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In Reply to: Re: Research Paper posted by John Swenson on February 7, 2007 at 15:40:51:
I must admit I am a bit surprised by your concerns about EMI. Can you share with us some findings that support the concern both in terms of typical EMI levels detected and audibility.I am not sure a well built PC would leak any more EMI than any other recent DSP based electronics component, such as AV receiver, DVD/BluRay/HD-DVD player, etc. And we know that some components these days (including Tivo boxes) are essentially PCs running Linux.
Do you have any data you would care to share?
Follow Ups:
Just thought I'd add that my SB3 generates some kind of EMI on the CAT5 line such that my DSL modem goes nuts and I get ~30% ATM packet errors. Even if I have a router between them. In fact the CAT5 line doesn't even need to actually be plugged into the SB, just within a few inches to cause me grief. Logitech says I should get the unit replaced, and I will try, but I suspect it's a design issue.
I have done some tests, I hooked up a 2 inch piece of wire to the probe on my 0-40MHz spectrum analyzer and moved it around components, wires etc. I get radically different amounts and spectrum of EMI from different devices and in different directions from the same device.Laptops as a group are pretty good EMI wise (at least the ones I tested), but every "quiet PC" I've tried has leaked like a sieve. Thats four separate PCs in either fanles or very quiet fan systems that are designed to be put on racks with other components. Two were commercial systems used as is from the factory and two were home built systems using cases designed for the purpose. I wouldn't put any of those in my rack.
I also did some testing of a couple other devices such as DVD playes, the SB3, power amp etc. These varied quite a bit. The 4 DVD players I tried were not too bad, about on par with the laptops. The expensive CD transport I have was terrible, maybe thats part of why I never liked it very much. Regular analog stereo components also emit EMI but most of that is power supply related, 60, 120, 180 Hz components. Even though the power transformers are burried inside a metal box I could still eaily pick up the fields from them.
On another question of yours the cables do radiate EMI, but it was usually far less than the boxes themselves put out. I can get the same level of sound degradation from those silent PCs without any connections to them other than the power cord.
I also tried el cheapo shielding (ie wrapping in aluminum foil) and it does actually help quite a lot, but its not really very good to the devices due to air flow restrictions. (its also really hard to see a display when its wrapped in aluminum foil, I need to get that formula from Scotty for transparent aluminum!)
I think its rather intrigueing that the mass market "consumer" items seem to have much lower EMI. They probably DO meet FCC class B, but the other items don't.
The tests I did were completely uncalibrated so I have no clue as to where they stood with regards to FCC regs, they were just to get some idea of the relative amounts and spectra of stuff coming out of some of the boxes in my system.
So I tried it out on my setup. I wrapped the analog audio daughter card (containing the ADCs and DACs and analog circuits) completely in aluminium foil, using paper in between the board and the aluminium foil (to prevent any short circuits).Too early to tell whether there is an improvement or not, but the sound is nice :-)
Can you name the four PCs (or types of cases) that you tried? It will be interesting to build a repository of good and bad PC designs for future reference.Do the EMI spectrum change much across models?
My understand (but I'm not sure why) is that there is "good" and "bad" EMI and techniques like spread spectrum can convert "bad" EMI to "good" EMI without necessarily reducing overall level. Kind of like good and bad cholesterol I guess.
I'm surprised that you did not get a lot of EMI from the cables, particularly the ribbon cable you were using for the I2S.
What I read John to say is that the average PC and other non audiophile quality devices--including the stock squeezebox--spew out more EMI than other hifi 2-channel audio components sitting in one's audio rack, and this is a concern if hi fidelity is one's goal...which was the context of the question in my post.key word search "emi" under John returned 4 posts (excerpts below):
"How is the EMI form this device? My mini-ITX system puts out enough EMI that I can't put it next to the rest of the audio omponents (which is fine, I have it next to me, across the room from the audio stuff)."
"BTW I'm running I2S from my SB3 into my DAC using a ribbon cable with standard 3.3V signal levels, but I think I'm going to go with LVDS ovber CAT5, it has lower jitter and much less EMI."
"We also found that the SB3 EMI was causing part of the problem. By wrapping the whole SB3 in aluminum foil the sound significantly improved."
And from the above post: "Different computers spew out different amounts of EMI so its hard to make absolute pronouncements, but I would not put any computer right next to a stereo system. Ultra low EMI computers can certainly be built, but almost everything on the market is shielded just to the point of meeting the FCC regs and no better."
John seems to have gotten better performance out of a SB3 i2s feed relative to his usb efforts, so my conclusion is to take reasonable care in finding ways to reduce emi potential, regardless of device, via distance or reduced transmission (replacing enclosure, etc.).
Your excerpts helped a lot.*** my conclusion is to take reasonable care in finding ways to reduce emi potential, regardless of device, via distance or reduced transmission (replacing enclosure, etc.). ***
I would certainly agree with this. I would also add that it doesn't hurt to look for ways to improve EMC (electromagnetic compatibility) in all your audio equipment.
Older equipment may not have good EMC, but newer equipment should meet FCC standards which means that in theory the sort of EMI generated by an FCC compliant PC (or any other consumer electronics device that is DSP based) should NOT pose a problem. Particularly since newer PCs implement spread spectrum on the high bandwidth busses (generally memory, graphics) which also help to reduce EMI and newer power supplies have active PFC.
I suspect a factor in John's case (and possibly yours as well) is that on both his Mini-ITX PC and SB3 he's transmitting digital audio signals across components.
The problem is that any cable carrying digital signals is a very strong EMI transmitter (and receiver!). It basically acts as an EMI antenna. Ribbon cables are terrible in this respect. CAT5 is not that great either.
In my case, I have gone for the opposite approach, which is eliminate digital cables where possible. In my setup, I try where possible to do D/A conversion in the source components, and carry analog downstream to the speakers. I'm relying on the EMI shielding in each source component and EMC in downstream components to minimise any issues. I also do a lot of power supply filtering, as that can be a good noise transmitter between devices as well.
If I had to carry digital signals (say, from a PC to an external DAC) I would recommend using quad shield coax cable (plus word clock sync, to reduce jitter).
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