Film-Tech Forum ARCHIVE: Speaker hiss When using WinDras

This is topic Speaker hiss When using WinDras in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Dean Kollet (Member # 1789) on 09-19-2004, 01:32 AM:

On one of our prints of "Sky Captain..." there has been some complaints of the Digital dropping out, so I hooked up my laptop to our DA20 to moniter the film. Well, the first show I played in Stereo while I checked the print (in which the SRD track seemed fine) so I played it the second time (while connected) in digital. I heard a complaint over the radio about a weird noise coming from the right stage speaker. I immediately unplugged the DA20 (and my laptop from it), plugged it back in without running WinDras and the noise was fine.

This may or may not be related, but this same speaker is actually blown (you can only tell at high volumes, or maybe it's something else now that I think of it). I'm using a Dolby Cat 700, the DA20 and an Ultrastereo JS-200. Thanks

Posted by Gordon McLeod (Member # 33) on 09-19-2004, 09:51 AM:

Did you plug the laptop into the racks power outlets or a different source and where was it sitting
It is possible that you created a issue with the systems ground and some clock noise from the laptop serial connection got superimpossed on the audio

Posted by Frank Angel (Member # 248) on 09-19-2004, 10:26 AM:

Yah, but hiss wouldn't be from a ground-loop, right? But then again, when people describe audio anomolies, I find many times non-technical folk use terms that tell you nothing, or, actually give you the wrong sense of the problem; witness: "....a complaint over the radio about a weird noise coming from the right stage speaker." That's certainly wouldn't give Dean any hint of it being a "hiss."

I get that all the time. Recently they kept telling me there was a "buzz" coming from the stage. The "buzz" was a low frequency hum, not something I would have deduced from the word "buzz." They also can't seem to distinguish noise that comes from the sound system from noise that comes from any other place in the theatre -- especially air conditioner noise.

Posted by Bruce Hansen (Member # 281) on 09-19-2004, 01:39 PM:

Computers use square waves. A square wave consists of the fundamental frequency, and "all" odd order harmonics. Because of this, square waves are very good at getting into audio equipment, and causing strange noises. This is why you will sometimes see iron chokes on computer cables, to keep noise from getting into other equipment, and keep the noise other equipment may make, out of the computer. Take an AM radio, tune it in between stations, and put it near your computer. In the early days of computers some people could diagnose computer problems by listening to the "noise" coming out of an AM radio held near that computer.

Posted by Dean Kollet (Member # 1789) on 09-19-2004, 11:19 PM:

I had the computer plugged into the sound rack with the laptop resting on top of a cart that houses the DA20. I'm guessing it's probably just interference. I'm going to try tomorrow again with the laptop away from the unit and plugged into the wall on a separate circuit. Thanks guys.

Posted by Michael Schaffer (Member # 1204) on 09-19-2004, 11:30 PM:

Or try it again in the same setup (with no audience of course) and see if you can recreate the effect. That would be interesting.

Posted by David Buckley (Member # 2600) on 09-20-2004, 05:40 PM:

Try it with the laptop not on top of the DA20.

My first guess would be inductive coupling.

Posted by Samuel Hynds (Member # 2610) on 09-22-2004, 10:37 AM:

If your computers power supply (A/C adaptor) has a third pin on the input side (the Ground), use a two pin to three pin adaptor. I have had the same strange noise problem with a DSP-3 unit running pink noise and my computer hooked-up. Found that when I un-plugged the power supply, the noise went away.
I believe it's the clock in the computer coming across the earth ground.

Good Luck [Cool]

Posted by David Buckley (Member # 2600) on 09-22-2004, 04:46 PM:

The computer in question was a laptop, and I've yet to see a laptop with a solid earth, even when you have a 3 pin mains connector, as my thinkpad does.

With a "proper" PC, of course, you are absolutely correct that the PC should be properly grounded thru a three pin mains plug. If it turns out that is a problem causing noise injection into the sound system, then an RS232 isolator is a much safer proposition than un-earthing something, a practice that is really dangerous. If something went wrong if you are lucky it'll fritz your processor, unlucky could mean bodies on the floor.

Suitable optical isolators are available from, amongst others B&B Electronics, model 9SPOP2 should do the job.

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-22-2004, 05:05 PM:

My last five Dell notebooks were all grounded from the wall to the actual notebook frame.

Others like IBM and Compaq only ground to the supply (a few models actually do ground to the machine too). The ground connection to the Compaq power supply is the most important since they have a habit of catching fire.

Posted by David Buckley (Member # 2600) on 09-22-2004, 06:08 PM:

*stunned*

But thinking - Did those Dells have integral power supplies, or the in-line jobbies? I could understand and applaud that. But I've never seen an in-line with earth continuity, and the thinkpads are unusual in having three prong power - most seem to have the figure-8 two pin connector or a physically non-compatible variation thereof...?

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-22-2004, 06:26 PM:

All the Dell's were in line supplies.

I've only seen very few IBM laptops or notebooks with a grounded supply. Most use the polarized figure eight connector you mention.

Most of the Compaq supplies I've seen do use the ground to the supply, which is a good thing considering how many of them (used to?) catch fire.

BTW, the optical RS232/485 optical isolators you mention can be handy. I've got them in use on a few robotic controllers.

Posted by David Buckley (Member # 2600) on 09-22-2004, 08:33 PM:

Its not just Compaq with the knack:

IBM recalls lots of thinkpad PSUs
http://www-307.ibm.com/pc/support/site.wss/document.do?lndocid=TPAD-RECALL

extract:

IBM is voluntarily recalling approximately 553,000 56 watt AC adapters made for use with notebook computers shipped worldwide between January 1999 and August 2000. No other AC adapters are affected by this recall notification.

IBM has received reports that some of these adapters have overheated, resulting in melting of adapter housing and charring of the circuit board inside the housing. None of the incidents has resulted in reports of personal injury. There have been some reports of minor property damage

Just happy that neither mine nor the missus's PSUs were in the affected batch. Though to call 553,000 units a batch is a tad crazy.

And for some reason I cant get the URL to display properly... odd.

[ 09-22-2004, 08:50 PM: Message edited by: Daryl C. W. O'Shea ]

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-22-2004, 08:51 PM:

Weird!

Yeah, I've seen a couple chared IBM ones as well, but not as many as Compaq. Didn't want to bring it up and offend the ThinkPad users! [Smile]

Posted by Alexander Alves (Member # 1746) on 09-24-2004, 02:09 AM:

Recreating the effect would be interesting really..Why would hiss emanate out of the right channel only, and not all the channels?

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-24-2004, 03:13 AM:

Why not (just the right channel)? You don't expect unwanted interference to know that it would 'make more sense' to infect both the left and right stage channels, do you?

It's just part of a circuit picking up noise. Another circuit on another part of 'the' board doesn't have to pick up the same noise, and usually doesn't.

Posted by Rick Long Jr (Member # 541) on 09-24-2004, 02:03 PM:

My current laptop (a Dell Latitude) does the same thing. Come to think of it, every laptop I've ever had except a Toshiba has induced some noise. If I have to use Dras during a show, I run off the battery.
I've also found some, but not all, Da-20's or CP500's don't like to boot up with the laptop connected.