, etc. I knew that it wasnt the AC, as all audio gear is run via a 30kw isolated ground transformer, and it surely wasnt the $90k Yamaha PM-5D! The problem disappered when the AC adapter was unplugged from the laptop. When the presenter finally persuaded me to try another non-IG source of AC power, the problem disappeared. He was happy that he was right....I was quick to note however that what had changed was the proximity of the unbalanced 1/8" audio cable to the low voltage AC adapter output! He had moved the AC adapter to make it stretch to the other AC outlet. The AC adapter for that laptop apparently emits an enormous amount of high frequency interference on the low voltage side. I am not sure if it charges the battery at some high frequency, or if it communicates charging info with the laptop as high speed digital. Moral of the story, keep the unbalanced, unshielded 1/8" output cable as short, and as far from the power adapter cable as possible! Take a miniplug to dual 1/4" directly into some DIs, which will output a balanced signal via their XLR jacks. The shielded cable and balanced signal will be far more immune to RF interference. If you are unsure, simply try running your laptop on battery power alone to see if this is the cause.

nmcgann · Post by **nmcgann** » Fri Nov 16, 2007 7:19 am

We generally use a Edirol UA25 (or the older UA5) and a stereo transformer DI (usually a Radial Pro AV2 now) with the ground lifts selected.

This works with all our laptops (Dell & HP), various desktops and Macbook Pros.

I never use a laptop internal audio output for any real show.

Neil

Mike Daniell · Post by **Mike Daniell** » Fri Nov 16, 2007 2:39 pm

Many thanks for these contributions. I'll collate this info and prepare an FAQ over the next few days. Any more suggestions still welcome.

trs · Post by **trs** » Sat Nov 17, 2007 5:00 am

The DI suggestion usually works for me.

Another thing to check is where the computer is plugged electrically in relation to the sound board. On occasions where I've had to plug both the board and computer into the same power strip, power conditioner, etc., I have found that if the computer is plugged into the last outlet on the power source, that sometimes cuts down on the noise. Weird, I know, but it has worked for me.

sound · Post by **sound** » Tue Nov 20, 2007 1:59 am

Mike,

A lot of useful information can be found at

http://www.jensen-transformers.com/apps_wp.html

http://www.jensen-transformers.com/faqs.html

Greetings,
Walter

sound14man · Post by **sound14man** » Wed Nov 21, 2007 9:26 am

Mike,
As this is my first posting I must say what a great product you have produced. I have used it for a number of years to great effect.

I have used a number of different Laptop/mixer combinations all have had problems with power supply noise even using Firewire and USB external sound cards has not helped. With the resulting complaints from the director. I have found that using a Behringer MicroHD (HD400) does the trick and removes the noise for very little expense.

http://www.behringer.com/HD400/index.cfm?lang=eng

Another solution is to run the show from batteries and then charge them during the interval. Not the best solution though.

Regards
Sound14man

Mike Daniell · Post by **Mike Daniell** » Sat Nov 24, 2007 1:55 pm

Thanks, everyone. The FAQ page as now been updated (see Q6).

kallek · Post by **kallek** » Wed Dec 05, 2007 12:08 am

Laptop always have to be connected to same power source than mixer/amp (not same with light board). Make sure connect projector to same mixer/laptop power because noise comes also if using different power outlet.

Kalle

paphianclive · Post by **paphianclive** » Sat Dec 15, 2007 5:18 pm

I have used a laptop for many years at our theatre without any problem. Recently a new member of the sound team brought a laptop, which gave terrible interference when the power supply was attached. When running on battery only, all was fine.
I eventually disconnected the earth wire from the plug into the power supply and everthing is perfect. Some laptops come without earth connections to the power supply (my Dell for instance), but some do. Try a lead with no earth connection and see if it works.

Great product Mike, use it for all productions.

jmgordon · Post by **jmgordon** » Sat Dec 15, 2007 10:43 pm

Unless the power supply is double-insulated, which your Dell's presumably is, it is most unwise for safety reasons to disconnect the earth in the plug.
A safer alternative is to find an outlet that does not create an earth loop or use one of the suggestions above, such as a DI box.

Malcolm