Topic: Cell phone interference: how can I shield against it / what's wrong with my rig?

[SparkE!]

Are you using a GSM phone?  I think the intereference problem has been targeted to GSM phones specifically.  Unless you or someone you know has T-Mobile, you probably are not using a GSM phone and thus are extremely unlikely to generate the interference.  Most of the world is on GSM but in the states almost no one is GSM.  It's too bad GSM causes the interference cuz all around its a better type of network than CDMA and the other garbage we use in the US.

Yep, Vodafone GSM, 900/1800 MHz. I didn't do any sophisticated testing, I was just curious whether to switch of my phone during recording to avoid those awful sounds. I turned on my rig, dial my phone number on another phone and simply moved my phone around the hardware.

Try sending email or SMS.  That's what most people are complaining about.

Simply dialing another number does not cause you to get a GSM channel.  That doesn't happen until you establish the call or a data session.  With a live call, you also have to produce noise on your end of the call because GSM will not use the channel to transmit audio that is essentially silence.

[kuba]

Try sending email or SMS.  That's what most people are complaining about.

Simply dialing another number does not cause you to get a GSM channel.  That doesn't happen until you establish the call or a data session.  With a live call, you also have to produce noise on your end of the call because GSM will not use the channel to transmit audio that is essentially silence.

Ah, I see, that's pretty obvious, I'm a dumbass    Still, I've never caught an interference on my recordings.

[SparkE!]

thanks for piping in.

i'm pretty sure its the mic caps that are the problem tho (going by the jump in volume of the interference according to the proximity of the phone to the caps), so while RF-protected cables may help, but it won't eliminate the issue.

I haven't managed to get to the post office during the day yet, but will do so soon cos i'm keen to see if Chris can work some magic.

What's the status of this issue?  Did the magic work?  Are you interference-free now?

[it-goes-to-eleven]

Been wondering.. Anyone done any interference testing with the r09?

[SparkE!]

thanks for piping in.

i'm pretty sure its the mic caps that are the problem tho (going by the jump in volume of the interference according to the proximity of the phone to the caps), so while RF-protected cables may help, but it won't eliminate the issue.

I haven't managed to get to the post office during the day yet, but will do so soon cos i'm keen to see if Chris can work some magic.

What's the status of this issue?  Did the magic work?  Are you interference-free now?

Status guys? Did the mods work?  If so, what did you do?

[whatboutbob]

thanks for piping in.

i'm pretty sure its the mic caps that are the problem tho (going by the jump in volume of the interference according to the proximity of the phone to the caps), so while RF-protected cables may help, but it won't eliminate the issue.

I haven't managed to get to the post office during the day yet, but will do so soon cos i'm keen to see if Chris can work some magic.

What's the status of this issue?  Did the magic work?  Are you interference-free now?

Status guys? Did the mods work?  If so, what did you do?

Nope. No luck. Not sure what Chris tried though.  Seemed inappropriate to ask.

Am shopping around for better shielded mics now. 'tis difficult cos I don't know exactly what's poorly shielded about my current mics.  The only mics I can find that actively advertise being shielded are the new AT's.

[whatboutbob]

Try sending email or SMS.  That's what most people are complaining about.

Simply dialing another number does not cause you to get a GSM channel.  That doesn't happen until you establish the call or a data session.  With a live call, you also have to produce noise on your end of the call because GSM will not use the channel to transmit audio that is essentially silence.

I'm guessing Australia's network works differently because I get interference simply by dialling/receiving.

[SparkE!]

Try sending email or SMS.  That's what most people are complaining about.

Simply dialing another number does not cause you to get a GSM channel.  That doesn't happen until you establish the call or a data session.  With a live call, you also have to produce noise on your end of the call because GSM will not use the channel to transmit audio that is essentially silence.

I'm guessing Australia's network works differently because I get interference simply by dialling/receiving.

Dialing a number will not create interference until it is transmitted.  On most phones, that should not happen until you hit the "Send" button.  Receiving text can cause interference too because your phone has to acknowledge the receipt of each packet in the message.  Sending text should cause more interference than receiving text.  Whatever the case, the channel access protocols are the same in Australia as anywhere else that uses GSM.

Sorry to hear that the fix did not work.  I think Chris probably put capacitors on the output of the mic amps, right where the output is attached to your mic cable.  That's a good place to start, but a better place to put the caps is probably on the input to the mic amp.  The problem with that is that it can affect your mic's sensitivity and may also affect the frequency response.  It depends on the topology of that amp.  Also, care should be taken to minimize the inductance of the ground path back to signal ground of the amp.  Feedthroughs on the board can represent 2 to 4 nH of inductance and a typical trace will have perhaps 1 nH per mm of length.  It depends on the thickness of the trace.  The smaller the trace, the higher the inductance.  Anyway, the parasitic inductance of the board traces can affect the effective impedance of the capacitors and move their self-resonant frequency to a lower frequency.  The caps were chosen so that they would have series resonance in the center of the GSM band, creating a virtual short to ground at that frequency.

