USB crackling noise (not a hum, but more like listening to a modem)

[ohangelo]

I have a strange configuration. I will try to describe it and why it's setup the way it is.
 
First, I run and host WebEx-style meetings for hours a day. My phone interface is an iPhone 4S since my landline does not do free toll calls, which drives most of my calls to my cell phone (unlimited minutes plan).
 
I have experimented with various BT headsets, but none seem to have the battery stamina to stay all day, and the quality of my voice going down the line is not always consistent, etc.  The Apple wired earbuds don't have bg noise cancellation, so they pick up everything, like my keys on my keyboard, and the click of my mouse. Too sensitive...
 
Having some studio equipment around in not in use, I mounted an EV RE-20, mic mixer (with EQ, line outs, etc.) an iRig to convert line level back to the correct level for iPhone in, and of course, some nice headphones for my own listening to what's coming back up the phone line.
 
Obviously, running this from a cell phone drains the batteries of a cell phone, and that's where the noise issue comes in. 
 
If I'm charging/powering the iPhone from the PC USB cable, I get what I can best describe as a high pitched crackling, almost like a modem sound, as background noise. It's not full volume, and it's not enough to make me pull my headphones out, but it is definitely there.  If I power the phone from an AC adapter, not connected to the computer. The noise is not a hum like a 60hz ground loop would sound like.
 
I would like to connect my mixer to the audio in on the computer so that I can use the mic/mixer and other sources to do VOIP and such, but if this noise is present in the USB bus, I'm not sure how nice any of the other audio in the computer will sound.  (BTW, Computer is a HP laptop, with docking station, with 1/8" TRS audio in and out, sound card is the onboard IDT High Definition Audio).
 
Everything seems grounded properly. (Perhaps that the problem?)
 
I've heard that computers and audio don't really get along all that well.
 
Is there a set of steps I can follow to make this go away? 

 

[PleasantSounds]

Quote:

[..]
 
If I'm charging/powering the iPhone from the PC USB cable, I get what I can best describe as a high pitched crackling, almost like a modem sound, as background noise. It's not full volume, and it's not enough to make me pull my headphones out, but it is definitely there.
 

 
This could be because your computer audio circuits are powered from the same source as the USB bus. The phone may be creating more load on the 5V power supply than it can deliver, and the sound system suffers. Does this crackling still happen when the phone is fully charged?
 
 

 
 If I power the phone from an AC adapter, not connected to the computer. The noise is not a hum like a 60hz ground loop would sound like.
 

 
Does your AC adapter have the earth plug? If not, then this shouldn't be a ground loop. Quite a few AC adapters do a very poor job converting to DC, as normally perfect DC current is not needed if it's only a battery to be charged. But sound devices are very sensitive to that.
 

 
I've heard that computers and audio don't really get along all that well.
 
Is there a set of steps I can follow to make this go away? 

 
 
What you can try is to find a better power source for your phone - maybe a powered external USB hub would do the trick?  I can't guarantee that this will work, but that would be the first thing I'd try.
 

 

[ohangelo]

Thank you for your well-thought reply. 
 
A couple notes here.
 
a) If I power the phone via a standalone charger (with or without a grounding pin), the noise is not there.
 
b) If the phone is fully charged or not, seems to make no difference; the noise is only present when the USB-iPhone cable is being used to power from the USB port (remember, this port will also carry data)...
 
I guess I can get away with never powering the phone via the USB port when I'm doing webcasts.
 
What I was hoping to avoid, and have not verified yet if it will be a problem, is if any connection of the computer into my audio chain will also carry this high frequency crackling/data bits sort of noise...  All my hopes of being able to do VOIP from my mic/mixer would be flushed away if that noise were there. (Hence, looking for the root of it now.)

 

[PleasantSounds]

Quote:

Thank you for your well-thought reply. 
 
A couple notes here.
 
a) If I power the phone via a standalone charger (with or without a grounding pin), the noise is not there.
 
b) If the phone is fully charged or not, seems to make no difference; the noise is only present when the USB-iPhone cable is being used to power from the USB port (remember, this port will also carry data)...
 
I guess I can get away with never powering the phone via the USB port when I'm doing webcasts.
 
What I was hoping to avoid, and have not verified yet if it will be a problem, is if any connection of the computer into my audio chain will also carry this high frequency crackling/data bits sort of noise...  All my hopes of being able to do VOIP from my mic/mixer would be flushed away if that noise were there. (Hence, looking for the root of it now.)

 
That means my theory was rubbish
Not sure how to solve your problem as I don't fully get your setup, but from my experience USB certainly is suitable for use even in very demanding audio setups. I've heard of some issues when devices are powered via the same USB connection, but never encountered that myself. Possible that one of the devices in your chain could be faulty.

 

[Roseval]

As it is a laptop, disconnect the AC. Sometimes the PSU is rather noisy.
Sometimes trying another USB port or using a USB hub helps.