How to avoid Firewire cable hum

Question

Level 1

127 points

How to avoid Firewire cable hum

Hi,

I am using a Firewire external drive to record from a video capture device that feeds into my Powerbook via a USB cable. I am noticing hum when I have both of these cables connected. I have noticed this before whenever I have a Firewire cable attached.

Are there Firewire cables that are shielded or are otherwise immune to causing hum? The two Firewire cables I have now are clear plastic and I can see a braided silver covering beneath it. Even though the Firewire and USB cables can come in on different sides of the Powerbook, I am still getting hum. I cannot separate the cables much more than they are now.

If there aren't cables that will prevent hum I am (bad word here). I have to record video to the Firewire external drive. I tried using a USB drive and had all kinds of problems.

Thank you.

John L

15" Powerbook/1.67 Ghz (DL/HR), Mac OS X (10.4.10), 1.5 gb RAM

Posted on Sep 21, 2007 11:31 AM

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Answer 1

Sep 21, 2007 5:24 PM in response to lipwak

If you are using a 6-pin fireWire cable, you could try a 6-pin to 4-wire cable with a 4-pin to 6-pin adapter. This will disconnect the power and ground when used with a powered FireWire drive.
<http://www.pccables.com/70931.htm>

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Answer 2

lipwak Author

Level 1

127 points

Sep 21, 2007 5:42 PM in response to Malcolm Rayfield

Yes, I am. Thanks!

Is this normal for this kind of cable? It's interesting that there are so few posts on this problem. I see 11 posts with the words firewire and hum in them (using the Boolean AND). I wondered if my two cables (both of them do it) are defective since I don't see other people having this problem.

There aren't any other issues if I do this, are there? The external drive is powered. It came from an old iMac and perhaps it was wired wrong when it was put in the enclosure. Trouble is, the same firewire cables caused problems with my old external commercially produced store-bought CD burner too so I concluded back then that I couldn't do audio if I had a firewire cable attached.

Cheers,

John L

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Answer 3

Sep 21, 2007 7:27 PM in response to lipwak

The problem may be in your USB audio interface, but only shows up when the FireWire cable produces the ground loop. Using 4-wire may provide a cheap fix by breaking the loop. I use an iMic audio interface, and have no hum problems, even though I a do have a FireWire device (Elgato EyeTV 200) connected. A good USB audio interface should electrically isolate the audio from the computer, to avoid noise problems. Of course you should check the the computer's power outlet has a good ground connection.

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Answer 4

lipwak Author

Level 1

127 points

Sep 21, 2007 7:45 PM in response to Malcolm Rayfield

Thanks but you say 'in my USB audio interface', meaning in this case either the Powerbook USB port or the USB cable I am using for the capture? Would that explain the hum with the older CD burner? The hum occurred with a previous Powerbook when I used that CD burner. I haven't used that CD burner with this Powerbook.

I too am using the EyeTV, in this case the 250 Plus. I will write to them over the weekend since I wrote a BIG note to them recently and I doubt they're in over the weekend, and I wouldn't want to bother them then anyway.

I also have a somewhat funky arrangement for my incoming audio to the EyeTV converter/computer. I'll work to be sure nothing is shorting there. (I have one mini to RCA cable that I use an A/B switch to change from audio in and out to/from the stereo, that connects to the Powerbook. I put the EyeTV box inbetween the stereo and the computer so I could keep this arrangement for feeding audio to/from the computer to/from the stereo. It's worked fine with no hum without any firewire devices connected. That audio cable is not shielded. Very susceptible to (what, RF?) from firewire cables I guess... I might want to rethink those connections and get shielded ones. Do they make shielded RCA cables? Monster probably does = expensive.

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Answer 5

Sep 22, 2007 7:01 AM in response to lipwak

What you have is a ground loop, not radiated interference. USB and firewire are digital interfaces, they are immune to EMI adding analog hum.

The problem is that you have two different grounds at differing potentials connected to the mac. The difference in current cause a voltage to flow which is causing the hum. This is why MIDI uses optical isolaters: There is no direct "wire" connection between equipment.

Here are the solutions:
Use the 6to4 FW cable. Use those grounding adapters that allow you to connect grounded equipment to ungrounded outlets on all plugs except one - i.e, only one plug should connect to the house ground. Sometimes - if it is possible - simply reversing the polarity of a non-grounded plug can help in these situations.

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Answer 6

lipwak Author

Level 1

127 points

Sep 22, 2007 10:19 AM in response to Keith Barkley

Well, I may have solved the problem. I moved the power plug for the Powerbook onto the same power strip that the Firewire drive is plugged into. The hum is now very minimal. I can definitely live with it. I did some net research and saw that some people are recommending having all similar kinds of equipment come out of one plug in the wall, which I can't do, or their own power strips. I tried that and it seems to work, or I've done something else that is lulling me into a false sense of security.

But for now, very little hum. I'm happy.

I will keep this thread in mind should the problem reappear.

Cheers and thanks.

John L

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Answer 7

lipwak Author

Level 1

127 points

Sep 22, 2007 6:48 PM in response to lipwak

9:42 pm
It's back! The hum is back (and EyeTV has been giving me trouble). I'm not a happy camper. I didn't change anything with the drive or power plugs. (I also have to check the USB ports on the Powerbook cuz now EyeTV will only work out of the left-hand one. My camera works fine out of the right-hand one leading me to believe it is the EyeTV cable but I will see if the Apple Hardware Test will check the ports...)

When it rains it pours...

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