Whatever you get for mics, good performance around GSM phones will rely on not only good shielding, but good rf bypassing around the active elements in the mic amp and good rf separation between shield ground and signal ground.

I don't know about how your mic cables are set up, but I'm assuming that they have a pair of signal wire (+ and -), a signal ground and a shield ground.  If so, it's common practice to isolate shield ground from signal ground at one end of the cable or the other.  If it's hum you're fighting, then you usually separate them at the mic end of the cable.  If it's rf you're fighting, then it's common to separate them at the far end (not the mic end) of the cable.  You might look into whether you can disconnect the shield from signal ground at one end of the cable or the other.

If on your cables, signal ground = shield ground, then maybe you can replace the cables and get better results, leaving shield ground separated from signal ground at the destination end of the cable.

Regardless, sorry to hear that the mods didn't work.  I was hoping for good results with a simple fix.

[whatboutbob]

Simply dialing another number does not cause you to get a GSM channel.  That doesn't happen until you establish the call or a data session.  With a live call, you also have to produce noise on your end of the call because GSM will not use the channel to transmit audio that is essentially silence.

Dialing a number will not create interference until it is transmitted.  On most phones, that should not happen until you hit the "Send" button.  Receiving text can cause interference too because your phone has to acknowledge the receipt of each packet in the message.  Sending text should cause more interference than receiving text.  Whatever the case, the channel access protocols are the same in Australia as anywhere else that uses GSM.

Ah...ok...I think we've got definitions of 'dialling'.  The 'dialling' I was likely incorrectly referring to is after I or the other person click 'send'.  So we are in agreement.  Having said that, I've also noted and been able to replicate (though not with my phone so it may be another network type) interference when no active transmissions have been going on - ie the incoming/outgoing texts/calls.  I think the phones are 'touching base' in an attempt to get a signal...why I've got no idea.

Sorry to hear that the fix did not work.  I think Chris probably put capacitors on the output of the mic amps, right where the output is attached to your mic cable.  That's a good place to start, but a better place to put the caps is probably on the input to the mic amp.  The problem with that is that it can affect your mic's sensitivity and may also affect the frequency response.  It depends on the topology of that amp.  Also, care should be taken to minimize the inductance of the ground path back to signal ground of the amp.  Feedthroughs on the board can represent 2 to 4 nH of inductance and a typical trace will have perhaps 1 nH per mm of length.  It depends on the thickness of the trace.  The smaller the trace, the higher the inductance.  Anyway, the parasitic inductance of the board traces can affect the effective impedance of the capacitors and move their self-resonant frequency to a lower frequency.  The caps were chosen so that they would have series resonance in the center of the GSM band, creating a virtual short to ground at that frequency.

Whoosh...I'm afraid most of that all went sailing over my head. I'll try to translate it some time soon when its not 2am.  Before sending the mics off to Chris, as per advice in this thread i I did try placing various caps across various pins with little success.

Whatever you get for mics, good performance around GSM phones will rely on not only good shielding, but good rf bypassing around the active elements in the mic amp and good rf separation between shield ground and signal ground.

I don't know about how your mic cables are set up, but I'm assuming that they have a pair of signal wire (+ and -), a signal ground and a shield ground.  If so, it's common practice to isolate shield ground from signal ground at one end of the cable or the other.  If it's hum you're fighting, then you usually separate them at the mic end of the cable.  If it's rf you're fighting, then it's common to separate them at the far end (not the mic end) of the cable.  You might look into whether you can disconnect the shield from signal ground at one end of the cable or the other.

If on your cables, signal ground = shield ground, then maybe you can replace the cables and get better results, leaving shield ground separated from signal ground at the destination end of the cable.

Regardless, sorry to hear that the mods didn't work.  I was hoping for good results with a simple fix.

To be honest, I'm not sure how my audix cables are setup (am yet to hack into 'em)...but they are the 'standard' cable that ships with the mics.  I do know my AT853s are in the '3 wire' config because I hacked them myself after being frustrated by brickwalling (blindly following instructions kindly laid out by Richard I believe).  What '3 wire' means has all been explained to me patiently in the past...but right now I can't recall what that means for ground.  I do know one of the first things I tried after getting sh!tty with the interference was to 'lift the ground' after doing some digging around ancient threads on here. Not even sure if I did it right tho.  I think I just disconnected the 'ground' from the pin at the amp-end.  This resulted in a hum and did nothing to improve the interefence (I think it actually got worse).  Argh...anyways...I'm off to bed.

Once again...apologies for my noobness and thank you very much for taking your time to talk me through these issues.